Many cats are living longer lives, and unfortunately, are acquiring diseases that were not commonly seen in the recent past. Hypertension (high blood pressure) is one of these diseases. High blood pressure can be primary, where the cause is unknown. In most animals though, it is secondary to some other disease.
The increase in blood pressure affects many organs, particularly the liver, eyes, kidneys, and heart. Hypertension hastens the progression of these diseases, and substantially predisposes your cat to blindness.
Dogs can get high blood pressure due to various diseases, but we see the problem more commonly in cats, especially as they age. This emphasizes the need for routine exams, blood panels, and blood pressure monitoring for cats as they get older.
The are two main factors that determine blood pressure:
It is the amount of blood pumped by the heart in a specific period of time. The determinants of cardiac ouptut are the heart rate (measured in beats per minute) and the stroke volume (the amount of blood in ml ejected with each beat of the heart).
This is how constricted or dilated the artery is as the blood is flowing through it. A dilated artery has a larger diameter, so less blood pressure needs to be generated by the heart for blood to flow through this dilated vessel. Arteries constantly constrict and dilate, all depending on the needs of the body overall and the specific organ they are supplying blood to. For example, the arteries to your muscles dilate when you exercise. This allows the muscles to receive extra nutrients and oxygen. When you are done exercising they start constricting and blood is diverted to other areas of the body where it might now be needed. Maybe now you are eating a meal and the digestive system needs the added blood flow.
As it turns out, is is cardiac output x vascular resistance that determines the blood pressure. If you get scared, adrenaline secretion will increase the heart rate and your blood pressure will rise due to the increased cardiac output. If you become dehydrated, the stroke volume might decrease due to a lack of fluid, and your blood pressure will decrease due to a decreased cardiac output. Older pets tend to have arteries that are not as elastic as when they were younger, the animal world version of arteriosclerosis. These blood vessels stay constricted more than dilated. This increases the vascular resistance, resulting in increased blood pressure.
Normal regulation of the blood pressure involves a complicated set of metabolic processes. Many body systems are involved, including the nervous system, the renal system, the cardiovascular system, and the endocrine system. It is a highly refined system that can make minute changes in rapid response to changing physiologic needs.
In a nutshell, the kidneys secrete a hormone called renin. This can be in response to a decreased blood flow to the kidneys, stimulation of the nervous system, secretion of hormones like adrenaline (epinephrine), or low sodium levels. Renin will activate the conversion of angoitensin I to angiotensin II in the lungs. Angiotensin II will constrict the blood vessels (increased vascular resistance) and stimulate the secretion of aldosterone. Aldosterone will increase water retention by its effects on sodium. Constriction of blood vessels (increased vascular resistance) and increased water retention (increased stroke volume) lead to an increased blood pressure. Pretty easy huh?
Abnormally high blood pressure causes blood vessel damage, particularly in the eye, kidney, heart and brain. These damaged blood vessels will bleed, cause clots, cause fluid buildup in the tissue surrounding the blood vessel, and even tissue death. The mechanism for this is complex.
Hypertension also places excessive strain on the cardiac (heart) muscle. The heart has to pump against more pressure (vascular resistance), causing further deterioration. It becomes a viscious cycle as the problem gets worse and worse. As it progresses the heart enlarges and a murmur might be heard with the stethoscope.
There are no specific set of symptoms of high blood pressure. That’s why its called the silent killer in people. What might appear are the symptoms of the disease that is causing the high blood pressure in the first place.
The primary symptom in cats some owners notice is a sudden onset of blindness, as evidenced by dilated pupils and bumping into objects. Cats that were apparently fine just a day or two earlier are now completely blind. Prior to the onset of blindness an owner might notice other symptoms. These might include weight loss, excess drinking and urinating, vomiting, change in appetite (up or down) and fast heart rate. Monitoring some of these parameters ahead of time is possible to look for subtle signs of diseases. This is explained in our In Home Exam section and our Wellness section.
Feline hypertension is almost always secondary to other problems, namely hyperthyroidism and kidney failure. The majority of cats with these two diseases will eventually develop hypertension. Any cat that has been diagnosed with one or both of these diseases should be monitored for hypertension every 3-6 months.
Every beat of the heart sends a significant amount of blood to the kidneys. As cats age the kidneys do not function properly, and through complex mechanisms mentioned in the physiology section, the blood pressure will elevate.
The increased level of thyroid hormone (thyroxine) in the blood stream causes the heart to increase its output of blood, leading to a racing heart and an increase in blood pressure.
