The effects of desflurane on the nervous system: From spinal cord to muscles

Monitoring of motor pathways via muscle contraction recording is sensitive to anesthetics, particularly volatile anesthetics. However, the specific ac tion sites of these anesthetics on the spinal cord and the peripheral nervo us system are not well known in humans. Therefore, we studied proximal and distal motor and sensory nerve conduction, neuromuscular junction transmiss ion, and spinal cord excitability (H/M amplitude ratio and F-wave amplitude and persistency) using standard neurophysiological techniques in 10 patien ts who underwent orthopedic surgery. Muscle potentials evoked by spinal cor d stimulation were recorded in five additional patients. Desflurane was int roduced to achieve end-tidal concentration of 3.7% and 7.4%, in 50% O-2/N2O and in 100% O-2. Measurements were obtained before desflurane administrati on and 20 min after obtaining a stable level of each concentration. Periphe ral nerve conduction and neuromuscular function were not significantly affe cted by desflurane. However, spinal cord excitability was significantly dec reased by desflurane administration (H/M ratio 37% +/- 9%, 12% +/- 5%, 7% /- 4% at desflurane concentration 0.0%, 3.7%, and 7.4% in 100% O-2 respecti vely). Muscle potentials evoked by spinal cord stimulation were abolished b y desflurane. These data rule out the possibility that desflurane specifica lly alters peripheral nerve conduction or synapse transmission at the neuro muscular junction. They demonstrate that desflurane acts preferentially at the level of the spinal motoneuron. Implications: We used neurophysiologica l techniques to assess the effects of desflurane on spinal cord conduction and excitability, motor and sensory peripheral nerve conduction, and neurom uscular transmission. Our data demonstrate that desflurane acts preferentia lly at the level of the spinal motoneuron, providing useful information for neurophysiological monitoring and immobilization during surgery and for mi nimum alveolar anesthetic concentration definition.