Can galvanic skin response be used as a quantitative estimate of sympathetic nerve activity in regional anesthesia?

To study the relationship between skin sympathetic nerve activity and changes in skin resistance (galvanic skin response--GSR), efferent sympathetic and sensory nerves to the hand were blocked by an axillary nerve blockade in 15 healthy subjects. Subsequently, intraneural electrical stimuli in the median nerve distal to the axillary nerve block were used to evoke changes in skin resistance and in water vapor partial pressure in the sensory and sympathetically denervated hand. With increasing frequency of stimulation, skin resistance decreased and water vapor partial pressure increased until stimuli exceeded 10 Hz. When an additional burst of impulses was added to the background stimulation, GSR amplitude varied in a nonlinear fashion with the background frequency. Stimulation-induced GSR was completely abolished in a dose-dependent manner by systemically (intravenously) administered atropine. The results indicate that GSR depends on the preceding level of nerve traffic in the sympathetic sudomotor nerve fibers. Consequently, skin resistance recordings cannot be used to quantify sympathetic nerve traffic and thus do not express the completeness of sympathetic blockade in regional anesthesia.