DEPRESSION OF HYPOXIC VENTILATORY RESPONSE BY HALOTHANE, ENFLURANE AND ISOFLURANE IN DOGS

The ventilatory responses to isocapnic hypoxia and hypercapniawere studied in six dogs each with a tracheostomy, awake andduring anaesthesia with halothane, enflurane and isoflurane(1–2.5 MAC). Isocapnic hypoxic ventilatory response (HVR)was expressed as the parameter A, such that the greater thevalue of A, the greater the hypoxic response. In the anaesthetizeddogs HVR (A) was reduced significantly from the awake valueof 2010±172 (mean+SEM) to 630±173 by 1 MAC halothane,495± 105 by 1 MAC enflurane and 952±157 by 1 MACisoflurane (P<0.05). All three anaesthetic agents producedsignificant depression of HVR at 1 MAC, but enflurane was moredepressant than isoflurane. At 1.5 MAC all three anaestheticsproduced equal and significant depression of HVR at equianalgesicconcentrations. Further increases in anaesthetic concentrationcaused no increase in depression. Hypercapnic drive, as measuredby the slope of the VE/PA_co2 response curve, was reduced significantlyfrom 9.75 litre min^–1 kPa^–1 ± 2.4 in awakedogs to 0.83 ± 0.56 after 1 MAC halothane, 0.68 ±0.53after 1 MAC enflurane and 1.58 ±0.75 after 1 MAC isoflurane.In addition, hypercapnia-induced augmentation of the hypoxicdrive was abolished by 1 MAC halothane or enflurane and diminishedmarkedly by 1 MAC isoflurane. It may be clinically significantthat hypoxia and hypercapnia during anaesthesia with these agentsdid not produce optimal stimulation of ventilation.