Performance of a low flow O2 Venturi mask: diluting effects of the breathing pattern.

To examine the performance of a new Venturi Mask (VM), PaO2 and PaCO2 were measured in 10 healthy volunteers before and while, breathing from a 0.24 O2 VM at 21/min of O2 flow for 20 min. FEO2, FECO2, VT, f, inspiratory (TI) and expiratory time (TE) and flow were also recorded during air breathing. Mean PaO2 increment observed during VM breathing was 1.8 kPa (SD 0.7). The FO2 measured at the face-piece of the VM while not in use was 0.247. The predicted increment of PaO2 corresponding to a FIO2 of 0.24 was calculated by the alveolar air equation and found to be 2.9 kPa (SD 0.2), indicating a diluting effect on the O2 concentration close to 40%, with an actual FIO2 of 0.228. The correlation coefficient (r) between dilution and a formula which includes initial acceleration of inspiration (VI 0.25), VT and the ratio TI/TT, was 0.94. Because the low O2 flow 0.24 VM model is markedly influenced by the patient's breathing, we conclude that it does not satisfactorily provide a stable and predictable concentration of supplementary oxygen. Theoretical considerations allow these results to be extended to other VM using low O2 flows for different O2 concentrations.