Scott A. Brown, VMD, PhD, DACVIM (Internal Medicine)
High systemic arterial blood pressure is commonly observed in cats with chronic renal failure and in a variety of other metabolic and endocrinological abnormalities. In veterinary medicine, systemic hypertension has been associated with ocular pathology, chronic renal failure, neurologic complications, and cardiovascular changes. There is a clear association between ocular injury and marked systemic hypertension in dogs and cats. Findings associated with hypertensive injury include hemorrhage within the retina, vitreous, or anterior chamber; retinal detachment and atrophy; retinal edema; perivasculitis; retinal vessel tortuosity; and glaucoma.
The kidney is susceptible to hypertensive injury. However, preglomerular arterioles usually constrict whenever blood pressure is elevated, serving to protect the renal glomerulus from hypertensive injury. In dogs and cats with renal insufficiency, these preglomerular arterioles are dilated and poorly responsive to changes in blood pressure. Thus the elevated blood pressure is transmitted directly to the glomerular capillary bed. This causes an increase in glomerular capillary pressure, referred to as glomerular hypertension, which may produce glomerular damage and a progressive fall in renal function unless hypertension is effectively treated.
Because the heart is working against an increased arterial pressure (i.e., afterload), left ventricular hypertrophy and secondary valvular insufficiency may be observed. Tachycardia is not a common finding with hypertension although some primary diseases that lead to secondary hypertension, such as hyperthyroidism, may also lead to elevated heart rate. Left ventricular hypertrophy may regress with antihypertensive treatment.
A diagnosis of systemic hypertension is based upon determination of systemic arterial blood pressure. Further, the indiscriminate use of antihypertensive therapy in the absence of reliable values for systemic arterial blood pressure is inappropriate.
While it is possible to measure blood pressure in all clinical patients, currently there is not sufficient rationale to do so in cats. If the veterinarian only measures blood pressure in those animals suspected of having complications secondary to hypertensive injury, the opportunity for early identification of hypertension and intervention is lost. Thus, patients with no evidence of hypertensive injury but known to be at risk for the development of systemic hypertension also should be assessed. In addition to routine screening of patients with chronic renal failure, hyperthyroidism, and advancing age, conditions in which blood pressure measurements are indicated include obesity, hyperadrenocorticism (endogenous or exogenous), mineralocorticoid secreting tumor, and pheochromocytoma.
Blood pressure may be measured by either direct or indirect methods. Direct blood pressure measurement is the "gold standard", and usually involves placement of a needle or indwelling catheter into a peripheral artery. The needle or catheter is attached to a pressure transducer, and pressure is displayed on a screen or recording chart. This technique is technically difficult in unsedated dogs and cats, may be painful, and complications, such as hematoma formation, may develop. The indirect techniques are more applicable to a clinical setting, since they require less restraint and are technically easier to perform. Indirect methods of blood pressure measurement include the auscultatory, ultrasonic Doppler, oscillometric, and photoplethysmographic methods.
All of these indirect techniques employ an inflatable cuff wrapped around an extremity. The pressure in the cuff is measured with the aid of a manometer or a pressure transducer. A squeeze bulb is utilized to inflate the cuff to a pressure in excess of systolic blood pressure, thereby occluding the underlying artery. As the cuff is gradually deflated, changes in arterial flow are detected by one of several means; the value for cuff pressure at various levels of deflation is then correlated with systolic, diastolic, and/or mean blood pressure. This detection method varies between different indirect methods. An oversized cuff may give erroneously low recordings; an undersized cuff a falsely high reading. In cats, indirect blood pressure measurement studies should employ a cuff width that measures 30-40% of the circumference of the limb. If the ideal cuff width is midway between two available sizes, the larger cuff should be used, since it will theoretically produce the least error. The cuff may be placed around the brachial, median, cranial tibial, or medial coccygeal arteries. Generally, for the Doppler technique the cuff is placed over the median artery and the transducer is placed between the carpal and metacarpal pad. Clipping of hair and application of acoustic gel at the site of transducer placement may enhance the signal. For the oscillometric technique, the median or coccygeal artery is commonly used. The cuff should be placed at the level of the aortic valve. If not, a compensation can be made for gravitational effect with a 1.0 mmHg rise in blood pressure expected for each 1.3 cm of vertical distance between the level of the cuff and the level of the aortic valve.
