The purpose of the cardiovascular system (heart and blood vessels) is to provide the cells of the body with oxygen, nutrition, and essential fluids. It also helps these same cells rid themselves of waste products, and distributes hormones and enzymes to allow for normal physiologic processes. It is even a big part of temperature regulation. all of this is no small feat when you consider the fact that the cardiovascular system must supply these needs to a body that contains billions of individual cells.
The cardiovascular system is a very complicated and does not lend itself to a simple explanation and categorization of its functions. This page summarizes this complex system. If you would like substantially more detail, go to our Heart page.
This page also has actual pictures of the heart and the organs of the chest. Most people will not be bothered by their graphic nature, and will actually find them fascinating.
We have a few short videos on this page, you will need QuickTime from www.apple.com to view them
This is the heart of a 50 pound dog. It is about the size of your fist. You can easily see some of the coronary arteries that supply blood to the heart muscle in the same manner that the heart supplies blood to the rest of the body. This organ starts beating before birth, and continues until death. Think of how many beats that is in the lifetime of any living organism!
Later in this page we will be referring to the right heart and left heart, which might give you the impression there are two hearts. There is only one heart- we do this only because it helps to understand the flow of blood through the heart.
We will be using some basic diagrams to explain the flow of blood through the 4 chambers of the mammalian heart. This will dramatically help in your understanding of the flow of blood. It will also help in understanding the anatomy later in this page when we show you actual anatomic structures of the dogs heart. It might be a good idea to come back to these diagrams when we go into more detail later.
Blood that has supplied the cells with oxygen, and now contains carbon dioxide to be eliminated from the body when we exhale, flows into the heart from 2 directions. From the head, it flows through the anterior vena cava (AVC) into the right atrium. From the back end of the body it flows through the posterior vena cava (PVC) and into the right atrium. When an adequate amount of blood has filled the right atrium (this takes only milliseconds) it contracts, and blood flows through the tricuspid valve (also called the right Atrio-Ventricular valve) and into the right ventricle. The tricuspid valve, like the other valves in the heart, is a one-way valve. In a healthy heart blood can flow only from the atrium into the ventricle.
The blood that is now in the right ventricle rapidly mixes with the small amount of residual blood that remains in the right ventricle from its last contraction. When the right ventricle is filled adequately (again, this takes only a few milliseconds), it contracts and the blood flows through another valve called the pulmonic valve, into the pulmonary artery, and eventually into the lungs. In the lungs the blood rids itself of carbon dioxide and absorbs a fresh supply of oxygen during exhalation and inhalation. The blood goes from poorly oxygenated to freshly oxygenated, and is now ready to supply the cells of the body with fresh oxygen all over again. Now it just has to get to all those billions of cells.
The freshly oxygenated blood in the lungs flows through the pulmonary vein and into the left atrium. Just like in the right atrium, when there is enough blood present, the left atrium contracts and the blood flows through the mitral valve into the left ventricle. Remember this mitral valve since we will be talking about it later in a common heart problem in the geriatric pet called endocardiosis. The mitral valve is a one-way valve also.
The blood that enters the left ventricle mixes with residual blood that remains from the last contraction. When the left ventricle is adequately filled with blood it contracts and ejects its blood through the aortic valve and into the aorta. Once in the aorta, a branch called the brachiocephalic trunk, supplies the head, while the rest flows down the descending aorta and to the rest of the body. a small branch (not shown in this diagram) off the aorta supplies the heart through the coronary artery.
The right and left hearts are doing their work at the same time, so coordination of all this blood flow is critical. This is especially apparent when you realize that a typical dog or cat heart is beating between 100 and 200 beats every minute. This means that for the average dog or cat, everything in this diagram below happens twice each second. Think of the coordination needed in a bird whose heart rate is 350 beats per minute! The heart valves have to open and shut very rapidly, the atria and ventricles have to expand and contract very rapidly also, and the rest of the body has to cooperate in the first place by bringing an adequate amount of blood into the heart through both vena cava’s. In addition, the respiratory system has to cooperate by inhaling air, exhaling air, and exchanging carbon dioxide and oxygen. Its a wonder that all of this can even be coordinated.
When the heart muscle contracts and ejects blood it is called systole. When the heart muscle is relaxing in between systoles, and filling up with blood in readiness for the next contraction, it is called diastole. Diastole is important to the heart muscle itself, since it is during diastole that blood flows into the coronary artery to supply the myocardium.
With such a complicated system it is no wonder that things can go wrong. We will learn more about heart pathology later, first there is more to learn about anatomy and physiology of the cardiovascular system.
Vascular Anatomy and Physiology
These vessels direct blood away from the heart and towards the cells of the body. They tend to lay deep in the body tissues, partially to protect them from trauma. Arteries have several layers; a tough outer layer, a middle layer of smooth muscle, and an inner layer of very smooth cells. The tough outer layer allows the artery to withstand the high pressure that occurs with each beat of the heart. Most normal people and pets have a blood pressure that ranges around 120-170 mm of Hg (mercury) when the heart contracts (systole). A giraffe has a blood pressure that goes up to 240 mm of HG- can you guess why?
The smooth inner layer of the artery gives red blood cells and the fluid surrounding the red blood cells (called plasma), a friction-free pipe to get to all of the cells of the body.
The muscular wall (the middle layer) of the artery helps the heart pump the blood. When the heart beats, the artery expands as it fills with blood. When the heart relaxes, the artery contracts, exerting a force that it strong enough to push the blood along. This rhythm between the heart and the artery results in an efficient circulation system.
The smooth muscle in the walls of arteries also allows them to selectively constrict and dilate. This is very important, because blood does not flow to all organs in the same amount consistently. For example, when you eat a meal, the arteries to your intestines dilate and more blood flows to them to aid in digestion. Or, when you are exercising, the arteries to your intestines will constrict and the arteries to your muscles will dilate. This process goes on continuously and in a highly refined process throughout life, all depending on the physiologic needs of individual cells at a specific moment in time.
The main artery from the heart is called the aorta. It is large, and has a thick wall because of the high pressure of blood that is flowing through it. The ascending aorta supplies the head with blood through arteries called the brachiocephalic trunk, eventually branching to the carotid arteries. The descending aorta goes through the thoracic cavity and supplies the rest of the body from within the abdomen. A branch of the descending artery, called the coronary artery, supplies the heart. Lets not forget the fact that the heart is an organ with millions of cells also, and they need oxygen and nutrients also if they are to perform their job.
This long white structure is an actual picture of a dog’s aorta as it comes off the heart (you cannot see the heart) on the left. The blood is flowing from left to right as it goes to the back of the body. The aorta is firmly embedded in a structure called connective tissue. This tissue gives it stability, and prevents it from tearing or rupturing during movement. A weakening anywhere along the aorta can lead to a bulge called an aneurysm. If an aneurysm ruptures death is almost instantaneous.
This radiograph of a cat’s chest shows the aorta as it leaves the heart in the same way as the picture above. The arrows show the direction of blood flow to the back of the body. You cannot see the branch off the aorta, called the brachiocephalic trunk, that supplies the head. This trunk branches off near the first arrow in the lower left.
As an artery gets further from the heart it gets smaller and eventually becomes an arteriole. An arteriole is smaller in diameter than an artery, and is found closer to the target organ. For example, a branch off the descending aorta, called the renal artery, supplies the kidneys. As the renal artery enters the kidneys it breaks up into many small branches called arterioles. The arterioles also are lined with smooth muscle, allowing further refinement of blood flow to a target cell.
These small blood vessels surrounding the kidney give you an idea of how they can branch into smaller and smaller sizes as they enter an organ
At the level of the cell the arteriole branches into even smaller vessels called capillaries. They do not contain smooth muscle, and cannot selectively constrict or dilate like arteries and arterioles. They are very small in diameter, so only one red blood cell can pass at a time. In fact, the capillary is so small that red blood cells literally have to squeeze their way through in many cases. The wall of capillaries are only one cell thick, all for a reason. It is at the capillary level that oxygen flows from hemoglobin, contained in the red blood cell, into the actual kidney cell or liver cell. At the same time, the hemoglobin picks up carbon dioxide that is coming out of the cell. This red blood cell, whose hemoglobin is now saturated with carbon dioxide instead of oxygen, eventually flows back to the lungs to rid itself of carbon dioxide and take on a new load of oxygen for delivery to some other cell in the body. After about 90 days the red blood cell wears out and is metabolized by the body.
The capillaries have many other functions besides the exchange of oxygen and carbon dioxide. The yellow around the red blood cells in the diagram above is fluid, called plasma, that flows in the bloodstream along with red blood cells. This fluid contains nutrients like fats, carbohydrates, proteins, and electrolytes that the cell needs to function. It also contains hormones, clotting factors, and drugs that we might administer. This fluid also flows though the wall of the capillary and into the cell.
Later on you will learn about pulmonary edema, which is fluid buildup in the lungs. In the case of the diagram above, the cell is the actual lung (called the alveoli) that becomes filled with excess fluid. This occurs because the fluid in the capillary is under higher pressure than normal, so more fluid flows out of it and into the actual lung cell. We will explain why this happens later in the pathophysiology section.
