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

To determine the time required for arterial oxygen partial pressure (Pao2) equilibration after a change in fractional inspired oxygen (Fio2) 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 Fio2. The Pao2 value measured at 30 minutes after a step change in Fio2 in both periods was accepted as representative of the equilibrium value for Pao2. The mean equilibration time was 8.26 ± 5.6 minutes and 4.5 ± 2.65 minutes for increases and decreases in Pao2, respectively (P = .003). The constant k values were .44 ± .31 minutes and .72 ± .7 minutes for increases and decreases in Pao2, 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 Pao2 to occur after an Fio2 change in ICU patients.