Clinical Pharmacology

Abstract

The effects of anesthetic drugs, hypothermia, and ischemia on segmentally evoked spinal cord potentials (segmental SCPs) and/or the longitudinally conducting evoked spinal cord potentials (conducting SCPs) were studied in adult patients undergoing surgery under general anesthesia. The segmental SCP was recorded from the posterior epidural space at the lumbosacral or cervical spinal enlargement in response to electrical stimulation of the posterior tibial nerve at the popliteal fossa or the ulnar/median nerve at the wrist, respectively. The segmental SCP consisted of an initial positive spike wave (P1) followed by a sharp negative wave (N1) and a slow positive wave (P2). It is generally agreed that the P1 wave is related to the arrival at the spinal cord of afferent volleys conducted through the dorsal roots and the N1 wave has its origin in the activity of interneurons in the dorsal horn. The characteristics of the P2 wave closely resemble those of the positive cord dorsum potentials (P wave) in animals, as well as the negative dorsal root potential (DRP-V), which is thought to reflect primary afferent depolarization (PAD) in the spinal dorsal horn (Kano et al., 1971; Shimoji et al., 1972, 1974; Kano and Shimoji, 1974). Following tibial nerve stimulation at the popliteal fossa, the segmental SCP was observed along with the M and H waves of the evoked electromyogram (EMG) recorded simultaneously from the calf muscle. After ulnar/median nerve stimulation at the wrist, the segmental SCP was observed, along with the ulnar/median nerve action potentials (NAP) at the elbow and the somatosensory evoked potentials (SEP) from the contralateral scalp. The NAP of the cauda equina was also simultaneously recorded in some patients through an electrode introduced into the lower lumbar epidural space, when the tibial nerve was stimulated at the popliteal fossa.