Some capillaries have specialized functions. If they line the intestines they will absorb nutrients like fats, carbohydrates, and proteins from the inside of the intestines directly into the bloodstream. If these capillaries line the kidneys they will help excrete waste products and regulate the metabolism of electrolytes. If they line the liver they will help in the metabolism of nutrients and the distribution of hormones.
As the capillary leaves the individual cells it is assigned to supply, and starts the journey back to the heart, it becomes a venule. Venules are small veins, and have a job similar to arterioles, although there are many more venules than arterioles. Their numerous branches drain an organ, eventually coalescing into veins on their trip back to the heart.
As the venules coalesce they eventually form veins and continue on their way through the cardiovascular system. Veins have 3 layers just like arteries, although each layer is thinner and not as strong. They don’t need to be as strong because the blood is under much lower pressure in the venous system. The blood in the veins is darker in color compared to the blood in the artery because they contain less oxygen. about 2/3 of the blood in the body resides in the veins at any one time.
Those veins in the back of the body eventually drain into the posterior vena cava, and into the right atrium of the heart. The veins that drain the head and upper part of the body eventually drain into the anterior vena cava and into the right atrium of the heart. You learned about this in the diagram at the beginning of this page. The pressure in the veins is much lower than in the arteries and arterioles. This can be a problem in the extremities. For example, if you stand for a long period of time, the blood in the veins of your lower legs needs to push against gravity to get back up into your heart. There is not enough pressure in these veins to do this by themselves.
To help get this blood back into the heart the veins have one-way valves so blood always flows towards the heart. Also, the skeletal muscle surrounding these veins continuously contracts in small amounts, further pushing blood in the right direction. You can see this yourself in your leg veins. Cross one of your legs and watch your calf muscle closely. You can see the small muscular contractions helping the veins.
If you can’t see this on your own leg we coerced a volunteer to film these muscle contractions on his leg. We tried to get him to shave his leg so you could visualize the muscle contractions even better, but he wouldn’t go for it!
Double click on the picture and in a few seconds you will notice subtle muscle contractions. This is the back of his calf muscle (gastrocnemius). His foot is on the left, his knee is towards the right.
Veins tend to lay at the surface of the skin, and are easily visualized. In addition to the functions described above, they are part of the thermoregulatory mechanism of the body. When they are at the surface and are dilated they rid the body of excess heat.
As you can imagine, nature is not always this simple. There is a special set of veins in the body called a rete mirable. It is a conglomeration (for lack of a better word) of arteries and veins, usually running adjacent but flowing in opposite directions, that allows for heat exchange. A classic example is the pampiniform plexus, one of the thermoregulatory mechanisms of the testicle. In mammals, the core temperature of the testicle has to be a few degrees cooler than core body temperature for sperm to be fertile. The warmer arterial blood coming from the body and supplying the testicle flows directly past the slightly cooler blood that is in the venous system draining the testicle. This allows the cooler blood from the venous system to absorb some of the heat from the warmer arterial blood, thus slightly cooling the arterial blood that enters the testicle (did that make sense to you?).
This is a picture of the pampiniform plexus (arrow) of a dog. We took this picture from our neuter page. It is difficult to differentiate the arteries from the veins because they are all wrapped together.
The body contains three different types of muscle. The first, called skeletal muscle, is the muscle type we usually think about when we mention muscles. Skeletal muscles provide locomotion and movement- the biceps muscle to move our arms, or the quadriceps muscles to move our legs. These muscles perform their action due to conscious control of our brains. Your arm does not automatically shoot a free throw until you tell it to do so (even when Dr. P tells his arm to shoot the free throw it doesn’t always work as planned).
The second type of muscle is called smooth muscle. Smooth muscles tend to perform their functions automatically without any conscious thought on our part. You don’t tell them what to do, they do it on their own, under the control of the autonomic nervous system (ANS). The muscles that surround arteries to make them constrict or dilate are smooth muscles. The muscles of the intestines that push the food along (a process called peristalsis) are made of smooth muscle.
The last muscle type is called cardiac muscle, and only resides in the heart. Cardiac muscles have their own supply of electricity, and “fire off” on their own without any stimulation.
This is a close-up view of the heart muscle (myocardium) in the left ventricle of a dog. It is thick because this dog’s heart has to spend a lifetime pumping blood to all the cells of the body. Later on when we talk about cardiomyopathy it is this muscle that is affected.
How does the heart beat all by itself? It does this because each heart cell supplies its own electricity. To make a complicated story a little simpler, it has to do with how the heart cells retain or excrete potassium, sodium, and calcium ions. When sodium and calcium are pumped out of the heart cell, potassium is pumped in. This eventually creates an imbalance in their equilibrium, with many more sodium and potassium ions outside of the heart cell than inside. This creates a “positive” charge outside of the heart cell , and the heart cell is now “polarized” (remember this word when we discuss electrocardiograms).
The body eventually wants to correct this imbalance of sodium, calcium, and potassium. So the opposite occurs. Potassium rushes out while the sodium and calcium rush in. The cell is now depolarized, and will stay that way until the positive charge outside the cell again reaches a threshold and the flow once again reverses. Every time this reversal of flow occurs, it generates a spark of electricity which races through the heart. It is this electrical spark that causes heart cells to contract and the heart to beat.
Even though they beat on their own, the electrical activity in each heart cell needs to be coordinated if the 4 heart chambers (atria and ventricles) are to pump an adequate amount of blood in the proper direction. Later on, when we talk about electrocardiograms, it is this electrical conduction we will be referring to.
At the beginning of the right atrium there is an anatomical structure called the sino-atrial node (Sa Node). It is this area of the heart muscle that originates the coordinated beating of the heart. When this Sa Node fires off it sends electrical impulses (the wires that carries these impulses are called perkinje fibers) through both atria, causing them to contract at the correct time. One of the signals from the Sa Node also stimulates the atrio-ventricular node (AV Node) located at the bottom of the right atrium. Stimulation of this node stimulates nerve fibers that surround the ventricles, causing them to contract in a rhythmic way. There are other factors involved, especially hormones and other parts of the nervous system.
The atrio-ventricular bundle (AV bundle) is also known as the bundle of His in honor of the man who discovered it. Since the heart in a dog or cat beats approximately 2 times every second, these nodes have to fire off rapidly if everything is to stay coordinated. From the time the SA Node fires it takes only 0.22 seconds for the ventricles to contract.
The SA node is called the pacemaker because it depolarizes at a faster rate than any other group of cells in the heart, and imposes that faster rate on the heart as a whole. If for any reason the SA node stops beating, the AV node, which has the next fastest rate of depolarization, would become the heart’s pacemaker. If the AV node failed, the AV bundle would take over. If it failed, the Perkinje fibers would start the heartbeat, and if they failed as well, a group of cells somewhere else in the heart would start pulsing. However, the further away the heart gets from its normal pattern and rate of beating, the less blood it pumps and receives, until eventually it can no longer sustain Life.
Disturbances in the heart’s normal rhythm, known as arrhythmias, are a common problem in heart disease. Arrhythmias can be minor and unimportant, or severe and life threatening. There are many different kinds of arrhythmias, including:
- Tachycardia- An abnormally fast heartbeat. If the hear beats too fast is does not spend enough time in diastole. Therefore the heart chambers do not fill up enough with blood, so the heart does not pump out an adequate amount of blood for the needs of the cells. In addition, the lack of time in diastole causes the heart muscle itself (myocardium) to suffer since it is in diastole that blood flows from the coronary arteries into the heart muscle.
- Bradycardia- An abnormally slow heart beat. If the heart beats too slowly the blood pressure decreases and it does not generate enough flow of blood to the cells. One of the first signs of this is called syncope, which is the same this as passing out and becoming unconscious.
- Heart block- Occurs when the electrical impulse has difficulty passing through the AV node.
- Atrial fibrillation- When the atria contract in an irregular way and blood does not flow out of them effectively
- Ventricular fibrillation- When the ventricles contract in an irregular and ineffective way, a condition which quickly leads to death unless corrected. This is a heart attack in people, and needs a defribillator to correct the problem. People who have serious arrhythmias can sometimes have an artificial pacemaker implanted in their chest or abdomen. This battery powered device delivers a rhythmic electrical impulse to the heart on either a constant basis, or only when the heart’s natural pacemaker temporarily fails to sustain a normal beat.
You will learn more about the SA node and AV node in the electrocardiogram section to follow, so try to keep their jobs in mind.
Cardiac Chambers (atria and ventricles)
The normal mammalian heart has 4 chambers (birds also have 4, reptiles have 3). The 2 smaller chambers are called atria, the larger ones are called ventricles. The diagrams at the beginning of this page described the flow of blood through these chambers. Now lets see what these chambers and valves actually look like. Click here to review the diagrams at the beginning of the page if you need to.
The following are necropsy (the same as an autopsy in people) pictures showing how these structures actually look. They are done tastefully and should not bother you. It will be obvious from these pictures that the real anatomy is much more complicated than the diagram pictures. We will be emphasizing the left ventricle and mitral valve, since that is the area of the heart that causes most of the problem as dogs age. We will trace the flow of blood from the left atrium, through the mitral valve, and into the left ventricle.
