Heads Up From Preanesthetic Evaluation

Normal values for systolic/diastolic and (mean) pressures quoted by the Veterinary Blood Pressure Society 2002 for awake healthy animals are for dogs 133/75 (94) mm Hg and for cats 124/85 (98) mm Hg. Whereas the systolic and diastolic pressures vary from publication to publication, mean arterial blood pressure (MAP) values are often 90-100 mm Hg. Hypotension is generally defined as a MAP of less than 65 mm Hg. MAP of 55 is seriously life threatening. Non-invasive method of blood pressure measurement using oscillometry provides a digital display of systolic, diastolic, and mean pressures. The mean pressure usually correlates well with the mean value obtained by direct arterial catheterization. The non-invasive Doppler shifted ultrasound method (Parks, Aloha, Oregon) provides systolic pressure. The diastolic pressure is difficult to hear in some patients. Experiments have shown that in cats and small dogs the first sound heard with the Doppler method is closer to the mean pressure. To improve accuracy of measurement, the width of the cuff used for both methods should be equal to 40% (40-60%) of the circumference of the limb or tail, and the inflatable cuff part should be placed directly on the medial side of the leg (not over a joint) or the ventral side of the tail. Although noninvasive methods of blood pressure measurement are not entirely accurate, these methods accurately predict hypotension 80% of the time. Consequently, a low pressure obtained by such means is an indicator for treatment.

Other measurements of cardiovascular function must be assessed alongside the BP. Heart rate may or may not be useful as hypotension may exist when heart rate is within normal limits, especially during anesthesia, because baroreceptor response is blunted or abolished by anesthetic agents. Capillary refill time (CRT) should be 1 sec and prolonged refill indicates decreased cardiac output. Hypotension can exist with pale or pink mucous membranes. Pink membranes usually indicate vasodilation, and this is frequently the cause of hypotension during anesthesia with isoflurane or sevoflurane. A patient that has a MAP over 64 mm Hg, pink gums and CRT of 1 sec probably has an acceptable cardiac output. Some clinical studies have included cardiac output measurement and generated the conclusion that cardiac output can increase or decrease in response to anesthetic or surgical manipulation but at the same time mean arterial pressure is unchanged. In absence of cardiac output measurement, we must rely on observation of changes in mucous membrane color and CRT with BP measurement to provide sufficient information to assess changes in cardiovascular function. Interpretation of membrane color after medetomidine administration is complicated by vasoconstriction.