The Validity of PaO 2 Values 3,6,and 9 Minutes after an FIO 2 Change in Mechanically Ventilated Heart-Surgery Patients

Abstract

When patients are being mechanically ventilated and a change is made in ventilator settings, it is important to know the resultant arterial blood gases and pH (ABG/ pH) as soon as possible. Commonly, it has been felt that 30 minutes may be required for a new steady state to occur, but two 1985 papers indicated that valid analysis could be made on blood drawn as early as 10 minutes after a change in FIO₂ in patients without COPD. Other evidence suggests that alveolar-to-arterial gas equilibration may be 97-98% complete as soon as 2 to 7 minutes after an FIO₂ change. I therefore performed a study to test the hypothesis that ABG/pH 3 minutes after an FIO₂ change would be as valid and useful as ABG/pH 30 minutes after the FIO₂ change. Methods: Blood for analysis of ABG/pH was drawn 3, 6, 9, and 30 minutes after an FIO₂ change on 17 occasions (68 samples) in 14 mechanically ventilated post-open heart surgery patients without pulmonary dysfunction. Descriptive statistics were calculated, and ANOVA tests were performed to determine variance between the 30-min control samples and the 3-, 6-, and 9-min experimental samples. Results: All the ABG/pH values were tightly grouped across the time intervals. There were no statistically significant differences between the 30-min control values and the 3-, 6-, and 9-min experimental values. ABG/pH at 3 min after an FIO₂ change were as valid and clinically useful to guide clinical judgments as were ABG/pH at 30 min. Conclusion: In mechanically ventilated patients without COPD or other pulmonary dysfunction, valid measurements of PaO₂, PaCO₂, and pH may be obtained from blood samples as early as 3 minutes after a change in ventilator FIO₂. (Respir Care 1987;32:1029-1034.)