Am. J. Respir. Crit. Care Med., Vol 152, No. 1, Jul 1995, 148-152.
Arterial oxygenation time after an FIO2 increase in mechanically ventilated patients
SA Sasse, MB Jaffe, PA Chen, KG Voelker and CK Mahutte
Long Beach VA Medical Center, California 90822, USA.
The time for arterial PO2 to reach equilibrium after a 0.2 increase in the
fraction of inspired oxygen (FIO2) was studied, using arterial blood gases
measured at 1, 2, 3, 4, 5, 7, 9, and 11 min in 30 stable, mechanically
ventilated medical intensive care unit (ICU) patients. Eight patients also
underwent a 0.4 increase in FIO2. Each patient's rise in PO2 over time
[PO2(t)] was fit to the following exponential equation: PO2(t) = PO2i +
(PO2f-PO2i) (1-e-kt), where t refers to time, PO2i and PO2f refer to the
initial and final equilibrated PO2. The time constant k and PO2f were
determined by a nonlinear curve fitting technique. The 90% oxygenation
times (t90%), defined as the time required to reach 90% of the final
equilibrated PO2, were calculated. The mean t90% (+/- SD) was 6.0 (+/- 3.4)
min for all patients (range 1.7 to 14.3 min); 7.1 +/- 2.1 min for 18
patients with chronic obstructive pulmonary disease (COPD) and 4.4 +/- 2.0
min for 12 patients without COPD (p < 0.05). In the subgroup of patients
undergoing both an FIO2 increase of 0.2 and 0.4, there was no significant
difference in the mean t90%'s for the two FIO2 changes (7.7 versus 7.7
min). We conclude that after a 0.2 or 0.4 increase of FIO3, a 15-min
equilibration time period is adequate for 90% of the increase in PO2 to
occur, in stable, mechanically ventilated medical ICU patients.
