Effect of low dose inhaled anaesthetic agents on the ventilatory response to carbon dioxide in humans: a quantitative review.
This paper reviews published studies (identified by a Medline-assisted search) on the effect of <or= 0.2 minimum alveolar concentration inhaled anaesthetic agents on the hypercapnic ventilatory response in healthy subjects. Each article was examined for the anaesthetic agent used, whether CO(2) was administered using a rebreathing or a steady state (step hypercapnia) technique, and subject arousal (by audiovisual stimulation or by pain). Analysis of variance was used to assess the influence of each of these factors on the standardised hypercapnic response (the hypercapnic ventilatory response in l.min(-1) x kPa(-1) in the presence of anaesthetic, expressed as a fraction of the response without anaesthetic). There were 21 separate studies embedded within 14 published articles. When considered together, the effect of anaesthetics on the hypercapnic ventilatory response was not significant (p = 0.089). Furthermore, when each agent was considered separately, no agent alone had a significant effect on the hypercapnic response. Neither subject arousal (p = 0.396) nor the method used to induce hypercapnia (p = 0.625) had any significant influence on the response. This suggests that the hypercapnic response is more resistant to the effects of anaesthetics than the hypoxic ventilatory response. These results indicate areas for future work. It would seem important to identify the cellular mechanisms which might underlie the difference in hypercapnic and hypoxic responses, and possible ways in which subject arousal might interact in the brain with the chemoreflexes.