Case Studies In Small Animal
Text from "Small Animal Cardiovascular Medicine"
By far the most common cause of systemic hypertension in dogs is renal disease. Renal disease and hyperthyroidism are the two most common causes of systemic hypertension in cats. The incidence of systemic hypertension in cats with renal failure ranges from 61% to 73%. Systemic hypertension ranges from mild to severe with most cats in the mild to moderate range (systolic pressure from 151 to 180 mmHg; diastolic pressure from 101-120 mmHg). Of the 39 cats with hyperthyroidism reported from one study, 87% (n=34) had systemic hypertension. None of the cats with renal failure or hyperthyroidism in these reports had isolated diastolic hypertension. All had either isolated systolic hypertension or both systolic and diastolic hypertension. In a study of dogs with renal disease, 61% had systemic hypertension. Of those that had glomerular disease, 80% had systemic hypertension. In this same study, 59% of dogs with hyperadrenocorticism had systemic hypertension. Recent findings would suggest that about 70% of dogs with diabetes mellitus have elevated systemic pressures. At our hospital, however, only about 50% have systemic hypertension. Hypertension in these dogs has been significantly correlated to the duration of the diabetes mellitus and glycemic control. Pheochromocytoma is a rare cause of systemic hypertension in dogs. Resting systemic arterial blood pressure may be elevated or it may be normal in these cases. Digital manipulation of the adrenal gland tumor during surgery may markedly increase systemic blood pressure.
The mechanisms by which renal failure causes systemic hypertension in dogs and cats have not been adequately studied. In the only study of the renin-angiotensin-aldosterone system in cats with renal failure, plasma renin activity was low, normal, or high while plasma aldosterone concentration was usually increased. In humans, an inability to excrete appropriate amounts of sodium and water with resultant hypervolemia appears to be the primary mechanism early in the course of renal disease although increased systemic vascular resistance may predominate in chronic renal failure. Reduced renal blood flow or localized renal ischemia leading to increased renin secretion also probably plays a role in some patients. Inadequate production of vasodilator substances, such as prostaglandins and components of the kallikrein-kinin system, by diseased kidneys has also been postulated. An inability to excrete sodium and water should be countered easily by administering a diuretic and systemic hypertension in humans is often responsive to diuretic administration or dialysis. Excess renin secretion with resultant aldosterone mediated sodium and water retention should be responsive to the administration of an angiotensin converting enzyme inhibitor and some human patients with chronic renal failure and hypertension are responsive to these agents. However, dogs and cats with renal failure rarely have a clinically significant response to diuretic administration and administration of an angiotensin converting enzyme inhibitor often produces no response or only a very mild response (< 10 mmHg decrease in systemic blood pressure). Dogs and cats with systemic hypertension secondary to renal failure, in general, are more responsive to and often require agents that dilate systemic arterioles. Amlodipine is currently the favorite drug for treating cats with systemic hypertension due to renal failure. Prazosin, hydralazine, and possibly amlodipine are the most effective agents used in dogs to treat systemic hypertension due to renal failure. Consequently, it would appear in dogs and cats with chronic renal failure that the primary mechanism for systemic hypertension is systemic vasoconstriction, not systemic volume overload.
Table 26-2 (Drugs Used to Treat Systemic Hypertension)
SBP = systolic blood pressure
©Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.