Oxygenation equilibration time after alteration of inspired oxygen in critically ill patients

To determine the time required for arterial oxygen partial pressure (Pao(2)) equilibration after a change in fractional inspired oxygen (Fio(2)) in intensive care unit (ICU) patients, a prospective study in a 7-bed university ICU was performed. Forty adult patients were examined using sequential arterial blood gas measurements after a .3 alteration in Fio(2). The Pao(2) value measured at 30 minutes after a step change in Fio(2) in both periods was accepted as representative of the equilibrium value for Pao(2). The mean equilibration time was 8.26+/-5.6 minutes and 4.5+/-2.65 minutes for increases and decreases in Pao(2), respectively (P=.003). The constant k values were .44 +/- .31 minutes and .72 +/- .7 minutes for increases and decreases in Pao(2), respectively. There was no significant difference between the increase and the decrease of 90% oxygenation times in the 2 groups (P=.150 and P=.446, respectively). The study confirms that a period of less than 10 minutes is adequate for 90% of the equilibration of Pao(2) to occur after an Fio(2) change in ICU patients.