Before we even get to the heart, there is a layer called the pericardium that surrounds it. In some diseases, fluid can buildup in between this outer layer and the actual heart muscle. This is the pericardium from a normal ferret. The fat at the bottom of this heart is normal.
This picture shows mostly the inside of the left ventricle of a dog (its the same heart you saw at the beginning of this page). You are looking into the chamber of the left ventricle. Note the thickness of the cardiac muscle (myocardium) surrounding the left ventricle, along with the smooth inner lining of the ventricle in the lower center of the picture. The lining needs to be smooth and relatively friction free for the red blood cells to flow through rapidly and not get ruptured or start clotting. The tip of the metal hemostat (see arrow) just barely poking out is coming from the left atrium (not visualized), through the mitral valve, and into the left ventricle. This is the normal direction of blood flow as it comes out of the left atrium and into the left ventricle.
Lets take a little closer look at the mitral valve. We have moved the hemostat a little further through the mitral valve in this picture. Again, the left atrium is not visualized. The white filamentous structures are called chordae tendinae. When the blood flows through the mitral valve these chordae tendinae are relaxed since there is no pressure on them. When the left ventricle contracts it exerts great pressure to get the blood through the aorta and to the rest of the body. This pressure pushes against the mitral valve, which is now shut since we do not want blood flowing backwards into the left atrium. It is these chordae tendinae that keep the mitral valve closed. a normal mitral valve can withstand this pressure, a diseased one cannot.
As we get even closer we can see the leaflets of the mitral valve clearly (we removed the hemostat so you can see the bottom of the valve now). The top arrow points to a normal leaflet, the bottom arrow points to a thickened and shrunken leaflet. This thickened leaflet is called endocardiosis (you will learn more about this in the disease section when we teach you about chronic atrioventricular valve disease). This thickening does not allow the valve to close fully, and blood regurgitates backwards into the left atrium when the left ventricle contracts. Since there is a huge difference in pressure (called a pressure gradient) between the left atrium and the left ventricle, this can have serious consequences. This regurgitating blood is turbulent, and is the source of the heart murmur we hear with this disease. If the leakage is significant the pressure will cause the left atrium to enlarge (can be seen on a radiograph), with the potential for this added pressure to impede the flow of blood from the pulmonary vein. If the blood in the pulmonary vein has a hard time flowing against this pressure in the left atrium, the plasma contained in the pulmonary veins will leak out of the capillaries and fill the lungs (the alveoli) with fluid. This is also called pulmonary edema, and is the “congestive” in congestive heart failure. We will discuss this in more detail later since it is an important aspect of chronic atrioventricular valve disease and congestive heart failure.
There are many other structures inside the chest (thorax) in addition to the heart and lungs. This next necropsy picture is from a dog, laying on its right side, with the head towards the left. We will be looking into the thorax, at the section of the thorax just before the abdomen.
Before we show you the necropsy picture lets get oriented. The dog is laying on its right side and the head is towards the left. The vertical white line on this radiograph points to the section of the thorax we will be looking at in the necropsy pictures to follow. The white arrow points to the horizontally running posterior vena cava (PVC) that is faintly visible. Use this landmark for your orientation when you look a the actual pictures below.
On the far right is the diaphragm (D), the muscle of respiration. It separates the thorax to the left of the diaphragm, from the abdomen on the right (the liver and stomach are just behind the diaphragm). The posterior vena cava (PVC) is visible as the horizontal blue structure at the bottom of the picture that is coming through the diaphragm. It is large because it has the job of returning almost all of the blood from the back end of the body to the heart.
The large horizontal pink structure above the posterior vena cava is the esophagus (E) as it goes through the diaphragm and enters the stomach behind the diaphragm on the right. You can see one of the posterior lung lobes above and to the left of the esophagus. If you look closely you can also see a white nerve running horizontally along the esophagus (vertical arrow). If you look even closer you can see a large white structure running horizontally just above the esophagus (horizontal arrow)- its the aorta embedded in tissue for protection.
With all of this anatomy packed into the thorax its a wonder we can even breathe at all!
This is the same picture as the previous one, only viewed from the top and not the side. The head is towards the top with the dog laying on its back, the diaphragm (D) is at the very bottom. The structures are labeled the same. Notice how much more lung is visualized. Look at the large veins to the lung lobes in the upper right. The posterior vena cava (PVC) is obvious as it runs vertically exiting the diaphragm at the very bottom of the picture and enters the right atrium at the top of the picture. On each side of the vena cava are lung lobes, then the esophagus (E), then the white aorta (a). Keep in mind these lungs are deflated. Think of how crammed this space is when the lungs are filled with air as we inhale. As a matter of fact, the negative and positive thoracic pressures that occur when we breathe have an influence on how this blood flows.
Now we are moving away from the diaphragm and going closer to the heart on a side view, with the head at the left again. The heart is the dark blue structure on the top. The pericardium (lining of the heart) is still around the heart, so it is not as apparent as you might expect. The right atrium cannot be visualized because the heart is covered with the pericardium. You can see the posterior vena cava (PVC) on the right as it enters the right atrium. You can also see the anterior vena cava (AVC) on the left as it also enters the right atrium. You can also see a nerve as it runs horizontally on top of each vena cava.
A normally functioning heart needs to be working in optimum condition, able to instantly adjust to the varying needs of the body. For this to happen everything needs to work in unison:
The blood vessels to the heart need to be functioning normally. A problem here (atherosclerosis) is a disease seen usually in humanoids, not animals. When these blood vessels do not supply the heart with an adequate blood flow, a myocardial infarct (MI) occurs. This means that a section of heart muscle dies because of a lack of blood supply.
The electrical conduction system has to be working in a coordinated fashion for the blood to flow efficiently through the heart chambers. If the problem is severe enough a heart attack can occur. In this condition the heart needs an external electrical charge (defribillator) to shock it back into normal rhythm.
All of the heart valves need to be working optimally so that blood can flow in the proper direction and in adequate amounts. A leaking valve causes regurgitation of blood backwards into the wrong chamber. This abnormal blood flow leads to turbulence, which is picked up by the stethoscope as a murmur. If severe enough the problem can lead to heart failure.
The heart chambers and muscles need to be the proper size for optimal flow of blood. Also, the septum that separates the right heart from the left heart needs to be intact. If not, blood can now flow directly from one ventricle to another, bypassing its normal route through the lungs. A dilated heart chamber leads to dilated cardiomyopathy, which is a heart muscle too weak to beat with enough force to supply the cells with blood. A heart chamber that is too muscular, called hypertrophic cardiomyopathy, leads to a ventricle chamber size that is too small to fill up with enough blood for the body’s needs.
The arterial and venous systems need to be able to constrict and dilate so that proper blood pressure is maintained and also so all of the cells of the body get an adequate blood flow.
The cardiovascular system of the body is truly a miracle. This series of pumps and pipes literally is able to supply the billions of cells in the body with all their essential needs, and it does this in an environment of constantly changing needs.
The whole point of the cardiovascular system is to provide the individual cells in each organ an adequate flow of blood (called perfusion) that gives them the nutrients and oxygen they need. When these cells have what they need they can perform their normal function. So for the kidneys, that means they can filter out waste products. For the muscles, that means they can initiate movement. For the liver, that means that liver cells can metabolize drugs we put into the body. This adequate perfusion equates back to how much fluid the heart can deliver to these cells. an adequate cardiac output is needed for this perfusion.
There is a difference between heart disease and heart failure. In heart disease the heart has some type of abnormality. If minor enough, the heart is able to deliver adequate perfusion to the cells, and there is no problem. In heart failure, the heart does not maintain an adequate perfusion for normal cell function. Pets that are relatively inactive may be able to stave off the effects of heart failure longer than active pets because they do not challenge the cardiovascular system. This has a bad side though, because by the time the symptoms of heart failure are finally apparent to an owner, the disease is well entrenched and more difficult to treat.
If the left heart becomes diseased it does not pump an adequate amount of blood (decreased cardiac output) through the aorta for distribution to the cells of the body. This inadequate flow of blood ((poor perfusion) prevents these cells from performing their normal functions The brain monitors this perfusion, and goes into action by regulating hormones and sodium in conjunction with the kidneys and the lungs. This increases the pressure in the arterial system as a whole, and satisfies the needs of the cells temporarily by supplying them with a greater flow of blood (better perfusion). This added blood pressure fills the diseased left ventricle with blood more than usual (increased preload), causing it to dilate and weaken further. It also increases the pressure the left ventricle has to pump against (increased afterload) to get the blood through the aorta and into the cells. These add further work to an already diseased heart, compounding the problem even further. Eventually, the blood presented to the left ventricle does not get pumped out effectively, which causes a back flow (added pressure) in the lungs. When the pressure reaches a certain point the fluid in the blood vessels in the lungs leaks out, causing pulmonary edema. This is congestive heart failure (CHF).
