Narcotic requirements for intravenous anesthesia.

MAC BAR, the minimum end-tidal alveolar anesthetic concentration that inhibits adrenergic response to a noxious stimulus in 50% of subjects, is a quantifiable physiological parameter that can be used to determine potency and therapeutic indices of volatile anesthetics. We wished to determine whether there is a minimal intraarterial plasma concentration (MIC BAR) of an opiate such as fentanyl that will prevent a hypertensive response to noxious stimuli in 50% of patients undergoing aortocoronary bypass surgery (ACBP). Forty-three patients with good left ventricular function were studied. All patients were premedicated with diazepam, morphine, and scopolamine. Five groups of patients were anesthetized with different fentanyl anesthesia protocols, each designed to produce different plasma fentanyl concentrations (PFC). A 20% increase in systolic blood pressure compared to control was considered an adrenergic response that related to the plasma fentanyl concentration inferred from each patient's PFC time-concentration curve. Only four patients became hypertensive with a PFC greater than 20 ng/ml. One patient became hypertensive at intubation with a PFC of 12.3 ng/ml. There was no statistically significant difference in the mean PFC in patients who became hypertensive at each event. During aortic dissection, when significantly more patients became hypertensive, there was no difference in the incidence of hypertension in patients with a PFC above or below 20 ng/ml. Two patients became hypertensive at skin incision with a PFC over 30 ng/ml. A MIC BAR could not be identified because of the great variability in the PFC of patients who became hypertensive.(ABSTRACT TRUNCATED AT 250 WORDS)