Reassessment of value of oxygen masks that permit rebreathing

Abstract

The relative efficiency of different types of personal equipment for the therapeutic administration of oxygen is considered. The highest efficiency is achieved by the use of a mask with a reservoir bag for the storage of oxygen during expiration. In one type of mask rebreathing into the bag is permitted; in the other it is prevented by a non-return valve situated at the neck of the bag. The effect of the design of the oxygen mask upon ventilation has been reassessed during the breathing of oxygen, using a mouthpiece assembly having volume and flow characteristics similar to those of these two types of oxygen mask. It has been found that for an oxygen flow of 3 1./min. the mean resting ventilation in six normal subjects and in 14 patients with chronic chest disease increased by 52%, from 7.5 to 11.4 and 8.6 to 13.2 1./min. respectively, when the non-return valve at the top of the oxygen reservoir bag was removed. The increase in ventilation is attributable to the rebreathing of expired carbon dioxide, which may also explain the discomfodt experienced by some patients when using this type of mask. It is suggested that masks employing the rebreathing principle should not be used for the administration of oxygen to patients liable to develop acute carbon dioxide retention, or for those with very low maximum ventilatory capacities. Where efficient usage is desirable, masks fitted with a valve at the top of the oxygen reservoir bag to prevent rebreathing are recommended. © 1956, British Medical Journal Publishing Group. All rights reserved.