Effect of general anesthesia on cardiac vagal tone.

Although it is accepted that general anesthetics alter cardiac vagal tone, the magnitude of this effect was difficult to quantify by noninvasive methods until recently. Twenty-eight mongrel dogs were anesthetized using one of four representative anesthetics: pentobarbital sodium (25 mg/kg iv), morphine (1 mg/kg sc) with alpha-chloralose (50 mg/kg iv), urethan (500 mg/kg iv), thiopental sodium (25 mg/kg iv), and halothane (2% inhalation). Heart period (R-R interval) was recorded, from which the amplitude of the respiratory sinus arrhythmias (RSAs; frequency 0.24-1.04 Hz) was determined by time-series analysis. Morphine with alpha-chloralose-urethan significantly (P less than 0.01) increased RSA (control 8.3 +/- 0.6 ln ms2, anesthesia 9.4 +/- 0.3 ln ms2), whereas thiopental and halothane both significantly (P less than 0.01) decreased RSA (control 8.7 +/- 0.4 ln ms2, thiopental 1.3 +/- 0.4 ln ms2; and control 8.5 +/- 0.6 ln ms2, halothane 3.6 +/- 0.8 ln ms2). Pentobarbital failed to elicit a consistent change in RSA. These data suggest that vagal tone was maintained during morphine with alpha-chloralose-urethan anesthesia but was reduced during thiopental and halothane anesthesia.