Effects of Systemic Deep Hypothermia and Subarachnoid Block on the Longitudinally Conducting Evoked Spinal Cord Potentials in Man

The present study reports the effects of systemic deep hypothermia (SDH) and subarachnoid block (SAB) on the longitudinally conducting evoked spinal cord potential (conducting ESCP) in man. Before induction of anesthesia, a pair of bipolar catheter electrodes was introduced to the epidural space: one at the level of the cervical enlargement and the other at the lumbosacral enlargement. The conducting ESCP was produced by electrical stimulation through the upper electrode and recorded through the lower electrode, and vice versa. SDH Study: Subjects were 6 patients who underwent replacement surgery of an aortic aneurysm with deep hypothermia anesthesia. The peak latency of the ESCP was gradually prolonged and the duration was widened with cooling via extracorporeal circulation. The amplitude of ESCP showed a biphasic change over the course of cooling with a turning point of around 30°C in esophageal temperature. The ESCP was well observed until blood temperatures as low as near 10°C. The result shows that ESCP is available as an intra-operative monitoring parameter of the spinal function even under SDH. SAB Study: Subjects were 7 patients, 6 of whom had SAB and the remaining 1 intravenous application of a local anesthetic. The conducting ESCP was markedly depressed or disappeared completely even after SAB with clinical doses of various local anesthetics, while it was hardly affected by the intravenous application. The result implies that SAB causes, at least partially, the conduction block within the spinal cord.