Oxygen delivery-consumption relationship in septic adult respiratory distress syndrome patients: The effects of positive end-expiratory pressure

The oxygen consumption/ delivery relationship (VO2/602) was studied in 18 sedated, paralyzed, septic adult respiratory distress syndrome patients. Different levels (0 to 15 cm H2O) of positive end-expiratory pressure (PEEP) were applied. DO₂ was calculated from cardiac index (thermodilution technique) and arterial oxygen content measurements. VO₂ was calculated using Fick's equation. Regression lines were obtained for each patient. The following results were obtained. First, patients with DO₂ at zero end-expiratory pressure ≤ 640 mL/min/m² showed a highly significant relationship between changes in DO₂ and VO₂ with PEEP (supply dependency). In all these patients PEEP decreased DO₂ by reducing cardiac index without significant changes in arterial oxygen saturation. All these patients developed multiple organ system failure and died. Second, changes in DO₂ and VO₂ with PEEP were not correlated in patients with a DO₂ on zero end-expiratory pressure ≥ 686 mL/min/m² (nonsupply dependency). As PEEP was applied, changes in DO₂ were compensated by changes in oxygen extraction ratio such as to keep VO₂ constant. On average, DO₂ decreased with PEEP, while oxygen extraction ratio and arterial-mixed venous oxygen difference increased as PEEP was applied. Only three of these patients developed multiple organ system failure and died (70% survivors).