Systemic Hypertension

Chapter 14 Systemic Hypertension



Rosemary A. Henik, Scott A. Brown



INTRODUCTION


Hypertension may exist transiently due to fear or excitement, or be sustained and pathologic. Small animal patients most commonly have secondary hypertension, with an underlying disease triggering increased blood pressure (BP). A diagnosis of systemic hypertension is established when reliable BP measurements demonstrate a sustained elevation of systolic (= 160 mm Hg) or diastolic (= 120 mm Hg) BP. The prevalence of idiopathic, hypertension in dogs and cats is unknown.



POPULATION AT RISK



Cats


• Chronic kidney disease, with or without proteinuria, and hyperthyroidism are the two most common causes of hypertension in cats. Although effective resolution of hyperthyroidism may reduce BP, some cats will become hypertensive only after therapy for this endocrinopathy.

• Occasionally, diseases such as hyperaldosteronism, pheochromocytoma, or other endocrine diseases may also be associated with hypertension.

• Many cats that are not hypertensive initially will become hypertensive when fluid therapy, steroids, erythropoietin, or vasoconstricting drugs are given.

• Some clinicians are incorporating the routine measurement of BP into geriatric wellness exams. This practice may occasionally identify a hypertensive cat with subclinical kidney disease; however, it should be realized that the risk of a false positive result (i.e., high BP measurement) increases when the prevalence of a disease is low (as in a healthy population).

• BP measurement is advised in all cats with:
• Kidney disease

• Endocrinopathy

• Compatible ocular signs (e.g., hemorrhage, detachment, retinal vessel tortuosity)

• Neurologic signs (e.g., nystagmus, head tilt, other cranial nerve signs, or seizures)

• A cardiac murmur or gallop

• Radiographic cardiomegaly

• Echocardiographically determined cardiac wall hypertrophy (although this is not a consistent change in hypertension)


Dogs


• Chronic glomerular disease or tubulointerstitial disease associated with proteinuria are most commonly associated with hypertension in dogs.

• Endocrine diseases including hyperadrenocorticism, diabetes mellitus, pheochromocytoma, and hyperaldosteronism may be associated with hypertension.

• Metabolic abnormalities including obesity or hypercholesterolemia may result in hypertension.

• Resolution of the endocrinopathy may not restore normal BP in dogs.

• BP measurement is advised in any dog with:
• Kidney disease

• Endocrinopathy

• Ocular signs or neurologic signs as described previously

• Cardiac hypertrophy (although rare in the dog as an acquired change), which should also prompt the clinician to perform BP measurement


Key Point


Chronic kidney disease and endocrinopathies are most commonly associated with systemic hypertension in cats and dogs.



CONSEQUENCES AND CLINICAL SIGNS OF HIGH BLOOD PRESSURE



Ocular Signs


• Severe systemic hypertension can lead to ocular injury. 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.

• A sudden onset of blindness due to retinal hemorrhage and detachment is a common presenting complaint (Figure 14-1).

image

Figure 14-1 A cat with acute blindness due to systemic hypertension secondary to an aldosterone-secreting adrenal tumor. This cat presented with a systolic blood pressure > 300 mm Hg. There are severe ocular hemorrhages bilaterally.



Neurologic Signs


• Signs consistent with cerebrovascular hemorrhage (head tilt, depression, seizures) have been seen clinically in cats and dogs with uncontrolled hypertension, and are often associated with a poor prognosis.

• Rarely, cats suffering from severe systemic hypertension (systolic BP > 300 mm Hg) develop a syndrome of progressive stupor, head pressing, or seizures that rapidly resolves with effective antihypertensive therapy. This syndrome is probably due to cerebral edema caused by high intracapillary hydrostatic pressure that develops once the systemic arterial pressure exceeds the autoregulatory range.


Renal Signs


• Persistent elevation of systolic BP to values > 160 mm Hg is associated with progressive renal injury in dogs and the severity of renal injury has been correlated to the degree of elevation. Potential renal pathologic changes induced by systemic hypertension include both glomerular and tubulointerstitial changes and can result in ischemia, necrosis, atrophy and exacerbation of proteinuria. These gradual and additive changes may be difficult to quantify in living animals with preexisting kidney disease, but systolic hypertension at the time of presentation (> 160 mm Hg) increases the odds of uremic crisis and of death in dogs with chronic kidney disease. Hypertension with systolic BP > 160 mm Hg is also likely to be a risk factor for progressive renal damage in cats.

• Proteinuria is perhaps the best marker that high BP is injurious to the kidney. The presence of microalbuminuria or an elevated urine protein-to-creatinine ratio (>0.5 in dogs, > 0.4 in cats) in a hypertensive animal (systolic BP > 160 mm Hg) should be considered an indication of ongoing renal damage.


Cardiac Signs


• Because the heart is working against an increased arterial pressure (i.e., afterload), diastolic dysfunction, left ventricular hypertrophy and secondary valvular insufficiency may develop. Changes may regress with antihypertensive treatment.

• Cardiac murmurs or gallops commonly occur.

• Congestive heart failure secondary to systemic hypertension is rare.


