Variability of arterial blood gas values in stable patients in the ICU

To establish guidelines for the interpretation of changes in arterial blood gas (ABG) values, we studied 29 clinically stable ICU patients for spontaneous variability in PaO₂, PaCO₂ and pH. ABGs were sampled six times over a 50-minute period, during which all patients received a fixed FI(O₂) of 0.5 via endotracheal tube and underwent no therapeutic interventions. Each sample was analyzed in duplicate with careful attention to method of collection and measurement. The range separating the lowest and highest PaO₂ varied from 1 to 45 mm Hg (16.2±10.9 mm Hg [mean ± SD]). For PaCO₂ this range was from 1 to 8 mm Hg (3.0±1.9 mm Hg). Coefficient of variation for PaO₂ and PaCO₂ averaged 5.1±3.2 percent (mean ± SD) and 3.0±1.5 percent respectively. pH varied within 0.03±0.02 units. Percentage change in PaO₂ between sequential intrapatient samples averaged 5.3±2.8 percent (mean ± SD) and 7.1±7.9 percent over ten- and 50-minute intervals, respectively. Various clinical features were analyzed by multiple regression analysis for their relation to PaO₂ variation. Only leukocyte count and mean arterial oxygen content were statistically significant associations (p <0.05), but together explained less than 35 percent of the variation observed. Because considerable spontaneous variation occurs, even in stable patients, clinicians should base therapeutic decisions on trends in PaO₂ values rather than on isolated changes interpreted without appropriate clinical correlation.