Propofol impairs the central but not the peripheral part of the motor system

Propofol provides some degree of muscle relaxation. Previous studies have investigated the effects of propofol on either the central or peripheral parts of the motor system. In this study, we simultaneously assessed both central (spinal) and peripheral effects. In 15 patients, general anesthesia was induced and maintained with fentanyl and midazolam. Neuromuscular blocking drugs were not administered. To investigate the central portion of the motor system, we monitored spinal F waves, an electrophysiologic variable of alpha-motoneuron excitability. Direct electrophysiologic muscle responses (M waves) and mechanomyography were studied to detect the peripheral effects of propofol on neuromuscular transmission or muscle contraction strength. After baseline recordings, 3 IV boluses of propofol (2 times 1 mg/kg followed by 2 mg/kg) were administered at 5-min intervals. Mean F-wave amplitudes were significantly reduced compared with baseline measurements (mean +/- SD, 0.22 +/- 0.13 mV) after the first (0.13 +/- 0.08 mV; P < 0.05), second (0.08 +/- 0.09 mV; P < 0.05), and third (0.03 +/- 0.04 mV; P < 0.01) propofol injections. M-wave amplitudes and mechanomyography signals remained unchanged. Our data suggest that the central part, but not the peripheral part, of the motor system is impaired after bolus administration of propofol.