Comparison of the effects of sub-hypnotic concentrations of propofol and halothane on the acute ventilatory response to hypoxia

To compare the effects of sub-anaesthetic concentrations of propofol andhalothane on the respiratory control system, we have studied the acuteventilatory response to isocapnic hypoxia (AHVR) in 12 adults with andwithout three different concentrations of propofol and halothane. Targetdoses for propofol were 0, 0.05, 0.1 and 0.2 of the effective plasmaconcentration (EC50 = 8.1 micrograms ml-1). Target doses for halothane were0, 0.05, 0.1 and 0.2 minimum alveolar concentration (MAC = 0.77%). Thedoses achieved experimentally were 0.01, 0.06, 0.13 and 0.26 of the EC50for propofol and 0, 0.05, 0.11 and 0.20 MAC for halothane. During theexperiment subjects breathed via a mouthpiece from an end-tidal forcingsystem. End-tidal PO2 (PE'O2) was held at 13.3 kPa for 5 min, and then at6.7 kPa for 5 min. End- tidal PCO2 (PE'CO2) was held constant at 0.13-0.27kPa greater than the subject's natural level throughout. The mean valuesfor AHVR with propofol were: 12.8 (SEM 2.4) litre min-1 (0.01 EC50), 10.0(1.9) litre min-1 (0.06 EC50), 9.8 (2.3) litre min-1 (0.13 EC50) and 4.9(1.2) litre min-1 (0.26 EC50). The values for AHVR with halothane were:11.9 (2.4) litre min-1 (0 MAC), 7.8 (1.6) litre min-1 (0.05 MAC), 5.9 (1.2)litre min-1 (0.11 MAC) and 3.2 (1.6) litre min-1 (0.2 MAC). The decline inAHVR with increasing dose for both drugs was statistically significant(ANOVA, P < 0.001); there was no significant difference between the twodrugs with respect to this decline. Normoxic ventilation with propofoldeclined from 13.2 (1.6) litre min-1 (0.01 EC50) to 8.3 (0.9 litre min-1(0.26 EC50), and with halothane declined from 13.5 (2.0) litre min-1 (0MAC) to 11.8 (1.6) litre min-1 (0.2 MAC). This was significant for bothdrugs (ANOVA, P < 0.001).