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

Abstract

The ventilatory responses to isocapnic hypoxia and hypercapnia were studied in six dogs each with a tracheostomy, awake and during 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 the value of A, the greater the hypoxic response. In the anaesthetized dogs HVR (A) was reduced significantly from the awake value of 2010±172 (mean+SEM) to 630±173 by 1 MAC halothane, 495± 105 by 1 MAC enflurane and 952±157 by 1 MAC isoflurane (PVE/PAco₂ response curve, was reduced significantly from 9.75 litre min⁻¹ kPa⁻¹ ± 2.4 in awake dogs to 0.83 ± 0.56 after 1 MAC halothane, 0.68 ±0.53 after 1 MAC enflurane and 1.58 ±0.75 after 1 MAC isoflurane. In addition, hypercapnia-induced augmentation of the hypoxic drive was abolished by 1 MAC halothane or enflurane and diminished markedly by 1 MAC isoflurane. It may be clinically significant that hypoxia and hypercapnia during anaesthesia with these agents did not produce optimal stimulation of ventilation.