Effects of hypothermia on short latency somatosensory evoked potentials in humans

Short latency somatosensory evoked potentials (SSEPs) elicited by median nerve stimulation were monitored in 14 adult patients undergoing cardiac surgery under cardiopulmonary bypass and induced hypothermia. SSEPs were recorded at 1-2 degrees C steps as the body temperature was lowered from 37 degrees C to 20 degrees C to determine temperature-dependent changes. Hypothermia produced increased latencies of the peaks of N10, P14 and N19 components, the prolongation was more severe for the later components so that N10-P14 and P14-N19 interpeak latencies were also prolonged. The temperature-latency relationship had a linear correlation. The magnitude of latency prolongation (msec) with 1 degree C decline in temperature was 0.61, 1.15, 1.56 for N10, P14 and N19 components, respectively, and 0.39 and 0.68 for interpeak latencies N10-P14 and P14-N19, respectively. The rise time and duration of the 3 SSEP components increased progressively with cooling. Cortically generated component, N19, was consistently recordable at a temperature above 26 degrees C, usually disappearing between 20 degrees C and 25 degrees C. On the other hand, more peripherally generated components, N10 and P14, were more resistant to the effect of hypothermia; P14 was always elicitable at 21 degrees C or above, whereas N10 persisted even below 20 degrees C. The amplitude of SSEP components had a poor correlation with temperature; there was a slight tendency for N10 and P14 to increase and for N19 to decrease with declining temperature. Because incidental hypothermia is common in comatose and anesthetized patients, temperature-related changes must be taken into consideration during SSEP monitoring under these circumstances.