Effect of positive end-expiratory pressure on intrapulmonary shunt at different levels of fractional inspired oxygen.

In 10 patients undergoing ventilation, venous admixture was measured at different values of positive end-expiratory pressure (PEEP). The measurements were performed at the level of fractional inspired oxygen (FIO2) at which each patient was ventilated, and at FIO2=1. In patients ventilated at FIO2 between 0.21 and 0.3 venous admixture was not modified by PEEP, while in patients ventilated with FIO2 between 0.4 and 0.6, venous admixture decreased significantly (p less than 0.01). With FIO2=1, increased PEEP produced a reduction in venous admixture in all cases (p less than 0.05). These observations suggest that in patients similar to ours, PEEP does not reduce venous admixture at low levels of FIO2 (0.21--0.3), and the observed reduction with PEEP at FIO2=1 may be misinterpreted.