Is transcutaneous electrical stimulation a realistic surrogate for genuine surgical stimulation during spinal anesthesia for cesarean delivery?

Several studies have investigated differential block during spinal anesthesia using transcutaneous electrical stimulation (TES) applied to patient's skin. These TES stimuli are claimed to be a surrogate for surgical stimulation, but TES has never been shown to be a realistic surrogate for a surgical stimulus during regional anesthesia. We investigated whether patients could appreciate nonpainful TES at the same time as they were undergoing painless cesarean delivery surgery. We applied a nonpainful TES (10 mA, 50 Hz, 1-s duration) to the skin, at 5 different dermatomal levels, in 20 women undergoing elective cesarean delivery during spinal anesthesia. During surgery, all the women were totally pain free but we noted that the level of block to TES was variable: in 30% of women, TES could be felt at the T10 dermatome or more caudally. The first appreciation of touch was consistently at T6 or above. The fact that a nonpainful TES stimulus could be appreciated within the dermatomes directly involved in transmitting surgical stimuli, at a time when the patients were totally pain free, suggests that TES at the tested levels is of little value as a surrogate surgical stimulus.