Level of consciousness affects the excitability of spinal motor neurones during propofol sedation in humans

Background. To investigate the relationship between the depression of spinal motor neuronal excitability and the sedative level induced by propofol infusion, we simultaneously analysed the suppressive effect of propofol on the F wave and the sedative level during propofol infusion. Methods. After spinal anaesthesia, sedation was achieved using a propofol target-controlled infusion (TCI) system to achieve a score of 4 on the Wilson sedation scale. The excitability of spinal motor neurones was determined by measuring the left median nerve F wave. F-wave persistence and the F/M ratio were recorded at pre-sedation as the control, during sedation, at arousal by mild physical stimulation and at post-sedation. Results. Wilson sedation scores increased significantly corresponding to the increase in the target propofol concentration (Cpt), and a Cpt-producing Wilson sedation scale 4 ranged between 1.2 and 1.8 mu g ml(-1). The F-wave persistence and F/M ratio before propofol infusion were 80.7 (8.6)% and 9.5 (3.9)%, respectively. At Wilson sedation scale 4, F-wave persistence and F/M ratio were 17.6 (12.8)% (0-37.5%) and 4.3 (4.1)%, and, at return of consciousness by mild physical stimulation, significantly increased to 71.3 (7.9)% and 10.0 (5.0)%, respectively. Conclusion. We demonstrated that the excitability of spinal motor neurones was suppressed during sedation by propofol TCI, but this suppressive effect vanished at return of consciousness by mild physical stimulation even at a constant Cpt. Our data suggested that the effect of propofol on the excitability of spinal motor neurones might be affected by consciousness level rather than propofol Cpt in humans.