Positive end-expiratory pressure may decrease arterial oxygen tension in the presence of a collapsed lung region.

The effect of positive end-expiratory pressure (PEEP) on PaO2 during collapse of a single pulmonary lobe was investigated in seven dogs. Increasing PEEP to the ventilated region of lung increased the proportion of cardiac output flowing to the collapsed lobe and decreased PaO2 and pH. PaCO2 increased at the onset of lobar collapse but was not significantly altered by changes in PEEP. In impaired lungs, the benefits of PEEP may be offset by an increase in blood flow to the unventilated region and hence lead to a decrease in PaO2.