In light of the uncertainty and the difficulties associated with blood pressure measurement in dogs and cats, only those animals with marked elevations of indirectly measured blood pressure and/or with clinical abnormalities directly attributable to hypertensive injury should be considered candidates for treatment. Because of the association of marked systemic hypertension with ocular injury, the authors consider antihypertensive treatment to be indicated in any dog or cat with a sustained systolic blood pressure > 200 mmHg or diastolic blood pressure >120 mmHg, regardless of other clinical findings. In both species, an animal with a systolic/diastolic blood pressure that consistently exceeds 170/100 mmHg should be considered for treatment if clinical evaluation has identified abnormalities (e.g., retinal lesions or chronic renal disease) that could be caused or exacerbated by systemic hypertension. In animals in which the blood pressure is moderately elevated (systolic/diastolic blood pressure that consistently exceeds 170/100 mmHg) and no clinical abnormalities related to systemic hypertension are identified, the rationale for therapy is less clear. Currently, some clinicians recommend treatment for animals in this range, while others do not.
It is usually not possible to restore blood pressure to normal values when treating a hypertensive animal. It should be the veterinarian's goal to lower the blood pressure to within 2550 mmHg of the normal ranges for blood pressure, thus lowering pressure (systolic/diastolic) to < 170mmHg/100mmHg.
Though poorly studied, the usual recommendation is to initially institute a low sodium diet which provides <0.25% sodium on a dry weight basis. A diet with a low chloride and a moderately elevated potassium content may provide further blood pressure-lowering benefit.
Obesity can elevate systemic arterial pressure in human beings and dogs and, perhaps, in cats. Consequently, weight loss is desirable in obese, hypertensive animals. However, the effect of obesity on blood pressure is relatively modest and by itself would be difficult to appreciate with devices that indirectly estimate blood pressure. It is likely that weight loss will be of some benefit to these animals and should be a long-term goal of medium priority in obese, hypertensive cats and dogs.
Medical treatment of hypertension in dogs and cats has, until recently, been extrapolated from human protocols. Recommendations for medical therapy have included vasodilators, betablockers, and diuretics; these agents are generally given in concert with dietary sodium restriction. In human beings with systemic hypertension and renal disease, vasodilator therapy is the preferred initial choice because of the renoprotective effects of certain classes of these agents (angiotensin converting enzyme inhibitors and calcium channel).
Some drugs classified as calcium channel antagonists reduce total peripheral resistance, leading to a decrease in blood pressure. Amlodipine besylate, a long-acting dihydropyridine calcium antagonist, has been used successfully as a single agent in hypertensive cats at a dosage of 0.625 mg/cat orally every 24 hours. Larger cats (>4 kg) often require 1.125 mg orally every 24 hours. An inhibitor of angiotensin converting enzyme (e.g., 0.5 mg enalapril/kg orally every 12 hours or 0.25-0.5 mg benazepril/kg every 12-24 hours) will lower blood pressure in some hypertensive dogs and the co-administration of an ACE inhibitor and a calcium channel antagonist may prove efficacious when monotherapy is not sufficient in lowering blood pressure. The co-administration of a calcium channel antagonist and an angiotensin converting enzyme inhibitor apparently blocks any adverse effects of calcium channel antagonism alone, at least in diabetic dogs.
FOLLOW-UP CARE AND ADDITIONAL MEDICATIONS
In all animals treated for systemic hypertension, the routine examination should include a fundic examination, evaluation of any underlying diseases and measurement of body weight, blood pressure, and serum concentrations of creatinine and electrolytes. The owner should be questioned for evidence of drug toxicity, which may include lethargy, increased time spent sleeping, ataxia, or anorexia. Animals on multiple drug regimes are more likely to exhibit adverse effects than are those on a single antihypertensive agent. Once blood pressure is controlled, the animal should be evaluated at three month intervals. A complete blood count, biochemical panel, and urinalysis should be evaluated at least once every 6 months.
Systemic hypertension has been causally associated with damage to the eyes, brain, cardiovascular system, and kidney.
Because of the reportedly high prevalence of systemic hypertension in dogs and cats with chronic renal failure, the nature of the relationship between chronic renal failure and systemic hypertension is crucial. While chronic renal failure can lead to systemic hypertension, the role of high blood pressure as a cause of renal injury in dogs and cats is unknown.
Systemic hypertension is often observed in dogs and cats with chronic renal failure and other metabolic and endocrinological abnormalities. High systemic arterial blood pressure has been associated with chronic renal failure, ocular injury, neurologic complications, and cardiovascular changes. Recent advances in our knowledge of the prevalence and consequences of systemic hypertension dictate that proper diagnosis and treatment of this problem should become a component of routine therapy for many of our patients.
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