[Objective quantification of spinal anesthesia by somatosensory evoked potentials after tibial nerve stimulation]

Eight patients (ASA status I) who had to undergo minor surgery of the knee received spinal anaesthesia by 80 mg hyperbaric mepivacaine. Clinical parameters of the block were compared with somatosensory evoked potentials (SEP) after tibial nerve stimulation. Depending on the degree of nervous block, the early components P40, N50 and P60 of the cortical tibial SEP underwent characteristic changes. After the injection of the local anesthetic, the latencies of the early components rapidly increased. The tibial SEP completely disappeared after 15 min (median). At this time pin prick testing revealed a complete somatosensory block in the corresponding dermatomes. With one exception, motor block was also complete. After 115 min (median) the early components could be recorded again, with latencies significantly increased. The maximum latency shifts were 22.2% (P40), 19.6% (N50), and 11.8% (P60). When the latencies recovered to less than 10% of the starting value, anaesthesia became clinically insufficient. The recovery of the afferent pathway was complete within 5 hours. Because of the good correspondence with the clinical blockade, the latency shift of P40, N50 and P60 is useful in objectively evaluating a spinal anesthesia with mepivacaine. The use of the method is limited by increased equipment and staffing requirements; however, it facilitates the development of optimised dosing strategies.