Influence of isoflurane anesthesia on motor evoked potentials elicited by transcortical, brainstem, and spinal root stimulation.

Electrical stimulation over the motor cortex, base of the skull, and cervical spine motor roots was performed in 9 male rats (410 +/- 86 g) before and after induction with isoflurane at 1 MAC concentration. The mean latency and amplitude of descending spinal evoked potential (DSEP) from spinal cord and motor evoked potentials (MEPs) from forearm muscles obtained after motor cortex, brainstem, and cervical root stimulations were calculated and compared. The electrical current intensity to elicit the MEPs after cortical, brainstem, and spinal roots stimulation were 23.4 +/- 7.6, 7.0 +/- 3.1, and 1.4 +/- 0.8 mA, respectively. The brainstem stimulation activated descending motor pathways with a latency midway between that produced by electrical stimulation over the motor cortex, and by electrical stimulation over the cervical enlargements. The latency difference between cortical (8.8 +/- 3.2 msec) and brainstem (5.7 +/- 1.2 msec) stimulation was 3.1 +/- 2.3 msec in all forearm extensor muscles. The latency difference between cervical (3.6 +/- 0.9 msec) and brainstem stimulation (5.7 +/- 1.2 msec) was 2.3 +/- 1.1 msec for the same muscles, suggesting the brainstem stimulation activates the descending motor neurons at the level of cervical-medullary junction. The amplitudes were 189 +/- 141, 672 +/- 354, and 765 +/- 389 microV for cortical, brainstem, and cervical root stimulations. The inhalation anesthesia isoflurane at 1 MAC (1.2%) completely abolished the cortical and brainstem MEPs within minutes, while the MEPs elicited by direct stimulation of the cervical spinal roots remained unchanged. Our results indicate synaptic-dependent MEPs elicited at motor cortex or brainstem levels are highly sensitive to isoflurane anesthesia.