Quantification of intraoperative somatosensory evoked potential

Cortical (C) and spinal (S) somatosensory evoked potentials (SEP) were measured and quantified in 30 patients undergoing scoliosis surgery during the following periods: I, preincision; II, hypotension; III, instrumentation; IV, postinstrumentation; and V, skin closure. Paired two-tailed t tests were performed on all commonly measured SEP parameters comparing values obtained at periods I and II with each subsequent period. CSEP were obtained with Cz-Fz recording sites of the international 10-20 system while SSEP were obtained with recording electrodes at C7-Fz, following bilateral posterior tibial nerve stimulation at the ankles. From period I, CSEP P1 prolonged significantly across all periods but not from period II to subsequent periods. N1 remained stable from either period I or II until period V when latencies increased. P1-N1 amplitude decreased significantly between period I and other periods until period V when near base value was regained. Compared to period II however, P1-N1 amplitude did not differ significantly until period V when it increased beyond base. CSEP P2 and N2 latencies and amplitudes were less distinct and had high variability under our anesthetic technique which consisted of N2O-O2 and isoflurane 0.25%-0.50%, narcotics, nondepolarizing blocking agents, and induced hypotension. When attainable, they behaved in a similar pattern to the early CSEP. SSEP were obtained in 23 cases. The stability of latencies and amplitudes compared favorably with CSEP P1 and N1. Graphs of relative percent changes were developed for clinical use. It is concluded that the noninvasive monitoring technique described is practical, and that P1 and N1 CSEP and SSEP values are reliable monitoring parameters.