High blood pressure is an extremely important concern in human medicine. A high stress lifestyle, smoking, and high salt diet all contribute to this potentially dangerous condition and virtually everyone in the U.S. knows how serious it can be. But what about our pets? They don’t smoke or worry about the mortgage and they don’t deposit cholesterol in their blood vessels. They do, however, get high blood pressure, especially in age, and here is what you probably should know.
What does High Blood Pressure Do?
Problems from high blood pressure arise when a blood vessel is simply too small for the high pressure flow going through it. Imagine attaching a garden hose to a fire hydrant. The pressure would cause the garden hose to explode and that is what happens to a blood vessel too small for the pressure going through it. Instead of water going everywhere, as in the garden hose analogy, bleeding results. Since the affected vessels are small, the bleeding may not be noticeable but a lot of little bleeds and a lot of blood vessel destruction can create big problems over time.
The retina of the eye is especially at risk, with either sudden or gradual blindness often being the first sign of latent high blood pressure. The kidney also is a target as it relies on tiny vessels to filter toxins from the bloodstream. Kidney disease is an important cause of high blood pressure and also progresses far more rapidly with it.
High blood pressure also increases the risk of embolism: the formation of tiny blood clots that form when blood flow is abnormal. These clots can lodge in an assortment of inopportune locations including the brain.
What Causes High Blood Pressure in Pets?
There are numerous diseases in pets that are associated with high blood pressure:
- Chronic renal (kidney) failure In one study, 93% of dogs with chronic renal failure and 61% of cats with chronic renal failure also had systemic hypertension. More recent studies suggest this may be an over-estimation but the percentages are still significant and patient screening is very important.
- Hyperthyroidism In one study, 87% of cats with untreated hyperthyroidism had systemic hypertension. (Note: hyperthyroidism is a feline disease; dogs are not affected.)
- Glomerular disease is one of the kidney filtration system in which protein is lost in urine. It is important to screen pets with high blood pressure for urinary protein as control of protein loss is important to survival time.
- Cushing's disease (an adrenal cortisone excess)
- Diabetes mellitus (inability to properly reduce blood sugar)
- Acromegaly (growth hormone excess)
- Polycythemia (an excess in red blood cells)
- Pheochromocytoma (an adrenaline secreting tumor of the adrenal gland)
In humans, high blood pressure is usually considered primary, meaning there is no underlying disease causing it. In animals, primary hypertension is unusual; there is almost always another disease causing it and if routine screening does not identify the problem, more tests may be in order.
How is High Blood Pressure Identified?
In human medicine, high blood pressure is called the silent killer because most people have no reason to think they might be hypertensive. To find high blood pressure in people, we screen for it. This means that virtually any time you see a doctor of any kind, a nurse will take your blood pressure. Similarly, in pets, a great deal of high blood pressure is identified by screening. If a pet has one of the above conditions, blood pressure is generally checked. It has recently been recommended that older pets have their blood pressure checked whenever they have a physical examination. There is some disagreement among experts as to which patients should be screened. Because of inherent insensitivity of the equipment commonly used in veterinary practice, not every pet necessarily needs to be screened. Certainly, any pet with a predisposing condition such as one of those listed above should be screened. Ask your veterinarian if your senior pet should get a blood pressure measurement.
The other time high blood pressure is discovered is when it makes its presence known. This usually means some degree of blindness or some other obvious eye problem. The retina of a hypertensive patient develops tortuous-looking retinal blood vessels. Some vessels may even have broken, showing smudges of blood on the retinal surface. Some areas of the retina simply detach. Sometimes the entire retina detaches. With early identification, some vision may be restored.
Do not let minor vision changes go unreported. Let your veterinarian know if you think your pet’s vision is not normal.
Retinal changes can be complicated to interpret. Do not be surprised or alarmed if your veterinarian recommends referral to a veterinary ophthalmologist.
A sudden neurologic condition could indicate a stroke or vascular accident in the brain or spinal cord. It used to be taken for granted that dogs and cats did not throw blood clots and get strokes like human beings can, but the advent of MRI technology has shown otherwise. A sudden neurologic deficit, especially a non-painful one, is another possible indication to screen blood pressure.
How do we Measure Blood Pressure in Pets?
