Influence of moderate hypothermia on posterior tibial nerve somatosensory evoked potentials

Posterior tibial nerve somatosensory evoked potentials (PTN-SSEP) were recorded in eight patients during cardiac surgery with cardiopulmonary bypass and moderate hypothermia (25-28°C). There was no correlation between changes in amplitude and temperature; however, latencies of potentials recorded over the tibial nerve in the popliteal fossa, the lumbar spinal cord, and the cortex increased linearly as temperature decreased. Latency changes correlated well with nasopharyngeal temperature, but only poorly with rectal and lower limb muscle temperatures. During perioperative monitoring of spinal cord function by means of PTN-SSEP, an increase of the first positive cortical peak (P1) greater than 3 msec is considered an indication for intervention. In this study P1 prolonged 1.15 msec/°C (r = 0.89, P < 0.001). This implies that a temperature decrease of 2-3°C may prolong P1 latency by more than 3 msec.