Determinants of F i,O 2 with oxygen supplementation during noninvasive two-level positive pressure ventilation

To maintain arterial oxygen saturation ( S a,O 2 ) above 90% in patients with acute respiratory failure, oxygen (O 2 ) is often added to the circuit of two-level noninvasive positive pressure ventilation (NPPV). However, the final inspiratory oxygen fraction ( F i,O 2 ) is not known. To clarify this issue, the effect of different inspiratory positive airway pressures (IPAP) of the oxygen tubing connection site and the flow rate of O 2 , on F i,O 2 was assessed. The effects of the tidal volume ( V T ) and the respiratory rate on the F i,O 2 were then clarified in a model study. The F i,O 2 varied depending on the point where O 2 was added to the circuit. When all other variables were constant, the connection closest to the exhaust port (ventilator side) gave the highest F i,O 2 . Increases in IPAP led to decreases in F i,O 2 . Finally, F i,O 2 increased with O 2 flow, although it was difficult to obtain an F i,O 2 >0.30 unless very high O 2 flows were used. Paradoxically, NPPV with low IPAP values and without O 2 supplementation led to a F i,O 2 12 cmH 2 O, oxygen flows should be at least 4 L·min −1 .