Attenuation of the vagolytic effect of atropine during high thoracic epidural anesthesia by heart rate fluctuation analysis.

Fifteen surgical patients received an epidural injection of 12 ml of 1.5% lidocaine through a catheter placed at C7-T1, followed by further injection as required. An intravenous bolus of 0.5 mg of atropine sulfate was administered simultaneously with the initial epidural injection. The high-(HFC: 0.15-0.4 Hz) and low-frequency components (LFC: 0.05-0.15 Hz) of the power spectrum of heart rate fluctuation, and the LFC/HFC ratio were calculated. At 30 min after the initial lidocaine and atropine injection, the HFC decreased to 21% of the baseline value and the LFC decreased to 11%. At 90 min, the HFC showed gradual recovery to 69% whereas the LFC remained low (22%). These results indicate that 0.5 mg of intravenous atropine reduces the autonomic imbalance that occurs under high thoracic epidural anesthesia, but its duration is too short to be effective throughout the course of anesthesia.