Preoperative autonomic nervous system analysis may stratify the risk of hypotension after spinal anesthesia

Studies on pregnant women undergoing cesarean delivery or elderly men scheduled for prostate brachytherapy have demonstrated the predictive value of heart rate variability (HRV) analysis for hypotension during spinal anesthesia. We conducted a prospective observational study to investigate if preoperative HRV analysis may have a role in identifying the risk of hypotension following spinal anesthesia in otherwise healthy patients.

The study investigated 47 ASA physical status I-II patients aged between 18-50 years that underwent subarachnoid anesthesia for lower abdominal or orthopedic scheduled surgery. ECG was recorded from all subjects before the subarachnoid block. We analysed the autonomic nervous system modulation, measured by HRV analysis. The variables that were be considered were preoperative HRV total power, low frequency (LF) and high frequency (HF) heart beat oscillations and LF/HF ratio. The LF/HF ratio was dichotomized according to the median for sensitivity analysis. The lowest arterial pressure value between spinal anesthesia and the end of surgery was recorded.

The median LF/HF before anesthesia was 2.3. We considered two groups of 23 (LF/HF<2.3, group LOW) and 24 (LF/HF>2.3, group HIGH) patients respectively. Both groups had similar baseline demographic and hemodynamic variables. A high preoperative sympathetic outflow and loss of vagal modulation, as stated by LF/HF>2.3, was correlated with a relative risk of 7.7 (95%CI 1.04 to 56.6, p=0.023) of post-spinal hypotension.

Preoperative analysis of autonomic nervous system modulation might be useful to stratify the risk of post-spinal hypotension and it might indicate the need for careful monitoring or prophylactic fluids.