CARDIOVASCULAR-RESPONSES TO NOXIOUS STIMULI DURING ISOFLURANE ANESTHESIA ARE MINIMALLY AFFECTED BY ANESTHETIC ACTION IN THE BRAIN

We investigated the ability of isoflurane to ablate cardiovascular responses to a noxious stimulus (dew-claw clamp) in 10 goats during isoflurane delivery throughout the body and with selective delivery to either the head or torso. Whole-body minimum alveolar anesthetic concentration (MAC) and the concentration blocking autonomic responses (MAC-BAR) were determined. Next, cranial blood (jugular vein) was drained into an oxygenator and reinfused into the head (carotid artery). In six animals (Group I) isoflurane to the torso was maintained at 0.2%-0.3%, and the cranial isoflurane concentration adjusted to determined MAC-BAR; in three animals (Group II) cranial isoflurane was maintained at 0.2%-0.3%, and the end-tidal isoflurane adjusted to determine MAC-BAR. Whole-body MAC was 1.4%+/-0.3%; whole-body MAC-BAR was 3.7% +/- 0.4%, a concentration range associated with hypotension (46 +/- 8 mm Hg). During bypass, MAC-BAR was 5.6% +/- 2.4% for Group I (P < 0.05) and 2.2% +/- 0.7% for Group II (P < 0.05). Changes in blood pressure, but not heart rate, correlated with the isoflurane concentration, both during the whole-body period and bypass period for Groups I and II. We conclude that isoflurane suppresses the arterial blood pressure response to noxious stimuli, but only with concomitant hypotension, and that the brain has little influence on this response, as its MAC-BAR substantially exceeded whole-body MAC-BAR.