Skin Conductance Responses During Spinal Analgesia

Skin conductance responses (SCR, “sympatho-galvanic reflex”) were measured before and during spinal analgesia in 17 patients scheduled for transurethral surgery. Responses were provoked by standardized electrical stimulation over the clavicle opposite to the recording side; alternatively, a short deep breath, pinching, verbal stimuli or sharp sounds were used. Measuring sites (two electrodes 6 cm apart) were the hand, levels T5, T9, T12-L1 and the foot. Spinal analgesia reached a median cephalad level of T4 (mean T4, range ⋅ 3 segments) 20–25 min after injection. SCR was markedly depressed in the foot in 15 of 17 patients, at T12-L1 in 12 of 17, at T5 in 10 of 17, at T5 in 9 of 16 and in the hand in 6 of 17. Total abolition of the SCR in the foot was accomplished in only seven cases and sympathetic activity reappeared long before regression of analgesia or motor blockade was observed. In four cases of five with an analgesic level T1-T2, the SCR was preserved in the hand. No consistent correlation between blood pressure change and SCR-change was seen. The conclusion from this study is that preganglionic sympathetic B-fibres are more difficult to block than A-fibres during spinal analgesia. The duration of sympathetic blockade was far shorter than analgesia and motor blockade. Thus, sympathetic blockade during spinal analgesia seems to be far less extensive than that described in the literature.