Motor evoked potentials in the dog: effects of global ischemia on spinal cord and peripheral nerve signals.

Motor evoked potentials (MEPs) in cats, rats, and humans have been reported. They appear promising as a test of central nervous system function, and they are sensitive not only to mechanical injury but also to ischemia. In mechanical trauma, the peripheral nerve response is much more sensitive to damage than the cord response, with a lower threshold and an earlier disappearance. We are reporting that the MEP can also be produced in the dog and that, under conditions of cardiac arrest induced by fibrillation, the peripheral nerve response disappears first at about 30 seconds and then the spinal cord response disappears at about 10 to 13 minutes. The late disappearance of the spinal cord response raises serious questions about its role as an adequate injury monitor. The most useful warning feature of the spinal cord response is an increase in amplitude during the critical first 2 minutes of arrest. Latency changes in the cord and peripheral nerve response did not seem as useful as amplitude changes in terms of providing adequate detection of injury. We also evaluated the peripheral nerve signals to determine whether they are partially volume-conducted weak muscle responses, and evidence substantiates their nonmuscle origin.