Page 243 - Small Animal Internal Medicine, 6th Edition

CHAPTER 11 Systemic Arterial Hypertension 215

Oscillometric method. The indirect oscillometric meth- considered the preferred method for systolic BP measure-
od uses an automated system for detecting and processing ment in cats and small dogs.
VetBooks.ir cuff pressure oscillation signals. With these systems, the flow Treatment and Prognosis
occlusion cuff is inflated to a pressure above the systolic pres-
sure and then slowly deflated in small pressure decrements.
with severe hypertension, animals with clinical signs pre-
The microprocessor measures and averages the resulting Antihypertensive therapy definitely is indicated for animals
pressure oscillations characteristic of systolic, diastolic, and/ sumed to be caused by hypertension, and animals with evi-
or mean pressures (depending on the system). Accurate dence of target-organ damage. It is unclear whether all
results with oscillometric methods depend on careful ad- asymptomatic dogs and cats with moderate hypertension
herence to the directions for use and an immobile subject. (e.g., repeatable systolic pressures of 160-180 mm Hg) benefit
Because muscle contraction can produce oscillations, the from specific antihypertensive treatment. Nevertheless,
limb used should not be bearing weight. At least five readings patients with high BP that persists after treatment for the
should be obtained; the lowest and highest are discarded, primary disease should generally be treated. The goal of
and the remaining measurements are averaged. The oscil- therapy is to reduce the BP to below 150/95 mm Hg. The
lometric method may be difficult to use effectively in small expense and time commitment required for long-term
dogs and cats; poor agreement in systolic BP values (specifi- (usually life-long) antihypertensive therapy and monitoring,
cally underestimation) compared with direct measurements as well as the potential for adverse medication effects, are
has been reported with various oscillometric devices. considerations and should be explained clearly to the owner.
Doppler ultrasonic method. This method employs the Although some cases constitute hypertensive emergencies
frequency change between emitted ultrasound and returning that require immediate therapy and intensive monitoring
echoes (from moving blood cells or vessel wall) to detect (discussed in more detail later), most hypertensive animals
blood flow in a superficial artery. This frequency change, can be managed more conservatively (Box 11.3). Gradual
the so-called Doppler shift, is converted to an audible signal. reduction in BP could be safer in patients with long-standing
One system commonly used in animals is designed to deter- hypertension. Chronically high BP leads to vascular adapta-
mine systolic pressure by detecting blood cell flow (Ultra- tions in the cerebral autoregulatory process; if BP is suddenly
sonic Doppler Flow Detector, Model 811, Parks Medical reduced, cerebral perfusion may be adversely affected.
Electronics, Inc, Aloha, OR). Several drugs are used as antihypertensive agents in dogs
Effective locations for pressure measurement include the and cats (Table 11.1). Usually one drug is administered ini-
dorsal metatarsal, palmar common digital (forelimb), and tially, at a low to moderate dose, if there is not a hypertensive
median caudal (tail) arteries. The probe is placed distal to crisis; then the BP is rechecked in 7 to 10 days to assess
the occluding cuff. A small area of hair is clipped over the efficacy. The dose of the initial agent can be increased within
artery for probe placement. Ultrasonic coupling gel is applied recommended guidelines, if needed; if this is not sufficiently
to the flat Doppler flow probe to obtain air-free contact with effective a second antihypertensive agent can be added. The
the skin. The probe is positioned so that a clear flow signal drugs used most often are angiotensin-converting enzyme
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is heard but not held so tightly that it occludes flow. The inhibitors (ACEIs), the Ca -blocker amlodipine, and (less
probe must remain still to minimize noise; it can be taped in commonly) the β-adrenergic blocker atenolol. Therapy with
place. A low-volume setting on the Doppler unit or a headset a single agent is effective in some cases, but combination
is used to minimize patient anxiety caused by loud audio therapy may be necessary for adequate BP control in others.
signals. An ACEI is recommended as the antihypertensive drug of
The flow-occluding cuff is attached to a sphygmomanom- first choice for all dogs. If the ACEI does not provide ade-
eter and inflated to about 20 to 30 mm Hg above the point at quate BP reduction even at maximum recommended dosage,
which arterial flow ceases and no audible signals are heard. amlodipine is added. Amlodipine is not recommended as
The cuff is slowly deflated (by a few mm Hg per second). the sole antihypertensive therapy in dogs, because amlo-
During deflation, characteristic pulsatile flow signals from dipine administration in dogs activates the RAAS (and thus
blood cell (or arterial wall) motion return during systole. requires concurrent ACEI). In cats, amlodipine often is
The systolic pressure is the pressure at which blood flow recommended as the initial antihypertensive drug, unless
first recurs (indicated by brief swishing sounds). A change hyperthyroidism is the underlying cause, in which case
in the flow sound from short and pulsatile to a longer, more atenolol could be considered first or in combination with
continuous swishing often can be detected as cuff pressure amlodipine. However, an ACEI also has been recommended
diminishes; the pressure at which this change occurs approxi- as initial therapy for cats with renal disease and proteinuria
mates the diastolic pressure. However, Doppler estimation of as the suspected underlying cause of hypertension. Other
diastolic BP is less accurate because of its subjective nature. strategies are indicated in certain other specific condi-
The change in flow sound is not always detectable, especially tions, such as sympathetic antagonists for pheochromocy-
with small or stiff vessels. As with the oscillometric method, toma or an aldosterone antagonist (e.g., spironolactone) for
it may be difficult to obtain measurements in small or hypo- hyperaldosteronism.
tensive animals with the Doppler method. Patient move- Ancillary strategies might be helpful in patients with
ment also interferes with measurement. Overall, Doppler is hypertension, although alone they are unlikely to markedly