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Determinants of F_i,O2 with oxygen supplementation during noninvasive two-level positive pressure ventilation

¹ Emergency Dept and ² Pneumology Dept, Cliniques Universitaires Saint-Luc, Université Catholique de Louvain, Brussels, Belgium

CORRESPONDENCE: F. Thys, Service des Urgences, Cliniques Universitaires Saint-Luc, Université Catholique de Louvain, Avenue Hippocrate 10, B-1200, Bruxelles, Belgium. Fax: 32 27641620. E-mail: Thys@rean.ucl.ac.be

Keywords: inspiratory oxygen fraction, noninvasive positive pressure ventilation, oxygen

Received: July 24, 2001
Accepted September 10, 2001

To maintain arterial oxygen saturation (S_a,O2) above 90% in patients with acute respiratory failure, oxygen (O₂) is often added to the circuit of two-level noninvasive positive pressure ventilation (NPPV). However, the final inspiratory oxygen fraction (F_i,O2) 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 O₂, on F_i,O2 was assessed. The effects of the tidal volume (V_T) and the respiratory rate on the F_i,O2 were then clarified in a model study.

The F_i,O2 varied depending on the point where O₂ 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,O2. Increases in IPAP led to decreases in F_i,O2. Finally, F_i,O2 increased with O₂ flow, although it was difficult to obtain an F_i,O2 >0.30 unless very high O₂ flows were used. Paradoxically, NPPV with low IPAP values and without O₂ supplementation led to a F_i,O2 <0.21 at the circuit-patient interface. V_T and respiratory rate did not appear to influence F_i,O2.

To conclude, when using noninvasive positive pressure ventilation with two-level respirators, oxygen should be added close to the exhaust port (ventilator side) of the circuit. If inspiratory airway pressure levels are >12 cmH₂O, oxygen flows should be at least 4 L·min^–1.

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