MEASUREMENT OF BLOOD PRESSURE



Patient Selection


• Currently there is no evidence to suggest that BP should be measured in all animals.

• BP should be measured in those animals that present with clinical signs attributable to high systemic arterial BP, such as blindness, hyphema, seizures, ataxia, or sudden collapse (signs compatible with cerebral vascular hemorrhage, edema, or stroke).

• Animals with azotemic kidney disease, hyperadrenocorticism, hyperthyroidism, pheochromocytoma, mineralocorticoid-secreting tumor, marked obesity, or cardiac hypertrophy should also be evaluated for hypertension.


Methods


• BP may be measured by either direct or indirect methods. Direct BP measurement is the “gold standard,” but is technically difficult in unsedated dogs and cats, may be painful to the patient, and may be associated with hematoma formation and other complications.

• The indirect techniques are more applicable to a clinical setting, as they require less restraint and are technically easier to perform. Indirect techniques of BP measurement include the auscultatory, ultrasonic Doppler, oscillometric, and photoplethysmographic methods.

• All of the indirect techniques employ an inflatable cuff wrapped around an extremity. The pressure in the cuff is measured with a manometer or pressure transducer. A squeeze bulb is used to inflate the cuff to a pressure in excess of systolic BP, 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, or mean BP. This detection method varies among different indirect methods.

• For the auscultatory method, a stethoscope is placed over the artery distal to the cuff, and the listener hears a tapping sound when the inflation pressure falls below systolic pressure. In animals, the arterial (Korotkoff) sounds are low in both amplitude and frequency, and the auscultatory technique is difficult in dogs and cats.

• Doppler flow meters detect blood flow as a change in the frequency of reflected sound (Doppler shift) due to the motion of underlying red blood cells. BP is read by the operator from an aneroid manometer connected to the occluding cuff placed proximal to the Doppler transducer.

• Devices utilizing the oscillometric technique detect pressure fluctuations produced in the occluding cuff resulting from the pressure pulse. Machines using the oscillometric technique generally determine systolic, diastolic, and mean arterial pressures as well as pulse rate.

• Another device for measuring BP indirectly is the photoplethysmograph, which measures arterial volume by attenuation of infrared radiation, and is designed for use on the human finger. It can be employed in cats and small dogs weighing less than 10 kg.

• We have evaluated the ultrasonic Doppler and oscillometric methods in conscious dogs and cats. In both species, the oscillometric devices tend to underestimate BP by increasing amounts as pressure increases. Another problem with the oscillometric devices is the excessive time required to obtain readings in cats. The major limitation of the Doppler technique is the imprecise discrimination of the sounds designating the diastolic, and therefore mean, pressures. Owing to this fact, the Doppler method of BP measurement may be unreliable for the routine diagnosis and surveillance of patients with diastolic hypertension.

• On the basis of our studies in conscious animals, devices using the Doppler principle are recommended for use in cats; either the oscillometric or the Doppler devices are recommended in dogs. For comparative purposes, the same device should be used each time in an individual animal.


Key Point


Doppler is the preferred method of measurement in unsedated cats; the oscillometric technique may be used in dogs.



Cuff Size and Placement


• A complete range of pediatric cuff sizes should be available for an optimal patient limb circumference to cuff width ratio.

• For dogs, a cuff width that measures 40% of the circumference of the limb should be used; for cats, a width of 30% to 40% of the limb circumference should be used. The cuff width should be noted in the medical record for future reference.

• An oversized cuff may give erroneously low recordings; an undersized cuff may give erroneously high recordings. If the ideal cuff width is midway between two available sizes, the larger cuff should be used, because it will theoretically produce the least error.

• The cuff may be placed around the brachial, median, or cranial tibial artery, or around the medial coccygeal artery.

• Generally, for the Doppler technique, the cuff is placed over the median artery, and the transducer is placed between the carpal and the metacarpal pads (Figure 14-2). Clipping the hair and applying acoustic gel at the site of transducer placement may enhance the signal, but clipping the hair may increase stress-artifact.

• For the oscillometric technique, our studies demonstrated that the coccygeal or cranial tibial artery in dogs may provide more reliable values than other sites (Figure 14-3).

• For comparative purposes, the same site for cuff placement should be used each time in an individual animal and recorded in the medical record.

• 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 mm Hg rise in BP expected for each 1.3 cm of vertical distance between the level of the cuff and the level of the aortic valve.

image

Figure 14-2 Demonstration of the technique to obtain systemic blood pressure measurements using a Doppler transducer in a cat.


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Figure 14-3 Demonstration of cuff placement over the cranial tibial artery in a dog. The oscillometric unit is most reliable in medium- and large-breed dogs.



Environment and Personnel


• Provide an environment that is quiet and away from other animals.

• The owner may be present to help calm the animal.

• Allow for a quiet equilibration time for the animal of 5 to 15 minutes.

• The same individual (preferably a technician) should perform all BP measurements following a standard protocol.

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Aug 15, 2016 | Posted by in SMALL ANIMAL | Comments Off on Systemic Hypertension

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