by Barry B. Burtis D.V.M.
Blood Pressure in Pets
There are few more familiar tests for people receiving a health examination than to have their blood pressure measured. Yet for a variety of reasons this has not been a common test for pets when they get a checkup. Have you ever wondered why this so?
The cardio-vascular system works the same for all mammals. When the heart, acting as a pump, causes blood to flow out into the arteries a pressure gradient is created. This pressure must be maintained from the arteries to the capillaries, the smallest blood vessels that deliver blood to the tissues of the body. A certain pressure is necessary to ensure body parts are supplied with proper nutrients, to exchange and excrete waste products produced in these parts and to supply oxygen to body tissues. Arterial blood pressure (BP) is influenced primarily by three factors: heart factors that influence the output of blood into the arteries; blood vessel factors that affect the resistance to the outflow of blood from the heart; factors that affect the viscosity or resistance to flow of the blood itself.
Blood pressure is regulated throughout an animal's body by the integration of some very complex mechanisms in the brain and nervous system as well as the kidneys and the heart. Some of these means of regulation can act almost immediately - as would be required for an athletic greyhound at the start of a race. Others act more slowly, perhaps in response to disease, such as chronic kidney failure in an aged cat.
Well, so much for the physiology or normal functioning of BP, but what happens when something goes wrong? Essentially, one of two problems can result - low blood pressure (hypotension) or high blood pressure (hypertension). In humans, primary hypertension or high BP is quite a common disorder. A number of genetic, dietary and lifestyle factors probably play a role in this fact. In veterinary medicine, primary hypertension is much less commonly a problem for our patients. This is likely the first reason why BP measurement has not been a common part of health examinations for pets.
However, there are many diseases that affect cats, dogs and other pets that cause changes in BP. Kidney failure, hyperadrenocorticism (Cushing's disease), diabetes, hyperthyroidism, hypothyroidism, hypoadrenocorticism (Addison's disease), heart failure and shock are examples of such diseases. Many medications and anesthetics used for pets can lead to changes in BP. Persistent change, whether it is high or low BP can significantly affect the function of a number of body organs. It can also seriously damage some organs - for example, eyes, heart, kidneys, and the central nervous system. Therefore, veterinarians need to be aware of
BP and be able to assess it for their patients.
The ease of checking blood pressure in an animal is the other reason it has, historically, been less frequently done for pets. There are two methods to measure arterial blood pressure, either the direct or the indirect method. In the direct method, a catheter must be placed directly within an artery. It is considered the gold standard method but it is expensive to do and can cause the animal discomfort. As a result, it is seldom performed in clinical practice. If done correctly, the indirect method will give very reliable blood pressure measurements. Fortunately, within the last ten years equipment has become available that allows these methods to be used for our pets. There are two indirect methods that can be used. Both use an inflatable cuff that is placed around a leg or the tail. In the oscillometric method, pulse pressure in an artery below the cuff is read as the diameter of the artery changes in diameter. With the other method, the Doppler technique, ultrasound waves are used to detect and make audible blood flow in an artery beyond the blood pressure cuff. This method is quite similar to the one used to check a person's blood pressure. The cuff around the pet's leg is inflated until the sound of blood flowing through the artery stops. The cuff is then gradually deflated until blood flow returns, allowing measurement of BP. Normal blood pressure for a dog is 180 mm Hg/ 100 mm Hg and for a cat is 170-180 mm Hg / 120 mm Hg.
Next time, I want to discuss some reasons your veterinarian may recommend your pet's blood pressure be checked.