Six commercially available devices were tested to determine the alveolar oxygen concentration that is achieved and the volume of rebreathing that can occur in conditions simulating clinical use. All were tested at zero oxygen flow, since failure and interruption of the oxygen supply to such devices can occur in clinical use. In patients whose cardiorespiratory reserve is exhausted, any increase in dead space will be detrimental; for such patients devices should be selected which cannot cause rebreathing under any circumstances. Three of the devices tested could cause appreciable rebreathing, i.e., Oxyaire (B.L.B. pattern), Pneumask, and the M.C. mask. Claims made for the Pneumask and the M.C. mask were not substantiated in this study. With administration by nasal catheter changes in alveolar oxygen concentrations were found to be dependent on the percentage of mouth breathing by the subject. The Ventimask produced a consistent elevation of the alveolar oxygen percentage and did not cause any increase in the dead space.
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