The Science Journal of the American Association for Respiratory Care

2003 OPEN FORUM Abstracts

OXYGEN CONCENTRATION DURING BiPAP, EFFECTS OF OXYGEN FLOW AND SYSTEM LEAK.

Eriko Miyoshi MD, Y Fujino, A Uchiyama, T Mashimo, M Nishimura. Intensive Care Unit, Osaka University Hospital, JAPAN.

To increase FIO2 (inspiratory oxygen concentration) during non-invasive positive pressure ventilation (NPPV) with older generation NPPV ventilators, supplemental oxygen administration is required. We evaluated the effect of mask leakage on the actual FIO2 compared to that calculated with a formularef commonly used to estimate FIO2.

Methods:
A BiPAP S/T-D ventilator (Respironics) was connected to a TTL model lung (Michigan Instruments) with compliance of 50mL/cmH2O and resistance of 5cmH2O/L/sec. Ventilator settings were T mode, IPAP 15cmH2O, EPAP 5cmH2O, rate 20/min and %IPAP 40%. Holes of different size were made at the airway opening of the test lung to simulate leakage: minor, 1.1L/min; medium, 11.3L/min; major, 34.5L/min at an airway pressure of 5cmH2O. Supplemental oxygen was administered at 3, 6, 9, 12 and 15 L/min into the ventilator circuit near the ventilator itself. Oxygen concentration at the airway opening of the model lung was evaluated by mass spectrometry. For later analysis, data was sampled and recorded on a personal computer at a rate of 100 Hz. The measured FIO2 was compared with FIO2 predicted by the formula: 21%+3%×supplemental oxygen flow (L/min).

RESULTS:
FIO2 varied greatly. The table shows the minimum and maximum FIO2 and the mean FIO2 during the inspiratory phase at each setting.

O2 flow(L/min)® 3 6 9 12 15
leakage¯ maximum/minimum FIO2(%)
mean FIO2 of inspiratory phase(%)
none 39.9/23.8 58.5/28.2 78.8/34.8 90.3/41.7 97.7/50.0
29.2 38.6 50.5 59.8 70.5
minor 38.8/23.4 56.9/27.3 76.7/33.1 89.0/38.7 95.8/45.3
28.5 37.3 48.3 57.4 65.6
medium 34.0/22.6 47.1/25.5 60.0/29.2 70.3/32.4 80.4/36.2
26.1 32.6 40.0 46.1 53.0
major 28.9/21.7 35.0/24.0 45.5/26.6 53.2/28.9 59.7/31.4
23.0 26.2 30.1 33.3 37.5
predicted FIO2 30 39 48 57 66



Conclusions:
Even without leak FIO2 varied greatly at each liter flow of oxygen. The greater the leak, the lower the measured FIO2. With a major leak, FIO2 fell far below predicted values.

Reference: Crit Care Med 1997; 25: 620-628

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