Effects of abdominal surgery on somatosensory evoked potentials during nitrous oxide-enflurane anesthesia

The effect of abdominal surgery on median nerve somatosensory evoked potentials (SEPs) was studied in 8 enflurane and nitrous oxide anesthesia (GOE) patients. We further compared the effect of epidural anesthesia. The first recording was done immediately prior to induction. Anesthesia was then induced with 5 mg.kg(-1) i.v. of thiopental and maintained with 1.0% enflurane, 66% N(2)O and 33% O(2). Before skin incision for abdominal surgery, the second recording was performed under GOE anesthesia and the third recording during surgery. Then 2% lidocaine was injected into the epidural space through a preinserted catheter, and after 15 min the fourth recording was obtained. The latencies of peaks N1, P2 and N2 and the amplitudes of N1-P2 and P2-N2 were measured. The latencies of N1, P2 and N2 increased and the amplitudes of N1-P2 and P2-N2 deceased significantly after the induction of anesthesia compared with the control values. During abdominal surgery the latencies of N1 and P2 decreased and the amplitudes of N1-P2 and P2-N2 increased. After epidural anesthesia, however, the latencies of N1 and P2 increased and the amplitudes of N1-P2 and P2-N2 decreased significantly and returned almost to the values recorded under preoperative GOE anesthesia. These phenomena indicated that the excitations produced by surgical stimulation in nerve ending might have been transmitted to the central nervous system via spinal nerves and blocked by epidural anesthesia.