Arterial oxygenation time after an FIO2 increase in mechanically ventilated patients

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.