Spontaneous variability of arterial oxygenation in critically ill mechanically ventilated patients

Objective: To assess the magnitude of spontaneous variability of arterial oxygenation and oxygen tension-based indices over time in medical intensive care unit (ICU) patients and to study whether high positive end-expiratory pressure (PEEP) or inverse inspiratory-to-expiratory (I:E) ratio ventilation (IRV) results in a greater variability than low PEEP with conventional I:E ratio ventilation. Design: Prospective study. Setting: Medical ICU in a tertiary medical center. Participants: 23 patients requiring a pulmonary artery floating catheter for hemodynamic monitoring. Intervention: After being completely sedated, patients were randomized to receive pressure-control ventilation at setting A: high PEEP (15 cmH₂O) with conventional I:E ratio (1:2) and setting B: inverse I:E ratio (2:1) with low PEEP (5 cmH₂O) alternately and then at setting C: low PEEP (5 cmH₂O) with conventional I:E ratio (1:2). Each ventilation setting lasted 1 h. Measurements and results: The arterial and mixed venous blood samples were measured simultaneously at baseline (time 0), and at 15, 30, 45, and 60 min thereafter. The coefficient of variation (CV) of arterial oxygen tension (PaO₂) over time was 5.9% for setting A, 7.2% for setting B, and 6.9% for setting C, ANOVA showed no significant differences in CVs of PaO₂ between the three settings. Oxygen tension based indices, alveolar-arterial oxygen difference (A-aDO₂) and PaO₂/PAO₂ (alveolar oxygen tension), displayed CVs equal to that of PaO₂; the CV of A-aDO₂/PaO₂ was significantly greater than that of PaO₂. Conclusions: In critically ill medical ICU patients, despite sedation, the spontaneous variability in PaO₂ over time is substantial. A high PEEP or IRV does not contribute to the increased variation in PaO₂.