Effects of selective parasympathetic blockade with atropine on power spectral analysis of heart rate variability during subarachnoid block

Power spectral analysis (PSA) of heart rate variability (HRV) is being used to assess autonomic function. We have examined the effect on HRV-PSA of subarachnoid block (SAB) and acute selective block of the vagus (parasympathetic) by intravenous atropine. Six patients, aged 64-83 years, received SAB. Atropine (0.6 mg) was given after 30 minutes. Using the electrocardiogram signal, PSA was performed on 512 R-R intervals using an autoregressive model (500 points), which was displayed on-line. HRV data was collected at baseline, during SAB and after atropine. Spectral densities were calculated for two frequency bands, low (0.04-0.15 Hz) and high (0.15-0.40 Hz), and comparisons made using paired t-tests. Heart rate decreased by 13 (5-20) b.p.m. following SAB and increased by 16 (10-24) b.p.m. After atropine. Low band frequencies increased by 236(128)% following SAB and then decreased to 39(48)% after atropine. Similarly, high band frequencies increased by 549(322)% and then decreased to 14(13)%. (all P<0.05). Following SAB, spectral activity increased (high band>low band). These increases appeared vagal as they were greatly reduced by atropine.