Why Is My Dog Or Cat’s Blood Pressure High or Low?

Ron Hines DVM PhD

It is exceptionally hard for veterinarians to obtain accurate blood pressure measurements for your dog or cat. Much more so than from us humans. That is because of the excitement or apprehension and anxiety that almost all of our pets experience to some degree in an animal hospital setting. The best we can do is rely on the veterinary doppler or oscillometric blood pressure measuring machines currently available. None are as accurate as we desire. Veterinarians hope that in taking a series of measurements during which we allow your pet to calm down (the “white coat” syndrome) we might obtain a more accurate reading. A second confirmation of high blood pressure a week or two later is always a good idea. (read here & here) The most accurate readings are likely to be obtained by a house call veterinarian measuring your pet’s blood pressure at home.

Your Pet’s Blood Pressure – up Hypertension, low = Hypotension

Most veterinarians consider that the maximum normal blood pressure in a relaxed dog is 140 mm Hg. In cats, veterinarians are less in agreement. Some say their blood pressure should be between 120-130 mm Hg, others below 140 mm Hg. Still others say that your cat’s sex, neuter status and body weight factor into what is normal as well. (read here) In us humans, blood pressure often rises as we age and our general health suffers from that later. Veterinarians do not believe that that type of arterial “hardening” (arteriosclerosis/ atherosclerosis) is common in dogs or cats. Even in miniature schnauzers with a genetic predisposition to high blood triglycerides, high blood pressure or heart issues are not a known to result. (read here)

Our pets do sometimes suffer from secondary high blood pressure (secondary hypertension) that is the result of health issues that indirectly affect blood pressure within their circulatory system. You can read about some of those problems here.

As in people, the early effects of persistent high blood pressure can be easily missed. Perhaps your veterinarian might notice increased snake-like loops and curves in the blood vessels deep within your pet’s eyes (“retinal capillary tortuosity” or TOD). More likely, your vet discovered the problem using a blood pressure monitor during a routine examination or after learning that your pet had kidney issues. But as I mention elsewhere, elevated blood pressure reading at animal hospitals can be due to no more than excitement, apprehension, or anxiety.

Confirmed sustained high blood pressure has a particularly destructive effect on the eyes of your dog or cat. With time, it can lead to blindness, detached retinas, hemorrhage within the eye, retinal edema, glaucoma and retinal degeneration. Elevated blood pressure eventually affects the brain and nervous system as well. It might begin with behavioral changes (altered mentation), decreased awareness (cognitive dysfunction) and “clumsiness” (ataxia). That can progress to seizures.

Health Problems That Are Thought To Cause High Blood Pressure (= systemic hypertension) In Dogs And Cats:

Chronic kidney failure is thought to be the most common cause of elevated blood pressure in dog and cats. Up to 80% of dogs with kidney (glomerular) disease are reported to have higher than normal blood pressure. Sudden kidney failure (acute renal failure) is a less common problem – but it can have the same effect. However, what medications might be helpful in preserving your pet’s remaining kidney function and lower its blood pressure and at what dose has not been adequately determined. (read here) Veterinarians are most likely to prescribe an ACE inhibitor such as benazepril along with amlodipine to lower your pet’s blood pressure. If the medications are helping, the amount of protein in your pet’s urine (microalbuminuria) and the amount of creatinine in your pet’s blood should go down – or at least not increase and its blood pressure reading should decrease. I suggest that when you give any medications in an attempt to lower your pet’s blood pressure, begin at a conservative drug dose(s), observe the effects, and, when required, increase or decrease the dose(s) gradually.

In cats, hyperthyroidism is high on the list of causes of abnormally high blood pressure. In dogs, an overactive adrenal glands (Cushing’s disease) can also result in high blood pressure. In one study, over half of Cushing’s disease dogs were said to have high blood pressure. Adrenal gland or pituitary gland tumors (those that produce aldosterone or growth hormone respectively) can also elevate your cat or dog’s blood pressure. Increased blood pressure can also be a side effect when your pet has diabetes. On some occasions, dogs and cats have elevated blood pressure readings when no underlying health problem can be found.

Health Problems That Can Cause Low Blood Pressure (Hypotension) In Your Pet:

Unlike high blood pressure which can exist unnoticed in your dog or cat for an extended period of time, low blood pressure in cats and dogs is usually a crisis situation. It could be a shock reaction, it could be due to severe trauma, heatstroke and/or dehydration, perhaps a vaccination reaction, a chewed electrical cord or an overdose of medications. Pets suffering from low blood pressure have a weak (“thready”) or irregular pulse. Their pulse can be fast (tachycardia) or slow (bradycardia). The pet’s gums are usually pale and slow to refill pink with blood when they are pressed with a finger (slowed CRT). These pets’ attitude is usually dull and apathetic. Their feet and ears often feel cold to the touch. Pets with low blood pressure do not pass much urine. They often end up in emergency clinics in need of immediate support including oxygen.