If the right heart becomes diseased, a similar set of physiologic sequences occurs. The higher blood pressure that results when the cells send their emergency signals to the brain results in a greater amount of blood being presented to the right heart (increased preload). Eventually, the weakened right heart cannot pump blood into the lungs faster than the venous system is presenting blood to it. This causes back pressure to build up in the venous system, especially the vena cava and other veins in the abdomen and even thorax. When the pressure gets high enough in these veins fluid leaks out, leading to ascites and pulmonary effusion. This problem can occur in both hearts at the same time, causing even more problems.
There are several factor, usually working in combination, that lead to cardiac disease:
They can buildup on the heart valves inhibiting their ability to flow blood in the proper direction through the heart.
They can directly affect the heart muscle (myocardium), causing cardiomyopathy. In the early years of parvo virus there was a form of the disease that attacked the myocardium and caused rapid death.
Taurine, an essential amino acid in cats, can lead to dilated cardiomyopathy. Humans and dogs can produce taurine naturally (non-essential) and do not need it in their diet. A deficiency of taurine in the cat will also cause degeneration of the retina. Taurine was not included in adequate amounts in cat food many years ago. This problem has been corrected in almost all commercial cat foods, so we rarely see this problem anymore.
An inadequate amount of the thyroid hormone thyroxin can predispose a dog to heart disease. This problem is diagnosed with a blood sample and corrected with thyroxin hormone replacement. This problem is almost exclusively seen in dogs.
This cat problem occurs when the thyroid gland has a tumor and secretes excess thyroid hormone (thyroxin). You can learn about it in detail in our hyperthyroidism page. In this disease the heart is racing as it tries to keep up with the increased metabolic rate of the organs as they respond to the increased thyroid hormone.
Congenital abnormalities of the heart valves, usually a problem in young dogs, can lead to significant heart disease. These can include any of the valves. We get an indication of this early in a pets life by the presence of a heart murmur. Young animals can have what are called “innocent murmurs”. These are heart murmurs that don’t cause any problems and eventually resolve. If these murmurs persist, are of high intensity, or the pet is ill, they should be pursued further with diagnostic tests.
Older dogs get a problem with heart valves that also can be significant. We will go into this in more detail soon because it is the most common cause of heart disease in the dog.
Some drugs are toxic to the heart. It is ironic that one of the drugs used to treat heart disease, called digitalis, can be toxic to the heart also. This drug is commonly used to help slow down a racing heart. When used, we monitor digitalis levels with a blood sample at least every l6 months to make sure it does not go into the toxic range.
A heart that receives a severe blow can have problems with adequate pumping of blood or the normal electrical beating.
The heartworm parasite (dirofilaria immitis) can cause severe heart disease. This problem is not uncommon in dogs, and has been diagnosed with increasing frequency in cats lately.
Lung Disease (cor pulmonale)
Problems with hypertension in the lungs can cause the right heart to fail. These problems can include cancer, heartworms, infections, trauma, toxins, hypothyroidism, and Cushing’s disease.
When vital internal organs like the kidney and liver are diseased there can be many changes that effect the heart. These include electrolyte abnormalities, calcium irregularities, and waste product buildup.
Some poisons selectively target the heart. Curare, the drug used in the tips of poison arrows and darts, is one of these drugs.
Typical symptoms of cardiac disease include:
- poor appetite (anorexia)
- coughing- initially caused by an enlarged heart pushing upwards on the mainstem bronchi (windpipe) when the left atrium enlarges. as time goes on, this coughing is exacerbated by fluid buildup in the lungs (pulmonary edema).
- passing out (syncope)
- difficult breathing (dyspnea)
- distended abdomen (especially if the right heart is diseased) called ascites
- pale or bluish colored gums (cyanosis)
- racing heart (tachycardia)
These symptoms are found in many other diseases also, especially of the respiratory tract. To determine which disease is causing these symptoms we need to strictly adhere to the diagnostic process.
In some heart diseases, notably cardiomyopathy in cats, there might be no symptoms prior to a sudden death. These cats seem fine until a stress causes their diseased yet compensating heart to reach its limit.
Several abnormalities might be noted in an animal with a heart problem. Most of these are due to poor perfusion of the target organ or fluid buildup due to changes in preload and afterload.
Difficult breathing or coughing due to fluid buildup in the lungs or thoracic cavity. This can be picked with the stethoscope during auscultation (listening to) of the heart and lungs. The term we use for an increased in lung sounds due to pulmonary edema is called rales.
Pale or bluish mucous membranes (cyanosis or anemia). This is noted when we look at your pet’s gums. Capillary refill time might be prolonged.
Distended abdomen (ascites) due to back pressure of blood not flowing from the back of the body into the right heart properly.
This Doberman with heart failure shows the typical appearance of a distended abdomen caused by ascites. There are other diseases that can cause this look, including Cushing’s disease.
Distended jugular veins due to back pressure of blood not flowing from the head to the right heart properly.
This short movie shows distended and pulsing jugular veins in the neck of a cat. The head is at the top (not seen). Click on it to play
Pulse deficit- A lack of synchrony between the beating heart and the pulse.
Dr. P is checking for a pulse deficit by listening to the heart with the stethoscope while simultaneously palpating the femoral pulse. The two should be exact. If the heart rate is faster than the femoral pulse there is a pulse deficit, indicating an arrhythmia.
An increased heart rate (tachycardia)
A heart murmur during auscultation with the stethoscope
Cool extremities to the touch
Reduced mental capacity
Many pets with heart failure are senior pets, so it is common for us to run a routine blood panel. There are no specific blood panel tests that diagnose heart disease in dogs and cats, although we can see indirect evidence of heart problems and their consequences sometimes. Also, the blood panel lets us assess problems in important organs like the kidney and liver that can occur simultaneously with heart disease in senior pets. This becomes important when we use medication to treat heart disease.
Our lab routinely performs a thyroid test on the blood panel for senior pets since a low thyroid level can be a cause of heart disease. This disease is called hypothyroidism, and occurs in dogs. A high level of thyroid hormone, called hyperthyroidism, occurs in cats, and can also cause heart problems. All pets 8 years of age or older should be checked for this hormone imbalance on a yearly basis.
A very important tool in the diagnosis of heart disease is the stethoscope. This simple yet elegant instrument allows us to hear the valves of the heart as they open and close, and also to hear the rhythm of the heart as it contracts. The name stethoscope comes from the Greek word “to spy on the chest”. It was designed in 1816.
We work with a wide variety of species, so we use different stethoscopes depending on the size of the pet. Each stethoscope has 2 different sized diaphragms, allowing us to hear heart sounds of different frequencies.
Animals have a normal arrhythmia, called sinus arrhythmia. This arrhythmia is an alternation in the rhythmic beating of the heart that coincides with breathing. This can easily be heard with the stethoscope. It is perfectly normal, yet does not occur in humanoids.
The stethoscope really comes into play for an animal that has a heart murmur. A heart murmur is a turbulence of blood flow as it goes through the heart valves (mitral or tricuspid). In a perfectly normal heart the blood flows past the valves silently, and all we hear with the stethoscope is the rhythmic lub-dub of the heart valves as they open and shut. If a murmur is present there is a whooshing sound mixed in with the lub-dubs, almost like the sound of a washing machine. This murmur is caused by blood becoming turbulent as it passes through an abnormal heat valve, although other problems can cause a murmur, the most notable being anemia. As we stated earlier, young animals can have an innocent murmur that resolves on its own and does not cause any problems.
Heart murmurs are graded from 1 to 6. One is barely audible with the stethoscope, 6 is so loud that you can feel the murmur with your hand on the chest. In general, the louder the murmur the more significant the disease, although this does not apply in all cases. Mitral murmurs tend to be louder due to the large difference in pressure (pressure gradient) between the left atrium and left ventricle.
In addition to their loudness, murmur have other audible characteristics. The sound can increase or decrease in intensity, a term we call crescendo or decrescendo. Murmurs that do not change in intensity are called holo.
If you follow this link you can hear a normal heart and one with a murmur.
As you already learned cardiac muscle (myocardium) is unique in its rhythmic beating. This rhythm is analyzed with an electrocardiogram. What the ECG measures is the depolarization (losing their electrical charge) and repolarization of the heart muscle as the cells contract. We talked about the mechanism of this in the cardiac muscle section in regard to sodium, potassium, and calcium. The ECG line that is produced measures the direction of the electrical activity along with its magnitude. Since the atria are smaller in size than the ventricles, their magnitude on the ECG strip is smaller.
The hardest part about performing an ECG on an animal is keeping it still enough to get a good reading. Sedatives are not used since they can interfere with the results. Interpreting the ECG is a medical art, and requires the familiarity of the electrical differences of many different breeds of dogs and cats. We used a service that allows us to transmit the ECG through the telephone or via fax to a group of veterinary cardiologists that specialize in ECG’s. In addition to interpreting the ECG, they also give us a cardiac consultation via fax the next day (or within a few minutes if it is an emergency).
This is one of their reports from a dog with a normal ECG that happened to have diabetes
We use the ECG in several situations. The first is for a dog or cat that is older or might have the symptoms of heart disease, especially coughing. We also use it prior to any anesthesia in high risk pets or when there is significant concern about anesthesia in general . We have a complete page on anesthesia to learn more.
