Anesthetic Doses Blocking Adrenergic (Stress) and Cardiovascular Responses to Incision—MAC BAR

Abstract

The reaction to stress, while vital to the conscious animal, may be detrimental to the surgical patient. To assess the stress-ablating action of different anesthetics (halothane, enflurane, morphine and spinal) and anesthetic doses, the responses in plasma norepinephrine, muscle movement, pupil diameter, heart rate and blood pressure to induction of anesthesia and incision in 170 unpremedicated healthy adults were studied. The age-adjusted dose (mean .+-. SD) of anesthesia that blocked the adrenergic response in 50% of individuals who had a skin incision (MAC [minimum alveolar concentration] BAR [blocking adrenergic response]) was 1.45 .+-. 0.08 MAC for halothane, 1.60 .+-. 0.13 MAC for enflurane, or 1.13 .+-. 0.09 mg/kg for morphine sulfate (each anesthetic was given with 60% N2O). No patient with a level of spinal anesthesia that blocked the pain of incision had an adrenergic response to incision. Increasing doses of halothane and morphine were associated with less of a cardiovascular response to incision (as measured by rate-pressure product); this was not true for enflurane. No patient with an adequate level of spinal anesthesia had a cardiovascular response to skin incision. The changes in heart rate, blood pressure, rate-pressure product and plasma norepinephrine content that occurred with induction of anesthesia tended to equalize these values between patients, regardless of anesthetic dose, and for all individual and combined anesthetics. If a patient''s heart rate while awake was below 63 beats/min, heart rate tended to rise 58% of the difference between heart rate while awake and 63 beats/min, and vice versa. The change in blood pressure with induction averaged 75% of the difference between systolic blood pressure while awake and 88 torr. The average for the change in rate-pressure product with induction was 79% of the difference between rate-pressure product while awake and 5917 torr .cntdot. beats/min. All the anesthetics tested can prevent the neuroendocrine response to skin incision at clinically attainable doses. Comparisons of neuroendocrine stress during surgery require quantitation of anesthetic dose. If adverse effects of surgery are related to the neuroendocrine stress that surgical manipulations induce, the hypothesis the less anesthetic the better may be wrong.