Here Are Some Common Causes Of Low Blood Pressure:

Failing hearts (congestive heart failure or cardiomyopathies) when your pet’s heart can no longer pump sufficient blood. Severe infections (septic shock). An Addison’s disease crises. Substantial blood loss resulting in anemia. Anaphylactic shock. Prolonged seizures (= status epilepticus). Severe trauma such as car accidents or animal fights that I mentioned previously. Severe diarrhea. Severe vomiting or any other cause of substantial dehydration (=hypovolemia). Burns and heatstroke can also result in low blood pressure. Low blood sugar (hypoglycemia) as occurs when too large an insulin dose is given or in rare instances, insulinoma. Moderately lowered blood pressure is also possible in hypothyroid dogs.

Even if your pet does not have heart disease, anything that restricts the flow of blood through its heart, such as a gastric volvulus, pressure on the diaphragm, a torn diaphragm, free air in the chest (pneumothorax) or fluid surrounding the heart (pericardial tamponade) can result in abnormally low blood pressure. In some cases of heartworm disease disease, even though the dog’s pulmonary (lung) blood pressure rises systemic (body) blood pressure falls (hypotension). Late-stage liver failure can also result in low blood pressure as can an abnormal heartbeat (arrhythmia), electrolyte imbalances and acid-base disturbances. A sudden episode of low blood pressure and shock (DIC) has the ability to activate the blood clotting process throughout your pet’s body by activating the coagulation cascade. That can result in dangerous blood clots.

Medications that your pet receives to treat heart conditions or kidney disease can also be responsible for low blood pressure readings. Those drugs include beta blockers such as metoprolol, propranolol and atenolol, Calcium channel blockers such as amlodipine, verapamil or diltiazem, and ACE inhibitors such as enalapril or benazepril. All anesthetics used for general surgery have the potential to lower blood pressure as well. A medication commonly dispensed to female dogs with urine incontinence, phenylpropanolamine, can also cause high blood pressure reading. Corticosteroid medications such as prednisone, prednisolone, and dexamethasone can have this same dose-dependent potential effect in both dogs and cats.

An overly large dose of amitriptyline in cats being treated for urinary spraying or in dogs being treated for anxiety can also lower blood pressure. Furosemide (Lasix®) and spironolactone diuretics in dogs and cats can also cause blood pressure to drop. So can sildenafil (Viagra®) given to dogs to treat pulmonary hypertension.

Health Problems That Can Cause Elevated Lung Circulation Blood Pressure (= Pulmonary Hypertension):

Another type of high blood pressure in pets is pulmonary hypertension. In that situation, blood pressure increases primarily in the blood vessels going from your pet’s heart to its lungs. In dogs in my part of South Texas, the most common cause for that is a massive heartworm infection (“caval syndrome”). However, birth defects (patent ductus arteriosis = PDA) and chronic lung disease can also cause pulmonary hypertension. On rare occasion, failing hearts or lack of oxygen supply to the pet’s kidneys (renal hypoxia) or kidney cancer lead to increased red blood cell production, elevated PCVs (= polycythemia) and result in thick (“overly viscous”) blood. That too can result in pulmonary hypertension.

Whichever the case in your dog or cat, pulmonary hypertension often results in rapid breathing (= hyperpnea), weakness, bluish gums (=cyanosis) and occasional, fainting or seizures after exertion or exercise.

Complementary Tests That Come To Mind:

The possible results of long-term elevated blood pressure in your dog or cat include kidney, heart and eye damage such as visual problems, large pupils, retinal detachment or blood leakage into the eye. Elevated blood pressure can also affect your pet’s ability to think clearly as evidenced by forgetfulness, aimless wandering, circling or seizures. So tests to check for the presence of those problems would be my next step. That would include repeated blood pressure measurement (you must confirm results on several occasions), CBC and a blood chemistry panel, urinalysis, examination of the dog or cat’s retinas for evidence of hemorrhage, screening test values that focus on kidney, adrenal gland and thyroid health. A review of all the medications and dose sizes is advisable as well. Many of the same tests would help your veterinarian determine the cause of low blood pressure. A doppler ultrasound examination by a trained veterinary specialist is always the best way to locate physical causes of both high and low blood pressure.

DxMe

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