An excellent tool in the diagnosis of heart disease is radiography. The size of the heart and pulmonary vessels, along with fluid buildup in the lungs, can be seen in a radiograph. We will show you some normal radiographs first, then some abnormal ones. You might one to come back to the normal ones to refresh your anatomy as you view the abnormal ones.
We will also show you radiographs of pets with breathing problems that are not the result of heart disease. This shows the importance of radiography in diagnosis, and emphasizes the need to follow thediagnostic process at all times. Don’t forget that changes in the lungs can eventually cause the right heart to fail (cor pulmonale).
Normal dog heart and lungs top view (called a DV view). The heart is the correct size, and it is surrounded by normal lung tissue. The lung tissue is black because it is filled with air. The white semicircular area at the bottom is the diaphragm.
Normal dog heart and lungs side view (called a lateral view). The head is towards the left. The trachea is in the correct location and is of the proper size. The dark lungs are readily visible along with the normal sized heart. The arrows are pointing to the diaphragm.
There is significantly more information in a chest radiograph than just the trachea, heart and lungs. This is a close-up of the radiograph directly above. In this radiograph you can see the aorta along with an artery (A), bronchus (B), and vein (V) to the cranial lung lobe. Blood is flowing from the heart through this artery to the cranial lung lobe which is the dark area on the lower left of the picture. The bronchus is the air passage from the trachea to this lung lobe. The vein is how the freshly oxygenated blood returns from the cranial lung lobe to the heart. Changes in the size of these veins and arteries give us clues as to whether heart disease is present along with the cause of the heart disease.
Normal cat heart and lungs side view. You can see the same structures as the dog radiograph. Do you see the artery, bronchus, and vein to the cranial lung lobe? They are smaller than the dog, but they are present. Do you also see the posterior vena cava? There is also fat under the liver.
Same cat from the top view. Its not easy to get a cat to sit still to take this radiograph!
An enlarged heart is called cardiomegaly. There are several parameters we use to give us an indication a heart is enlarged. These parameters are variable, and depend heavily on the breed of dog. This heart has cardiomegaly. We will label the abnormalities in the next picture.
The radiographic diagnosis is elevated trachea, mainstem bronchus compression, loss of caudal waste, loss of cranial waste, and increased sternal contact. The diagram below illustrates these changes.
The trachea is pushed upwards, instead of its more normal horizontal orientation. This is because the heart is enlarged in general. The top arrow on the right points to a bulge at the left atrium, the arrow below that and to the left points to a bulge at the right atrium. Both of these bulges are abnormal, and indicate that each of these atria are enlarged. The arrow on the bottom points to the amount of the heart that is touching the sternum. This dog has more than normal sternal contact, an indication of an enlarged right heart in general.
Do you remember earlier when we showed the pictures of the mitral valve? It is backflow of blood through a diseased mitral valve that causes the left atrium to enlarge, which you can now see radiographically.
This is a top view of the same dog. Compare how big and rounded the heart is to the normal top view at the beginning of the radiology section.
Radiographs are also used to diagnose pulmonary edema, which is fluid buildup within the lungs themselves. All the whitish area at the the top right of the lungs is the fluid. Normally this area should be black because it is filled with air.
This is a close-up of pulmonary edema in a different case.
This cat has ascites, which is fluid buildup in the abdomen. If you compare it to the radiograph above you can see that individual internal organs are not visualized. This is because the organs are surrounded by fluid. The dark areas are air in the stomach and in the intestines. Heart disease is not the only problem that can cause ascites. Several other conditions can give this radiographic appearance, especially FIP in cats.
We have one more fun thing to show you. It is the beating of the heart of a tortoise that is undergoing bladder stone surgery. Double click on it to view it and enjoy the beat.
An echocardiogram of the heart is almost indispensable in detecting a significant number of heart problems. Echocardiography refers to the imaging of the heart with ultrasound. Echocardiography may be divided into three types based on the instrumentation and application. These are M-mode, Two-dimensional, and Doppler Echocardiography.
This image is useful to measure the thickness of the ventricular walls along with size of the cardiac chambers. With these measurements a formula is used to determine cardiac contractility.
This image gives us an overall view of the heart. We can look into the chambers and see the valves in action. We can also see fluid around the heart if it is present.
This ultrasound picture shows the left ventricle (LV), aorta (AO) and left atrium (LA)
The Doppler view allows visualization of the blood as it literally flows through the heart. It is usually performed simultaneously with the Two Dimensional image. It can sometimes help us detect the degree of valvular disease by measuring the speed at which the blood flows through the abnormal valve.
In the top half of this ultrasound picture the 2 parallel running vertical lines are the doppler measuring the blood flow through the mitral valve. The “MR” at the bottom half of the picture is the doppler measuring the intensity of the mitral regurgitation that is occurring.
The color doppler makes it easy to visualize the blood flow through the heart valves
Double click on the movie below to watch the color doppler in action. Notice how fast this heart is beating.
Here is the report of the actual measurements obtained and calculated during an ultrasound on a cat with severe cardiomyopathy
There are a multitude of diseases that occur in the heart. We will go into detail into the 2 heart diseases we see most commonly. We will also briefly review other heart diseases we encounter on occasion.
Chronic atrioventricular Valve Disease (Myxomatous aV Valvular Degeneration)
This disease goes by several names. They include endocardiosis, mucoid valvular degeneration, chronic valvular fibrosis and acquired mitral regurgitation/insufficiency. It is the most common cause of heart disease and congestive heart failure (CHF) in the dog. Tricuspid valve regurgitation can also occur with this disease. It is rarely seen in cats.
Cause The exact cause is unknown. It effects the mitral valve more commonly than the tricupid valve, although both can be involved. It probably affects the mitral valve more commonly because of the high pressure in the left ventricle. As dogs age nodules form along the edge of mitral and tricuspid valves. When they get thicker the valves begin to leak (regurgitation), and as the disease progresses, the valves become deformed and shrink. You saw this previously in the necropsy pictures of the dog heart. As the problem progresses the chordinae tendinae might even rupture.
As the blood regurgitates abnormally backwards into the left atrium it increases the pressure in the atrium, making it harder for blood to flow from the lungs into the heart (from the pulmonary vein to the left artium). As the back pressure increases, the pressure in the veins of the lungs increases to a point (pulmonary hypertension) where the fluid leaks out, leading to pulmonary edema. This regurgitating flow of blood through the left atrium eventually causes it to enlarge (you saw this in the radiograph pictures). The regurgitating blood can go from the left ventricle through this valve and into the left atrium with such force that it causes “jet” lesions in the wall of the left atrium. The left atrium might also develop an arrhythmia as it continues to dilate. The enlarging left atrium can actually tear and cause blood to leak within the pericardial sac.
Since some of the blood that would normally be ejected by the left ventricle is now regurgitating back into the left atrium there is less flow of blood out of the left ventricle and into the aorta. This can begin the process of poor perfusion, leading to the cascading series of events culminating in congestive heart failure (CHF). Not every case of chronic atrioventricular valve disease will lead to heart failure though. Some dogs can have thickened and deformed valves and never show any symptoms.
You learned all about this in the anatomy, physiology, and pathophysiology sections-do you dare read it again?
It usually occurs in smaller breed dogs, notably:
- Miniature Schnauzers
- Fox terriers
- Cocker spaniels
- Boston terriers
- King Charles spaniels
- Lhasa apso’s
- Shetland sheepdogs
Symptoms might include exercise intolerance, weakness, syncope (passing out), cough, shortness of breath, and lethargy.
A heart murmur along with an arrhythmia is a clue to this disease. As the problem progresses the murmur becomes louder. In the later stages of the disease the fluid that builds up in the lungs (pulmonary edema) can be heard with the stethoscope also. An irregular rhythm and increased heart rate might also be present as the disease progresses. In the early stages of this disease there might not be any abnormalities on the physical exam.
Radiographs are used commonly to help in the diagnosis. The left atrium is enlarged, and sometimes the left ventricle. We might also find enlargement of the pulmonary veins and even evidence of pulmonary edema.
An echocardiogram can also give us significant information. The abnormal valves can be seen, along with rupture of the chordae tendinae in some cases. The doppler can actually view the blood regurgitating through the abnormal valve. The left ventricle might also be enlarged.
In the early stages the ECG might be normal. As the problem progresses arrhythmia’s might be present.
Surgery and balloon catheter dilation are helpful but may be of limited value. Medical therapy is used to treat this problem if it progresses to CHF.
Medical therapy is utilized to slow the progression of the disease (minimize the compensation mechanism we describe earlier), control the fluid that builds up in the lungs, and decrease the heart rate as the problem worsens. The advent of ACE inhibitors (afterload reducers) has given us the opportunity to help slow the progression of this incurable problem.
Therapy might depend on the stage of the disease. If there is a murmur in one of the affected breeds, but the left atrium is not enlarged, then minimizing sodium (salt) in the diet might be of help. As the murmur intensifies and the left atrium enlarges we will start Enalapril, and continue with the low sodium diet. As the murmur intensifies and the problem progresses we might increase the dose of Enalapril.
