Canadian Journal of Anesthesia, Vol 8, 501-511, Copyright © 1961 by Canadian Anesthesiologists' Society

A Modification of Ayre's Technique

¹ Department of Anaesthesia, University of Western Ontario, London, Canada

A modification of Ayre's technique, consisting of a piece of tygon tubing 1 cm. in diameter with an expiratory opening of the same diameter, was used in 119 cases with clinically satisfactory results. Carbon dioxide accumulation in the anaesthetic system is not possible, since there is no reservoir. The limiting factors are the expiratory resistance and the production of an adequate anaesthetic concentration in the lung. These two factors have been investigated using a model lung with variable changes in respiratory rate and volume; nitrogen represented the anaesthetic agent. The resistance was found to be acceptable. The concentration of nitrogen (anaesthetic gas mixture) in the lung was calculated from the measured changes in oxygen tension. Typical curves are presented for the effect of different rates of inflow of nitrogen and the influence of changes in respiratory minute volume on the nitrogen concentration in model lung. From these a graph was constructed from which the rate of flow of anaesthetic gas can be determined for a given respiratory minute volume in order to obtain an anaesthetic concentration of 75% in the lung (or 25% dilution of the anaesthetic mixture with air). The flow rates so determined were compared with data published by Ayre. It was concluded that this modification is only slightly less efficient than Ayre's technique, but has less bulk and weight than any other system in children. The technique is particularly useful in surgery of the head and neck in small children, where the tube will occupy a minimum of space and can be readily incorporated into the surgical drapes.

Note:

Presented at the Annual Meeting, Canadian Anaesthetists' Society, May 15–18, 1961.