Doppler Blood Pressure Monitor
Photo by MarVistaVet
When a person's blood pressure is checked, the nurse uses a stethoscope to listen for "Korotkoff sounds" arise and dissipate as an inflatable cuff slowly releases over an artery. In animals, the stethoscope is just not sensitive enough and an ultrasonic probe must be taped or held over the artery. Using ultrasound, the sound of the systolic pressure is converted into an audible signal. It is not possible to measure diastolic pressure in a pet without actually placing a catheter inside an artery so we make do with just a systolic measurement. In pets, this measurement should not exceed 160. A reading of 180 is considered by the American College of Veterinary Internal Medicine to indicate high risk for organ damage while readings of 150-159 are considered mildly elevated.
Blood pressure measurement is performed similarly to the way it is in humans. An inflatable cuff is fit snuggly around the foot, foreleg, or even tail of the pet. The cuff is inflated so as to occlude blood flow through the superficial artery. In a person, as the cuff is slowly deflated, a stethoscope is used to listen for the point when the blood pressure is adequate to pump through the partially occluded vessel. This point on the pressure gauge is the systolic blood pressure. The cuff is further deflated until the vessel is open and no more sounds are made. This point represents the diastolic blood pressure and the actual sounds are called "Korotkoff sounds" after the doctor who discovered them.
Some pets (obviously) are nervous at the vet’s office and this factor must be taken into account when reading blood pressure. It is possible for a pet to have high blood pressure at the vet’s office and normal pressure at all other times. You might think this would be a common situation but most pets are able to maintain normal blood pressure despite being surrounded by hospital staff. To account for the “White Coat Effect,” at least five measurements are taken so that the pet becomes accustomed to the process and understands that no pain is involved.
What Treatment is Available for Hypertension?
Ocular disease may require prescription eye drops depending on how much bleeding is in the eye and whether or not return of vision is likely.
When hypertension is identified, a search for the underlying cause is indicated. It may be that controlling the underlying disease totally reverses the hypertension (especially true for hyperthyroid cats).
Beyond these methods, as with people, medication to lower blood pressure is often in order. This typically involves some type of pill that dilates peripheral blood vessels, effectively making them larger so as to accommodate the high pressure blood flow going through them.
Amlodipine is a calcium channel blocker that dilates the systemic arterioles. It is the most effective drug for treating feline hypertension, with the fewest side-effects. The drug is also effective in dogs, but literature documentation is lacking. Amlodipine in dogs has always been expensive, but recently a generic version of the drug has become available, which should make it more cost effective.
Amlodipine can also be applied transdermally, although the clinical effect may be less than with orally administered amlodipine.
Hydralazine is a very effective antihypertensive agent in dogs, but can be associated with side effects such as reflex tachycardia and gastroinstestinal upset.
Prazosin has also been used as a vasodilator.
Angiotensin-converting enzyme inhibitors (ACE-I) do not appear to be very effective in reducing blood pressure in hypertensive dogs or cats. Beta blockers and diuretics are similarly unimpressive in reducing blood pressure in these species as sole agents. However, beta blockers may be used as adjunctive agents when reflex tachycardia occurs following the use of a vasodilator or if a patient is not responding well to a vasodilator.
Salt restriction in the diet is controversial; it seems to make sense but there is not enough data at present to whole-heartedly recommend it. Certainly, if there is kidney disease present the recommendation is less equivocal as these low salt diets are designed with other features more specifically for kidney disease. This generally means a dry or canned formula prescription diet if the pet will eat it or a diet limited to dry food if the pet will not accept prescription food.
Appropriate home cooked diets may be designed by a veterinary nutritionist or through the public site at www.balanceit.com. Several places provide such services through veterinary teaching hospitals or private endeavors.
After hypertension is controlled, patients should be rechecked every two to four months to keep their blood pressure in a healthy range.
Research on this Topic
Effect of Control of Systolic Blood Pressure of Survival in Cats with Systemic Hypertension. Jepson, R.E., Elliott, J., Brodbelt, D., Syme H.M. Journal of Veterinary Internal Medicine 2007; 21: 402-409.
In this study 141 pet cats with high blood pressure were studied. In these cats, 87% were found to have either evidence of renal failure (increased BUN or creatinine tests) or hyperthyroidism or both. Amlodipine besylate was used to treat hypertension in these cats and in 50% of the cats, the initial dose eventually proved inadequate and an increase was necessary. Blood pressure was stabilized within one to two recheck visits for 96% of cats, with a median time of 20 days required to achieve blood pressure stabilization. Blood pressure was more difficult to control in the long term for cats with higher urinary protein loss.
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