When a cough appears lasix will be used, and when the heart starts racing we might add digoxin. If pneumonia occurs or other lung problems become apparent we will also use antibiotics and bronchodilators.
Mildly affected dogs can have a good quality of life for years. It all depends on when the diagnosis is made and when therapy is instituted.
All dogs on therapy for CHF should be monitored every 3-6 months to adjust for changes. This includes a blood panel with thyroid, a chemistry panel, ECG, and chest radiographs.
Chronic atrioventricular Valve Disease can mimic infectious endocarditis, which is an actual infection of these valves caused by a bacteria. This is a serious disease that can cause significant illness.
Some of the more common bacteria are:
- Strep. spp.
- Staph. spp.
- E. coli
- Pasteurella spp.
These bacteria arise from infections of the gums, skin, urinary tract, prostate, lungs, and internal organs. The infection can spread to the spinal cord, causing a disease we call diskospondylitis. It is treated the same way, since CHF is present. In addition we will use antibiotics for up to several months to control the infection.
Cardiomyopathy is a disease where the actual heart muscle (myocardium) becomes weak and unable to contract with sufficient force (decreased contractility leading to a decreased stroke volume) to provide adequate perfusion (reduced cardiac output) for the cells of the body. Pets that get cardiomyopathy have a poor prognosis in general.
There are three types of cardiomyopathy:
The heart muscle is weak and flabby, and does not have the strength to contract with enough force to provide adequate perfusion of the cells. This form is rare in cats now because of supplementation with Taurine.
The heart muscle has become so thickened that the left ventricle chamber is too small to allow an adequate amount of blood to flow into it (diastole) before it contracts (systole) and perfuses the cells of the body. It is like a bodybuilder who is too buff to move efficiently. This increase muscularity of the left ventricle increase pressure in the left atrium causing it to dilate. This increases the pressure in the pulmonary veins leading to pulmonary edema. This is the form of the disease that is prevalent in cats.
The heart muscle might hypertrophy due to hypertension or problems with the outflow tract of the aorta.
3. Restrictive (Intermediate)
This form has characteristics similar to dilated and hypertrophic. Cats with this problem have scar tissue instead of normal heart muscle. The scar tissue decreased the strength of the heart, so it pumps out less blood at each beat (decreased stroke volume).
The diseased heart muscle beats irregularly (arrhythmia) and does not have the contractility to pump blood to the rest of the body (decreased cardiac output). This leads to the congestive heart failure we described in the pathophysiology section. In some cases there is sudden death from the arrhythmia that occurs.
Viruses, toxins, drugs, and taurine deficiency are suspected as causes. In most cases, especially in the breeds that are prone to this problem, the cause is unknown.
This disease occurs in large breed dogs like the Great Dane, Labrador, German Shepherd, St. Bernard, Irish wolfhound, English cocker spaniel, Scottish deerhound and Boxer. It is especially prevalent in Doberman pinschers. Most of these dogs are males and get the dilated version of cardiomyopathy.
Even though dogs of any age can get this problem, the tend to me middle aged. Dilated cardiomyopathy also occurs in Burmese, Siamese, and abyssinians.
The symptoms noted are those of congestive heart failure (CHF). Typical symptoms include weakness, poor appetite (anorexia), cough, breathing difficulty (dyspnea), weight loss collapse (syncope) and distended abdomen (ascites). In some cases sudden death occurs before any symptoms.
Some Doberman pinschers can have this disease and not show symptoms for several years. By then the prognosis is poor.
In cats the symptoms are similar. Unfortunately, a significant number of cats do not show any symptoms, they just have a sudden death. Cats also get paralyzed in their back legs because of a blood clot (thrombus) that enters the blood vessels to the back legs. This is called a “saddle thrombus” because it occurs at the point where the abdominal aorta branches into each of the arteries that supplies the rear legs with blood. The blood clot arises from the pooling of blood that occurs in the left atrium secondary to the hypertrophy of the left ventricle (described above in the hypertrophic section). These cats are in significant pain, and palpation of the rear legs might reveal a cooler temperature than the rest of the extremities. The prognosis is poor for this form of feline cardiomyopathy.
To get you oriented, this cat’s head is towards the left and the back legs are towards the right. Blood coming from the heart flows down the abdominal aorta (labeled as artery in this picture) and towards the rear legs. At the vertical arrow the abdominal aorta branches into the internal and external iliac arteries. From there it goes into the femoral arteries to supply the rear legs with freshly oxygenated blood. The poorly oxygenated blood returns to the heart through the femoral vein, then the iliac veins, and eventually the caudal vena cava (labeled as Vein in this picture). At the vertical arrow is where the saddle thrombus forms.
The thrombus (vertical arrow) is now apparent when we open up the arteries.
This is the way the artery is supposed to look when there is no thrombus
You can see why it is called a “Saddle Thrombus”
Abnormalities found during an exam are similar to other heart diseases and even other diseases in general. Auscultation of the thorax might reveal an arrhythmia and high heart rate. There might even be a pulse deficit if atrial arrhythmia is present. A murmur might be found in some pets with cardiomyopathy, although not every pet with a murmur has this disease.
Cats with a saddle thrombus might have weak or non existent femoral pulses. They will be painful and might not be able to use the rear legs.
The ECG can pick up arrhythmia’s caused by cardiomyopathy, in some cases prior to the onset of congestive heart failure. In breeds the are highly prone to this problem like the Doberman pinscher, it is advised to perform an ECG yearly to monitor for these changes.
On a radiograph the heart will appear enlarged because of the dilatation of all the chambers. Pulmonary edema and congested veins are indications of congestive heart failure secondary to cardiomyopathy.
The radiograph of this cat with cardiomyopathy and saddle thrombus shows the classic signs of an enlarged heart.
Some Boxers with cardiomyopathy will have normal appearing hearts on a radiograph.
In many pets an echocardiogram is the most conclusive means of diagnosing HCM.
This is an echocardiogram of the cat with the enlarged heart above. The echocardiogram showed an enlarged papillary muscle in the ventricles. We could even visualize the blood swirling around as it formed a pre-clot that would eventually become a thrombus and expelled from the ventricle to large in some artery in the body.
We could even visualize the thrombus at the end of the aorta just before it branches into the iliac arteries.
Routine treatment for congestive heart failure and arrhythmias is used. The effectiveness of therapy depends on the severity of the disease and how long the problem has been present. Some dogs and cats can do well for several months and even years if treated early enough.
Since the arrhythmia that occurs with cardiomyopathy can be detected before the onset of congestive heart failure, it is obvious that checking for this problem is important, particularly in Doberman pinschers and Boxers. Doberman pinschers with atrial fibrillation and dilated cardiomyopathy have a poor prognosis.
Cats might also be treated with aspirin to prevent a saddle thrombus. It is imperative that cats are kept in a relatively stress free environment.
The best long term approach for dogs and cats is not to breed lines that are prone to this problem.
Heartworm is caused by a parasite called dirofiliaria immitis. It is spread to dogs, and recently cats, by a mosquito. The mosquito is a necessary part of the life cycle.
In an affected dog or cat, female heartworms that reside in the pulmonary artery release small larval forms called microfilaria. Microfilaria circulate in the bloodstream for many months, and even sometimes years. If a mosquito bites a dog or cat that has circulating microfilaria it ingests them. These larva go through changes in the mosquito over the next 2 weeks. When this mosquito bites a dog it injects this larvae into a new dog. The larvae continue to develop in this dogs tissue and eventually become adult heartworms in the heart or pulmonary arteries 6-7 months later.
Traditionally this disease only occurred in hot and humid states that were essential to mosquito breeding. The disease occurs throughout our country now, although heatworm preventive medications have had a significant effect in lowering the incidence of heartworms.
The degree of heart disease that occurs depends on the number of heartworms, how long they have been present, and how the immune system reacts to them.
The actual presence of the worms in the arteries induces an inflammatory reaction causing blood clots (thrombi) to form. It also causes fluid to leak out of the artery into the surrounding tissue. Eventually pulmonary hypertension develops leading to an enlarged right ventricle as it works harder to pump the blood against all this pressure (afterload).
If a large number of worms are present they can literally fill up the right atrium and caudal vena cava. Not only does this interfere with the proper flow of blood through the heart, it also causes changes in the red blood cells and clotting system. In serious cases this can cause a bleeding disorder called disseminated intravascular coagulation (DIC).
The immune system can destroy the microfilaria released by adult heartworms in the pulmonary capillaries, resulting in a negative heartworm test. This is called occult heartworm disease.
Dogs and cats of any breed can be infected, although we tend to see it more in large male dogs. Since it takes at least 6 months from the time a mosquito injects microfilaria into a dog until these microfilaria become adult heartworms, puppies must be at least 6 months of age before adult heartworms are present. Most dogs are diagnosed at middle age.
Many dogs do not have any symptoms until heart failure progresses. This emphasizes the need for yearly testing. When symptoms occur they can include:
- Poor appetite
- Weight loss
- Difficult breathing
- Exercise intolerance
- Distended abdomen
These are the symptoms of general heart failure, yet they can also occur in other diseases besides heartworm.
