Effects of Isoflurane, Midazolam, and Etomidate on Cardiovascular Responses to Stimulation of Central Nervous System Pressor Sites in Chronically Instrumented Cats

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Abstract
The systemic hemodynamic actions of isoflurane (a volatile anesthetic) and etomidate and midazolam (intravenous anesthetics) have been well documented. However, few studies have investigated the actions of these agents on central cardiovascular control sites. The present investigation examined the actions of these agents on the responses of systolic arterial pressure (SAP), heart rate, infrarenal aortic blood flow, and lower body vascular resistance to central nervous system pressor site stimulation in chronically instrumented cats. Male and female cats (n = 23) were chronically instrumented with bipolar stimulating electrodes in the regions of the ventrolateral hypothalamus (anterior, 10.0 mm; lateral, 2.5 mm; depth, -4.0 mm) and mesencephalic reticular formation (anterior, 2.0 mm; lateral, 2.0 mm; depth, -1.0 mm). Control experiments consisted of stimulation sequences at 1x, 2x, and 4x threshold current levels to elicit pressor responses. Stimulation of the hypothalamic site produced current-dependent increases in SAP (6-85 mm Hg), in heart rate (3-56 beats/min), and in infrarenal aortic blood flow (0-85 mL/min). Reticular formation site stimulation produced graded increases in SAP (6-129 mm Hg) only. Isoflurane (1.5%, 2.5%, and 3.0%), etomidate (3.0-mg/kg bolus and 0.4-mg.kg-1. h-1 infusion), and midazolam (7.5-mg/kg bolus and 0.2-mg.kg-1.h-1 infusion) were then administered in separate experimental groups. After a steady hemodynamic state was established with each agent, stimulation sequences were repeated. Isoflurane produced an attenuation of the responses of SAP (from 85.1 +/- 8.2 to 17.8 +/- 6.1 mm Hg at 1.5%, to 7.2 +/- 2.0 mm Hg at 2.5%, and to 4.7 +/- 2.0 mm Hg at 3%, all P less than 0.05), heart rate (from 41.1 +/- 13.0 to 12.5 +/- 2.7 beats/min at 2.5% and to 6.2 +/- 1.7 beats/min at 3%, all P less than 0.05), and of the infrarenal aortic blood flow (from 72.6 +/- 14.3 to 11.8 +/- 4.2 mL/min at 1.5%, to 10.2 +/- 5.6 mL/min at 2.5%, and to 3.2 +/- 1.5 mL/min at 3%, all P less than 0.05) to the highest level of hypothalamic site stimulation. Isoflurane similarly produced an attenuation of the SAP response (from 128.7 +/- 10.3 to 15.4 +/- 8.1 mm Hg at 1.5%, to 0.2 +/- 1.1 mm Hg at 2.5%, and to 0.3 +/- 0.5 mm Hg at 3.0%, all P less than 0.05) to the highest level of reticular formation site stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

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