Intraneural recording of muscle sympathetic activity during epidural anesthesia in humans.

The extent and magnitude of sympathetic blockade during epidural anesthesia have previously been assessed only by indirect methods. In this study, direct intraneural recordings of muscle sympathetic activity (MSA) in the peroneal nerve was performed to determine the profundity of blockade of sympathetic fibers to the lower extremities during epidural anesthesia. Lumbar epidural catheters were inserted in nine volunteers. Multiunit postganglionic sympathetic activity was recorded in a muscle fascicle of the peroneal nerve before and after epidural injection of 4 mL of 2% mepivacaine followed by an additional 12-16 mL after 5 min. Apnea (30-60 s) was used to elicit transient sympathetic activation. The upper level of sensory blockade was T-3 to T-10. Muscle sympathetic activity decreased after epidural blockade with no spontaneous or apnea-induced sympathetic bursts observed later than 11 min after injection of the initial test dose. Sympathetic blockade was accompanied by increase in foot skin blood flow as well as loss of skin resistance responses to arousal. Results show that epidural anesthesia with sensory blockade above T-10 to T-11 blocks spontaneous peroneal MSA as well as the marked sympathetic activation induced by apnea.