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 30minutes 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±5.6minutes and 4.5±2.65minutes for increases and decreases in Pao₂, respectively (P=.003). The constant k values were .44 ± .31minutes and .72 ± .7minutes 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 10minutes is adequate for 90% of the equilibration of Pao₂ to occur after an Fio₂ change in ICU patients.