The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

CALCULATION OF OXYMASK FIO2 BY USING A MODIFIED VERSION OF THE ALVEOLAR AIR EQUATION

Tim Hubbard1, Amanda Brawley1, Doug Pursley1, Aaron Light1



Background: The OxyMask (Southmedic Inc, Canada) is an open-designed oxygen mask that, according to the manufacturer's website, is capable producing an FIO2 of 60-90% at 15 l/m. In this study, we sought to verify that assertion by calculating the mean FIO2 produced by an OxyMask at 15 l/m using a modified version of the alveolar air equation.

Methods: We recruited 15 healthy adults with a mean age of 24 years into our study. All participants were required to be less than 35 years of age, undergo DLCO testing and have a DLCO value of 90% of predicted or greater, be a non-smoker, and have no history of lung disease. Individuals meeting these inclusionary criteria were then seated, placed on a tight-fitting OxyMask at 15 l/m, and were instructed to relax, breathe normally, and not talk for a period of 15 minutes. During the testing period, all subjects were observed to perform quiet, restful breathing. At the end of the fifteen minute period, we performed a radial artery blood gas and measured pH, PaCO2, and PaO2 using a GEM 3000 blood gas analyzer (Instrumentation Laboratories, Lexington, MA). No air bubbles were observed in any of the syringes and all samples were analyzed within five minutes. Assuming that our subjects had normal cardiopulmonary anatomy and physiology, we estimated alveolar PO2 by dividing arterial PO2 by a normal arterial to alveolar ratio of 0.9 to reflect a ten percent higher partial pressure of oxygen in the alveoli than in arterial blood. Knowing approximate alveolar PO2, we then calculated FIO2 by the following formula: FIO2 = [(arterial PO2 � 0.9) + (arterial PCO2 x 1.20)] � (PB - 47).

Results: The mean PaO2 produced by an OxyMask at 15 l/m in our study was 232 mmHg with a range of 162-291 mmHg. This resulted in a mean calculated FIO2 of 44% and a standard deviation of 6%. The calculated FIO2 ranged from 34% to 55%. During the testing period, our subjects had a mean pH of 7.44 and a mean PaCO2 of 38 mmHg. The average DLCO of the fifteen participants was 116% of predicted.

Conclusion: Young healthy persons breathing 60-90% oxygen in Springfield, Missouri (PB 730 mmHg) should have a PaO2 of between 325-510 mmHg. The range of PaO2 produced by an OxyMask at 15 l/m in our cohort was considerably less. Therefore, using our method for determining FIO2, the OxyMask produced a lower value than that stated by the manufacturer. More studies need to be performed on the OxyMask to determine FIO2 capability at various flowrates.