In many cases the physical exam is normal. As the disease progresses the following exam findings might occur:
Cough when the windpipe (trachea) is palpated
Increased respiratory rate (tachypnea)
Increased lung sounds (crackles) when ausculting the chest with a stethoscope
Radiographic changes that might occur include an enlarged right heart along with an enlargement of the pulmonary artery. Several smaller arteries might be dilated, tortuous, or pruned. Other changes are similar to those that occur with right heart failure.
An EKG might show changes indicative of right heart enlargement.
A routine blood panel might reveal an elevation of certain types of white blood cells called eosinophils and basophils. There might even be an elevated protein level and evidence of kidney problems.
Microfilaria detection is the best and most accurate way to diagnose adult heartworms. We can see them microscopically when we look at a blood smear from an infected dog. Unfortunately, this diagnostic method is unreliable due to the small size of the microfilaria in relation to the large amount of blood in the bloodstream. Several tests were developed to increase our ability to see these microfilaria on the blood smear. They made a major improvement in diagnostic ability, but are still not as good as immunodiagnosis.
Immunodiagnosis involves looking for antigens of microfilaria or antibodies against the microfilaria using monoclonal antibody techniques. Several of these tests can be used in our office, therefore you get the report within 30 minutes. If the test is negative, then your dog or cat probably does not have heartworm. Unfortunately, reproductively inactive female adult heartworms, along with male heartworms, will also show up negative on these tests. This is called occult heartworm disease. On occasion, false positives can occur, particularly if other parasites (roundworms, etc.) are present or there is another nonpathogenic heartworm present called dipetalonema reconditum.
Approximately 15% of the heartworm infected dog have occult heartworm disease. In occult heartworm disease there are adult heartworm parasites in the pulmonary arteries yet there are no microfilaria circulating in the bloodstream. Since the primary method of diagnosis is detection of these microfilaria, either visually, or with monoclonal antibody tests, it is possible to miss a diagnosis.
There are several reasons why occult heartworm disease might occur. It happens early in the course of infection when adult female heartworms have not had enough chance to release microfilaria to any significant extent. At the other end of the spectrum, old adult heartworms might not release microfilaria. It can occur if mostly male heartworms are causing an infection, since they do not release microfilaria. It can also occur if adult heartworms are present and your dog is on heartworm preventive medication. Finally, it can occur if the immune system produces antibodies to microfilaria, suppressing their numbers so low that they are not detected with current monoclonal antibody techniques. Unfortunately, in this scenario there is significant pathology to the lung tissue because of this immune system reaction. These dogs usually present with severe breathing problems.
In general, we are starting to encounter dogs with adult heartworms that have minimal circulating larvae. This is making diagnosis more difficult.
This is a disease where the treatment is almost as bad as the disease. No treatment is completely effective although the dog or cat feels substantially better. The worms that are killed can block small blood vessels and cause thrombosis. If large numbers of heartworms are present there can be a massive amount of thrombosis. This can increase pulmonary hypertension and lead to additional fluid buildup in the lungs, even leading to disseminated intravascular coagulation. The inflammatory reaction that occurs can also affect the glomeruli of the kidneys. All treated dogs need to rest to minimize a reaction from the dead heartworms.
If a dog is already in heart failure it needs to be stabilized prior to treating the adult heartworms. This includes the routine treatment for CHF, particularly diuretics, digoxin, Enalapril, exercise restriction, and aspirin.
The primary drug used to treat adult heartworms is melarsomine. It has arsenic as its active ingredient, although side effects are less than previous arsenic treatment for heartworms. It still has the potential to cause problems with the kidneys (nephrotoxicity) and liver (hepatotoxicity), so close monitoring is vital. Dead heartworms can still cause a pulmonary allergic reaction with its associated problems. This problem is partially mitigated by giving melarsomine over a 2 day period of time instead of all at once. If your dog has severe heartworm disease it might be given one treatement, then sent home to rest for one month. At that time, it is given a full dose over 2 days.
Another treatment modality used by some veterinarians is to use heartworm preventive medications at a modified dose to slowly kill adult heartworms over 16 months. The theory is that the slow killing of the worms will minimize reaction from the death of adult worms. The disadvantage to this slow killing involves the damage the adult heartworms can cause during that time.
In some cases we use aspirin or cortisone as an adjunct to our routine therapy. These drugs minimize blood clots, platelet problems, and inflammation.
Once the adult heartworms are killed the microfilaria that are circulating in the bloodstream are our next target. The drug of choice, except in Collie breeds who can have a serious reaction, is Ivermectin. In Collie breeds we use a drug called Levamisole.
Since heartworm is such a serious disease, and treatment has potential toxicity, it is obvious that prevention is the best way to go. There are many products that are highly effective at preventing heartworms. They all should be started within one month of the mosquito season, and continued to the end of the mosquito season. Since many of these preventatives also control fleas and internal parasites, we recommend their use year round. Except for Revolution, your dog must be tested negative for heartworms prior to instituting these preventive medications.
These preventive medications need to be given monthly. If your dog does get adult heartworms because you did not follow the montly schedule you set up a diagnostic problem. These dogs will have so few circulating microfilaria that we might not be able to make an accurate diagnosis.
Some of the more popular ones are:
Heartgard and Heartgard Plus- They contain Ivermectin, and can be safely used in Collies over 10#. In addition, Heartgard Plus kills internal parasites. It is given on a monthly basis.
Interceptor- It contains milbemycin, which in addition to preventing heartworms also kills circulating microfilaria and sterilizes adult heartworms. It is also given on a monthly basis.
Sentinel- It contains milbemycin and lufeneron, so in addition to its ability to prevent heartworms, it controls roundworms, hookworms, whipworms and flea eggs.
Revolution- It contains the drug Selamectin, which is similar to Ivermectin, but much more broad spectrum in action. an advantage of Revolution is no adverse reaction if the dog already has microfilaria circulating from a current adult heartworm infection. In can be used safely in Collies, and treats and controls fleas in dogs and cats, ear mites in dogs and cats, sarcoptic mange and ticks in dogs. It even treats hookworms and roundworms in cats.
In general, the prognosis for dogs with heartworm disease is good. as is usually the case, it depends on the number of worms present, how long the disease has been occurring, and how the immune system is reacting. Although all of the worms are not usually killed, their decrease results in significant breathing improvement in almost all dogs.
Feline Heartworm Disease
Heartworm disease in cats is similar to dogs, yet there are some significant differences. Symptoms can range from none to sudden death or chronic illness. Most do not have any symptoms. If they are present, they usually include breathing problems (similar to asthma) and vomiting. They usually have less worms and more occult heartworm disease. Circulating microfilaria are not detectable with the microscope, and diagnosis using other means is difficult. Melarsomine is not used to kill adult heartworms, Thiacetarsamide, the original drug used to treat dog heartworms, is used in place of melarsomine. Preventive medication should be used for cats in addition to dogs. These include Revolution and HeartGard for Cats.
This disease of dogs arises when abnormal tissue near the aortic valve obstructs the flow of blood out of the left ventricle. If severe enough it can eventually lead to CHF and ventricular arrhythmias.
It usually occurs in large breed dogs, notably:
- Golden Retrievers
- German Shepherds
Symptoms include exercise intolerance, rear limb weakness, syncope (passing out), cough, shortness of breath, and even death.
A heart murmur along with an arrhythmia is a clue to this disease.
Radiographs and electrocardiograms are of help in the more advanced cases. Echocardiograms are helpful in the moderate to severe cases.
Surgery and balloon catheter dilation are helpful but may be of limited value. Medical therapy is used to treat CHF.
Mildly affected dogs can have a good quality of life. If the problem is more severe the prognosis is not as good.
This disease of dogs arises when abnormal tissue near the pulmonic valve obstructs the flow of blood out of the right ventricle. If severe enough it can eventually lead to an enlarged right heart and regurgitation of blood through the tricupid valve and into the right atrium. If severe enough right sided CHF might occur.
The following breeds are know to be prone to pulmonic stenosis:
- English bulldog
- Terrier breeds
- Spaniel breeds
Many dogs have no initial symptoms. If symptoms are present, they might include exercise intolerance, ascites, syncope (passing out), shortness of breath, and even death.
A heart murmur along with distended jugular vein might be present.
Radiographs and electrocardiograms might show right ventricular enlargement. Echocardiograms are helpful in many cases.
If the problem is severe enough surgery is used to correct the problem. This depends on exactly where the problem is in this valve along with other blood vessels to the heart. Balloon catheter dilation can also be used.
Medical therapy is used if CHF is present. When the CHF is stabilized surgery might be utilized.
Mildly affected dogs can have a good quality of life. If the problem is more severe the prognosis is not as good. These dogs should be monitored yearly for changes in their condition.
Ventricular Septal Defect
In this disease an abnormal opening occurs between the ventricles, which allows blood to flow directly from the left ventricle (higher pressure) to the right ventricle (lower pressure). This disease occurs in dogs and cats. The severity of this disease depends on exactly where the opening is located and how big it is. Eventually left sided CHF can occur because of an overload of blood to the left ventricle as more blood is shunted through the right ventricle, leading to more blood flowing through the lungs, leading to more blood flowing back into the left atrium and left ventricle.
