Vagal and sympathetic activity during spinal analgesia

Indices of vagal and sympathetic activity were studied in 30 elderly males, to elucidate their possible roles in causing hypotension during spinal analgesia. The technique of spinal analgesia and the regimen of intravenous fluids were standardised. An index of vagal activity was derived from the degree of heart rate variation (successive RR interval change) on ECG recordings. Sympathetic activity was evaluated by changes in the skin conductance (SCR) of 15 patients. Analgesia to pinprick reached a median dermatome level of T5-6 (range T2-T10) by 15 min. Hypotension was correlated with the level of analgesia, and was more likely when spinal analgesia was higher than T5. There was no correlation between vagal activity and the degree of hypotension. The depression of skin conductance responses was not correlated with the degree of hypotension nor with vagal activity. Vagal efferent activity, measured at the heart, does not seem to play a causative role in hypotension occurring during spinal analgesia.