Independently measured oxygen consumption during reduction of oxygen delivery by positive end-expiratory pressure

Oxygen consumption (VO2) is often assumed to be fixed by metabolic demand, but some investigators have found VO2 linearly dependent on O2 delivery (QT-CaO2) during positive end-expiratory pressure (PEEP) trials for the adult respiratory distress syndrome. However, in those studies VO2 was calculated using the same cardiac output (QT) and O2 content measurements used to calculate the O2 delivery to which it was being compared. We therefore obtained independent measurements of VO2, QT, and oxygen contents under 3 conditions in 21 dogs: 14 normal dogs, each receiving varying degrees of PEEP or central venous obstruction, and 7 dogs receiving PEEP after developing pulmonary edema from intravenously infused oleic acid. In all groups, VO2 measured from expired gas analysis remained unchanged until QT-CaO2 was reduced to below 13 ml/kg/min. The VO2 was then linearly dependent on QT-CaO2. We conclude that this reduction in VO2 is due to severe QT-CaO2 reduction and not to any special effect of PEEP or lung injury.