Background: We examined whether minimum alveolar anesthetic concentration needed to block adrenergic response (MAC-BAR) of sevoflurane with nitrous oxide (N2O) varies depending on body surface sites to which noxious stimuli are applied.
Methods: Seventy-seven ASA I adult patients, aged 18-50 years old, were anesthetized with sevoflurane and 66% N2O in O2, and their tracheas were intubated. The anesthesia was maintained with 66% N2O in O2 plus sevoflurane at predetermined end-tidal concentrations (0.8, 1.1, 1.4, 1.7, 2.0, 2.3, or 2.6%, n = 11 in each concentration) for at least 15 minutes. Heart rate (HR) and non-invasive blood pressure (BP) was recorded at 1-minute interval automatically. As a noxious stimulus, electrical tetanic stimulation with a 15 sec burst of 50 Hz, 0.25 msec square-wave, 55 mA electric current was applied at three different sites; forehead, abdomen, or thigh. A positive cardiovascular response was defined as an increase of either mean BP or HR by more than 15% from the prestimulation value. Logistic regression analysis was used to determine MAC-BAR.
Results: MAC-BAR of sevoflurane with 66% N2O obtained by stimulating forehead, abdomen, and thigh were 2.01% (95% CI: 1.70-2.57%), 1.71% (1.13-2.74%), and 1.31% (0.77-1.66%), respectively. MAC-BAR on the forehead was significantly higher than that on the thigh.
Conclusion: MAC-BAR of sevoflurane with 66% N2O varied depending on the body surface sites to which noxious stimuli were applied. These findings support our clinical impression that sensitivities to pain vary among body surface sites, and that anesthetic requirement to stabilize hemodynamic variables vary among surgical sites.