Effect of halothane on hypoxic and hypercapnic ventilatory responses of goats

We have measured the ventilatory responses to increased inspired carbon dioxide and to hypoxia in four goats awake and at 0.5%, 1.0% and 1.25% end-tidal halothane concentration. While maintaining PE'CO2 constant at each of three values (means 5.86, 6.45 and 7.2 kPa), PE'O2 was reduced rapidly from more than 25 kPa to 5.3-6 kPa for 3 min to record the increase in ventilation. Eleven sets of these 24 steady state points were obtained (2 PO2 x 3 PCO2 x 4 anaes. = 24). The mean isocapnic hypoxic ventilatory response (HVR) was 6.52 (SD 2.58) litre min-1 (n = 33) when awake, 5.62 (3.48) litre min-1 at 0.5% end-tidal halothane (ns), 3.05 (2.02) litre min-1 at 1% and 2.91 (2.12) litre min-1 at 1.25%, the last two being reduced significantly from awake and 0.5% halothane (P less than 0.05). With 1.25% halothane, HVR was reduced to 44.5 (18.6)% of the awake HVR. However, when HVR was expressed as % increase in ventilation produced by isocapnic hypoxia, it was 71 (19)% awake but 124 (65)% with 1.25% halothane, a significant increase with halothane (P less than 0.05). With 1.25% halothane, the carbon dioxide response slope decreased to 36.4 (26.4)% of control; hypoxia did not increase the slope significantly. Whereas previous studies in man have shown that halothane preferentially depresses hypoxic chemosensitivity and has a significant effect at 0.1 MAC, in the goat the hypoxic and carbon dioxide chemosensitivities were depressed equally. At 0.5% end-tidal concentration (about 0.5 MAC), halothane did not significantly depress hypoxic response.