Effects of midazolam on heart rate variability during surgery under spinal anaesthesia

Effects of midazolam on the cardiac autonomic nervous system were studied by power spectral analysis of electrocardiographic R-R intervals in patients undergoing elective lower abdominal surgery under spinal anaesthesia. Patients were randomly assigned into two groups: 10 patients in group A received spinal anaesthesia only and 10 in group S received spinal anaesthesia and midazolam of 0.05 mg/kg when surgery started. In the frequency domain power spectra, low (Lo; 0.04-0.15 Hz) and high (Hi; 0.15-0.40 Hz) frequency components were integrated to ascertain sympathetic and parasympathetic activity, respectively. There was no intergroup difference in starting time of surgery, ventilatory frequency, arterial pressure, heart rate, mean and variance of R-R interval, and cephalad level of analgesia. In spectral component, Lo decreased (P < 0.05) and Hi/Lo ratio increased (P < 0.05) relative to their baselines in group S. These were also different from group A (P < 0.05). We concluded that intravenous midazolam depressed sympathetic activity to produce a vagotonic cardiac autonomic nervous system under spinal anaesthesia.