As in people, hypertension is a silent disease. You don’t feel ill, and there are no obvious symptoms until it is too late. Fortunately, we have sophisticated medical equipment that will help us make this diagnosis.
Hypertension is found in cats and dogs. There is no breed predilection, but tends to occur more in males and older animals.
Older cats that have racing and pounding heart rates, along with blindness, give an indication of hypertension. Hypertension is suspected in cats that have been diagnosed with kidney or heart disease in addition to hyperthyroidism.
In most cases there are no obvious physical abnormalities. Blood samples that are easily obtained, or pulsate in the syringe when obtained, are another indirect indication of hypertension. The pupils might be dilated, the thryoid gland might be enlarged, the kidneys might feel abnormal, and there could be blood in the urine (hematuria) or nose bleed (epistaxis).
An examination of the retina by a veterinary ophthalmologist will sometimes give an indication of hypertension. There might be areas of hemorrhage or even detachment of the retina. Every blind cat should be seen by an ophthalmologist to check for hypertension, along with other causes of blindness like FeLV, FIP, fungal infections, and Toxoplasmosis.
An important tool in the diagnosis of hypertension is a blood pressure monitor. Cat arteries are very small, and the usual method to detect hypertension in people is not accurate in cats. A special blood pressure unit called a Doppler can be used. Its basis in principle is the Doppler effect (obviously). The Doppler effect is the change in frequency of a sound wave as an object comes towards you then moves away from you. The best analogy of the Doppler effect is the sound a speeding race car makes as it comes towards you, then passes away from you. Coming towards you it has one frequency of sound, going away it has another.
In the case of a Doppler blood pressure unit, it is the movement of red blood cells through the artery that is being measured. During the frequency change that occurs during this red blood cell flow the reflected sound wave goes from the ultrasonic to the audible range.
Taking a blood pressure reading on a cat is more difficult than in a person. Their arteries are very small, and of course, cats are covered with hair. Also, cats don’t sit still, and are easily stressed, which can lead to a false reading. We will routinely take several readings, discard the lowest and highest, and average the rest. Sometimes we find a cat with consistent readings each time, so not as many readings are needed. A cat with a systolic pressure of 160 mm of Hg is suspected of having hypertension, over 170 mm Hg is considered to be hypertensive. In dogs we consider over 180 mm Hg to be hypertension. Sight hounds, overweight, and older animals tend to have higher numbers. These are not hard and fast numbers, just a guideline for each individual case. In many cases of high readings we repeat the blood pressure readings later to check for consistency in the readings, especially in cats.
To obtain an accurate reading we prefer owners stay with their cat in one of our quiet exams rooms. After clipping the fur on the bottom of the rear foot (we also use the front foot and tail) we set up our equipment. We take our first pressure reading when everything is calm and all equipment is set up. Notice in the video below how calm the cat is and how quiet the atmosphere for an accurate reading.
Low blood pressure can be of significance in animals. Usually this is encountered during anesthesia. Our Doppler unit allows us to monitor the blood pressure during anesthesia and make corrections as needed. We also encounter low blood pressure during shock, trauma, bleeding, and from certain medications.
Underlying treatment of the disease that is causing the hypertension is sometimes all that is needed to prevent hypertension. In kidney disease the use of K/D food, with its decreased salt (sodium chloride) might be beneficial.
If your cat has hyperthyroidism we will treat for that, and usually the blood pressure returns to normal
If hypertension still persists after treating the primary problem then we sometimes will use specific medications to lower the blood pressure:
ACE inhibitors like Enalapril or Lotensin (prevent conversion of angiotensin to angiotensin II, thus decreasing stroke volume and vascular resistance)
Beta-adrenergic blockers like Propranalol or atenolol (decrease the heart rate)
Calcium channel blockers like Norvasc (decrease vascular resistance). This is the most common one we use in cats.
Lasix or aldactone (decrease the stroke volume)
Hydralazine (decrease vascular resistance)
Blind cats need to be placed on a drug called Norvasc to rapidly lower the blood pressure. There is a chance of regaining sight with the use of this drug, especially if utilized as soon as blindness is noted. Cats that are on this drug should have their kidney tests monitored 2 weeks after starting Norvasc, then every 3 months.
The best treatment for hypertension and its associated blindness is prevention. Any cat over 8 years of age, or diagnosed with kidney disease, diabetes mellitus or hyperthyroidism, should be monitored periodically for hypertension. This will allow diagnosis of the problem before it causes blindness.
All pets with hypertension should have their blood pressure checked every 3 months. In addition, blood panels, thyroid tests, urinalysis, and eye exams should be performed every 3-6 months.