Changes in heart rate variability under propofol anesthesia: a possible explanation for propofol-induced bradycardia.

We propose to study the bradycardia associated with propofol anesthesia. Ten women undergoing laparoscopy for benign disease were studied using ambulatory electrocardiogram monitoring. Anesthesia was induced with an intravenous bolus of propofol and maintained with an infusion. After ventilation using positive pressure via a mask for 5 min, relaxants (succinylcholine or vecuronium) and opioids (alfentanil or fentanyl) were administered and the trachea was intubated. Approximately 15 min later, the laparoscopic trocar was placed and carbon dioxide insufflated. Heart rate variability spectra using a fast Fourier transformation technique were determined from the recordings in four separate time periods (preinduction, postinduction, postintubation, and posttrocar placement). Total, high-frequency, and low-frequency power in each time period was determined. Induction of anesthesia with propofol was associated with a significant reduction in total, low-frequency, and high-frequency power. Maintenance of anesthesia with propofol alone resulted in further reductions in total and low-frequency, but not high-frequency, power. Placement of the laparoscopic trocar and insufflation of carbon dioxide resulted in a decrease in heart rate and an increase in high-frequency power. We conclude that high-frequency variability reflects parasympathetic tone. Propofol anesthesia reduces parasympathetic tone to a lesser degree than sympathetic tone. This autonomic milieu predisposes the patient to developing bradycardia in response to parasympathetic stimuli.