Fixed performance oxygen masks operate by supplying mixtures of oxygen and air at rates exceeding the inspiratory flow rate of the patient. In this study the oxygen concentration delivered by three fixed performance oxygen masks was determined non-invasively at various inspiratory flow rates. At low inspiratory flow rates all the masks studied acted as fixed performance devices. When the peak inspiratory rate increased the performance of all the masks showed some variability. The change from fixed to variable performance depended on the relation between inspiratory flow rate and the total gas flow delivered by the mask and was independent of the volume of the mask.Hence the use of low volume masks and high oxygen flow rates should produce more consistent results than high volume masks and lower flow rates.
Four recently introduced low-capacity3xed performance oxygen therapy masks have been compared with the established Ventimask design. Under conditions of varying peak inspiratoryflow rate the low-capacity devices all permit a variable amount of air admixture with a consequent fall in the inspired oxygen conceniration. It is concluded that low-capacity venturi masks are not truejxedperformunce devices under all circumstances. The Ventimasks satisfy their speciJications under all test conditions. A case is made for more rigorous assessment of new so-called pxed performance ' oxygen therapy devices before marketing is permitted.
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