Relationship between O2 Delivery and O2 Consumption in the Adult Respiratory Distress Syndrome

We evaluated the relationship between O2 delivery and oxygen consumption (VO2) in ten patients with the adult respiratory distress syndrome (ARDS) over the course of their illness (mean 7 +/- 5 days) while they were receiving positive mechanical ventilation with varying levels of positive end-expiratory pressure (PEEP). Mean values and standard deviations for O2 delivery and VO2 were 15.2 +/- 4.8 ml/min/kg and 4.1 +/- 1.2 ml/min/kg, respectively. In our ten patients, below an O2 delivery of 21 ml/min/kg, VO2 was linearly related to O2 delivery (VO2 = 0.32 X O2 delivery--0.53, n = 83, r = 0.76). However, at an O2 delivery rate higher than 21 ml/min/kg, there was no correlation between VO2 and O2 delivery. Mixed venous O2 content (CvO2) and arteriovenous oxygen content difference (C(a-v)O2) did not correlate with O2 delivery, nor was there a significant correlation between cardiac output (Qt) and mixed venous O2 tension (PvO2) or saturation (SvO2). We conclude that in patients with ARDS on mechanical ventilation with PEEP, VO2 and O2 delivery are linearly related except at high levels of O2 delivery. PvO2 and (C(a-v)O2) do not correlate with O2 delivery and are not sensitive indicators of tissue oxygenation in ARDS.