The H-reflex as a Measure of Anesthetic Potency in Man

The effects of different concentrations of halothane with oxygen and with 70 per cent nitrous oxide on the amplitude of the H-reflex (a spinal monosynaptic reflex) were studied in 12 volunteers. In six subjects on mechanical ventilation the mean amplitude of the H-reflex was 21.5 per cent of the awake response at 0.3 per cent halothane–N2O (MAC); 20 per cent at 0.8 per cent halothane–O2 (MAC); 7.5 per cent at 0.8 per cent halothane–N2O. In six subjects breathing spontaneously at a PaCO2 of 46.7 torr, the amplitude of the H-reflex was 23.5 per cent at 0.3 per cent halothane–N2O; 27.3 per cent at 0.8 per cent halothane–O2; 14.2 per cent at 1.5 per cent halothane–O2; 11.6 per cent at 0.8 per cent halothane–N2O; 4.25 per cent at 1.5 per cent halothane–N2O. Halothane 0.3 per cent with 70 per cent N2O and halothane 0.8 per cent with oxygen were equipotent in their effects on both the H-reflex and the response to noxious stimulation. Increase in PaCO2 depressed the H-reflex. Measurement of the H-reflex may provide a means of correlating central nervous system depression with anesthetic dose.