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Indirect BP Monitoring is Directly Applicable to Proper Patient Monitoring
Abnormalities in systemic arterial blood pressure (BP) have been associated with a variety of diseases in veterinary patients. Monitoring of BP provides the clinician with a parameter for determining the patient's volume status. Normal BP for dogs is systolic pressure of 110-190 mmHg and diastolic pressure of 55-110 mmHg. Normal BP for cats is systolic pressure of 120-170 mmHg and diastolic pressure of 70-120 mmHg. Since the duration of diastole is longer than that of systole, the mean arterial blood pressure (MAP) is not merely an average of the two values, and should be calculated as follows: MAP = [(systolic BP - diastolic BP) / 3] + diastolic BP.
Hypotension, defined as a systolic BP <80 mmHg or MAP <60 mmHg, is common in critically ill patients. These values are the minimum pressures required to maintain adequate perfusion to the brain and kidneys. Hypertension in the critically ill may be triggered by renal failure, neoplasia, hyperthyroidism, diabetes mellitus, hyperadrenocorticism, pheochromocytoma, medications, and is common secondary to pain and anxiety. Primary hypertension is rare in dogs and cats.
Blood pressure is most frequently measured non-invasively (indirectly), and may be performed with Doppler or oscillometric methods. These indirect methods rely on detection of blood flow beneath an inflated cuff, and therefore are less accurate during hypotensive states and/or severe vasoconstriction.
Both the Doppler and the oscillometric sphygmomanometry techniques (Dinamap or Cardell) utilize a cuff placed around an extremity to detect flow in a superficial artery. The Doppler method makes use of a small ultrasound probe placed on a peripheral artery to convert blood flow into an audible sound. The oscillometric method detects changes in oscillations produced by fluctuations in artery wall diameter. Measuring BP by direct and oscillometric techniques gives systolic, diastolic, and mean arterial pressures, whereas the Doppler gives only systolic values. Systolic pressures obtained using the indirect methods correlate well with direct pressure measurements in healthy conscious dogs; however, the systolic pressures of cats may be underestimated by 10-15 mmHg using the Doppler method. Doppler measurements at the metatarsal artery and the oscillometric technique performed at the coccygeal artery are the most accurate. The accuracy of the indirect methods decreases with small patient size, vasoconstriction, dysrhythmias, or movement. These techniques are inexpensive and technically easy to use. The most common mistake made is cuff size selection. The cuff should be 40% of the circumference of the limb or tail in dogs and 30% for cats. If the cuff is too small, a falsely high pressure will be obtained and if the cuff is too large, a falsely low reading will result.
For more information or to talk to one of
our specialists, please call 402-614-9000
or email info@midwestvetspecialists.com
Christopher G. Byers, DVM, DACVECC, DACVIM (SAIM)
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