The following breeds are know to be prone to PDA:
- English bulldog
- English springer spaniel
Many pets have no initial symptoms. If symptoms are present, they might include coughing and shortness of breath as CHF develops
A heart murmur is commonly present.
Radiographs and electrocardiograms might show left ventricular enlargement and distention of the pulmonary vessels. Echocardiograms are used to confirm the diagnosis.
Many pets need no treatment at all. Surgery is difficult to perform. Medical Therapy is used to treat CHF if it occurs.
Prognosis is good if the opening is small. Larger defects need to be monitored yearly.
Patent Ductus arteriosis (PDA)
In the uterus the mother of the dog supplies the dog with its oxygen since obviously a puppy in the uterus is not breathing. One of the mechanisms of the body to bypass the lungs of the pup is to flow the blood directly from the pulmonary artery to the aorta. When the pup is born, this direct communication between these vessels closes, the blood now flows from the pulmonary artery to the lungs to get its oxygen. This disease of dogs occurs when the communication between the descending aorta and the pulmonary artery fails to close after birth. This abnormal flow of blood overloads the left side of the heart, leading eventually to CHF.
The following breeds are know to be prone to PDA:
- English springer spaniel
- Bichon frise
- Yorkshire terriers
- Shetland sheepdogs
- German shepherds
Many dogs have no initial symptoms. If symptoms are present, they might include coughing and shortness of breath.
A heart murmur called a continuous (occurs during systole and diastole) murmur, is a clue to this disease.
Radiographs and electrocardiograms might show left ventricular enlargement and distention of the pulmonary vessels. Echocardiograms are helpful but not needed in many cases.
A surgical procedure that closes this abnormal connection is used to treat PDA.
Prognosis is good if surgery is performed before the symptoms of CHF start appearing.
The medical management of animals with CHF (congestive heart failure) is aimed at controlling the deleterious effects of the underlying pathology. These effects typically include pulmonary congestion and edema, cardiac arrhythmias, reduced cardiac output, and excessive vasoconstriction. Many medications are used to treat heart disease. In almost every case they can only control the symptoms, and hopefully, slow down the progression of the disease. In general, medications used to treat cardiac disease, especially CHF, are highly beneficial. The goal of therapy is to improved the quality of life of these pets while simultaneously minimizing the side effects of the drugs used. Most pets will not go back to being able to exercising vigorously, as a matter of fact, too much exercise can destabilize compensated dogs and cats.
Therapy needs to be tailored made to each pet, and again, constant monitoring is needed to adjust for any changes that are occurring. This means that blood panels, radiographs, ECGs, and blood pressure need to be checked weekly initially, then every 3-6 months, assuming the heart failure is under control.
Cardiac disease is commonly diagnosed in geriatric patients that have other problems simultaneously. Some of the medications used to treat heart disease can exacerbate other medical problems common in geriatric patients. Using these medications properly tends to be a balancing act, minimizing their deleterious affects while maximizing their advantageous properties. This is particularly true if a pet has kidney disease along with heart disease. This balancing act requires constant monitoring.
ACE (Angiotensin Converting Enzyme) Inhibitors
These drugs are a part of the treatment plan of almost every dog with CHF. The addition of Enalapril to conventional therapy in dogs with chronic congestive heart failure resulted in a significant decrease in clinical signs and a 92% increase in survival time. The use of vasodilators, such as Enalapril, increase the survival time of dogs with CHF. This is especially true in CHF from cardiomyopathy, but does not necessarily apply to dogs with CHF from mitral insufficiency. They add to the quality of your dogs life by minimizing the fluid buildup in the lungs (pulmonary edema) and abdomen (ascites). Some cardiologists believe they should even be used in heart disease cases prior to the onset of heart failure in dogs, except in cases where the problem is mitral insufficiency. In these dogs the use of ACE inhibitors prior to the onset of heart failure is of no benefit.
In general, ACE inhibitors lower the blood pressure (decrease afterload) and decrease salt and water retention. They work by inhibiting the conversion of angiotensin I to angiotensin II in the lungs. We discussed this in the Physiology section under the blood pressure heading. This decreases fluid buildup in the lungs, thorax, and abdomen, and it also makes it easier for a weakened or enlarged heart to pump the blood out into the bloodstream. Kidney values and electrolytes should be monitored while on ACE Inhibitors.
The more common ACE inhibitors we use are Enalapril, benazepril, and captopril. Enalapril is approved for use in the dog by the FDA, and is the most common ACE inhibitor we use. ACE inhibitors can take up to 7 days to produce beneficial effects, and need to be constantly adjusted to maximize their benefits.
Diuretics (usually lasix) are the mainstay of treatment for CHF, and should be used in conjunction with ACE inhibitors. Lasix is a loop diuretic. That means it has its effects on a section of the kidney called the loop of Henle. It is here that lasix causes more sodium (salt) to be excreted from the body. Sodium pulls water with it when it is excreted, so urination increases and fluid level of water in the bloodstream decreases. This decreases the blood pressure in the capillaries, so less fluid leaks out into the lungs (pulmonary edema) and abdomen (ascites). Another medication, called spironolactone, is also used with lasix, or in place of it.
Overuse of diuretics can cause dehydration and can interfere with normal kidney function. As was already mentioned, lasix use should be monitored with a blood panel every 3-6 months to assess kidney function. as a geriatric pet ages, the kidneys and heart can both have disease, so the use of lasix is a balancing act between these two diseases. It is rare for a dog to need potassium supplementation (K+) while on a diuretic, unlike people. Your pet will drink more than usual, so make sure fresh water is available at all times.
Digitalis is used in the later stages of CHF when the heart is racing and lasix and ACE inhibitors have lost their effectiveness. Digitalis is a positive inotropic agent. This means that it increases cardiac contractility. Digitalis does this by increasing the amount of calcium in the heart cells. It is used in some cases of CHF when the cause is cardiomyopathy or advanced degenerative valve disease (usually mitral insufficiency). Digitalis also decreases the heart rate. This can be beneficial when the heart is beating so rapidly that the chambers do not have enough time to fill with an adequate amount of blood. This decreases the stroke volume, which as you already know decreases cardiac output and all the cells of the body have reduced perfusion. Also, if the heart is beating rapidly it spends significantly less time in diastole, so the heart itself receives less blood. You can review this in the pathophysiology section.
The two types of digitalis used are digoxin and digitoxin. They come in tablets and liquids, so are easy to give to smaller pets. Digoxin is excreted by the kidneys, while digitoxin is metabolized by the liver. In either case, blood values need to be closely monitored.
Digitalis has the potential to cause toxicity, so its use has to be carefully monitored with a combination of ECG’s and blood digoxin levels. Common side effects include depression, anorexia, vomiting, diarrhea, and cardiac arrhythmias. Do not make changes in your pet’s diet when we first start this drug because not eating well is a sign of toxicity. It is important that you communicate with us when we start this drug, even though we routinely use modest doses initially to prevent any toxicity.
The digitalis level needs to be monitored 2 weeks after initial therapy, and then every 3-6 months to watch for toxicity
Minimizing salt in the diet is beneficial in preventing fluid buildup (ascites of pulmonary edema) only when the fluid is present. If you start a restricted sodium diet too soon you can actually cause the opposite effect.
Typical food include Hills K/D and H/D. Many pets eat these foods well, but if they have diminished appetite on these foods then they are of no benefit, since they will continue to lose weight and be lethargic. Do not change to this diet and start digitalis therapy ate the same time so that we can monitor the cause of a poor appetite if it occurs.
Supplementation with Omega 3 fatty acids might be of help, along with carnitine and taurine in specific cases of dilated cardiomyopathy. Carnitine will only be effective in a small number of dogs. To know for sure if a dog is carnitine deficient, a biopsy of the heart muscle is needed.
Coenzyme Q10 is also used, although this has not been proven to be effective either.
If your dog or cat has heart failure it needs to stay relatively inactive. The heart is already compromised and working at maximum capacity, and does not have the capacity to increase cardiac output.
Some pets are presented to us having severe difficulty breathing from pulmonary edema. They need to be handled very carefully since they are literally drowning in the own fluids and unable to get air. It would be like a person firmly holding a pillow over your face. These pets might need:
Nitroglycerin to dilate the blood vessels and decrease the afterload. We use gloves to put the nitrogylcerine on the ears since it is absorbed through the skin
Intravenous lasix to reduce pulmonary edema
Intravenous digoxin to slow the heart
Intravenous morphine to dilate the blood vessels and decrease the afterload
Intravenous sodium nitroprusside- potent arterial and venous vasodilator, needs constant blood pressure monitoring. Reduces afterload
We might use low levels of low sodium intravenous fluids to help the kidneys if they are failing also. We cannot thoroughly flush out kidney waste products because this much fluid given intravenously will exacerbate the pulmonary edema that is already present. Once the emergency is under control we might be able to slowly increase these fluids.
Oral medications have no place in these dogs since they are usually anorectic and likely to have minimal GI motility. In addition, they are unable to absorb the medication through their intestines and into the bloodstream because blood is being shunted away from the intestines and to the vital organs.