BACKGROUND: Total spinal anesthesia (TSA) anesthetizes cranial nerves as well as peripherial nerves, leading to specific circulatory perturbations related to autonomic imbalance between sympathetic and parasympathetic tone. Heart rate variability (HRV) result from moment-to-moment changes in sympathetic and parasympathetic activity in response to many conditions. Using a power spectral analysis of heart rate variability, we evaluated the effect of TSA on the changes in the autonomic nervous system. METHODS: Twenty-four Sprague-Dawley rats, during halothane anesthesia, were placed in a stereotaxic head holder. Polyethylene tubing (PE-10) was passed caudally from the cisterna magna, and these rats were anesthetized by a urethane intraperitoneal injection (1.5 g/kg). Succinylcholine was infused intravenously at 1 mg/kg/min. During mechanical ventilation, ECG signals and mean arterial blood pressure were recorded for 5 min after a period of 10 min of anesthetic stabilization (baseline). Lidocaine (40 mg/kg) was administered intrathecally and then two subsequent 5-min ECG signals and mean arterial blood pressure were recorded (TSA 0 5 min, 5 10 min). A power spectral analysis of the data was computed using a short-time Fourier transform. The spectral peaks within each measurement were calculated; low frequency area (0.25 0.75 Hz), high frequency area (0.75 3.0 Hz), total frequency area (0.25 3.0 Hz). RESULTS: Mean R-R interval increases progressively during the 5 minutes after TSA but mean blood pressure decreases to the level of blood pressure of TSA within 2 minutes after TSA (p < 0.05). TSA diminished HRV within 2 minutes after a spinal injection of lidocaine (p < 0.05). CONCLUSIONS: These results suggest that total spinal anesthesia depresses both sympathetic and parasympathetic tone within 2 minutes.