Mechanical Determinants of Inspired Oxygen Concentration in a Pressure-Cycled Ventilator1

The concentration of oxygen delivered by a Bird Mark 7 respirator with the air mixture control adjusted to full air dilution was evaluated under clinical circumstances. A mixture of carbon dioxide in air was substituted for oxygen to drive the respirator because the concentration of carbon dioxide in the inspired air could be measured by an analyzer with a short response time. One hundred and three single inspirations of 7 participants were analyzed for inspired concentration of carbon dioxide tracer gas; for peak and mean flows; and for pressure, volume, and cycling time. The concentration of delivered gas was found to be directly proportional to the pressure (P) and inversely related to the mean flow () with the following relationship: inspired concentration = 58.2757 − 0.4567 + 0. 520 P. The concentration of inspired driving gas varied from 31 per cent to 76 per cent. This is less than the inspired concentration predicted from lung—thorax analogs, probably indicating that performance of respirators nee...