Effect of PEEP on gas exchange and pulmonary perfusion in canine lobar pneumonia.

We investigated the effects of positive end-expiratory pressure (PEEP) on respiratory gas exchange and pulmonary perfusion in 10 dogs with left lower lobe (LLL) pneumonia. In 6 dogs, measurements were made with the chest closed and 12 cmH2O were applied; in 4 other open-chest dogs (group O), 6 cmH2O were used. Both groups showed similar changes, and the individual results for all dogs were averaged. With the animals breathing 100% oxygen, total pulmonary shunt (Qs/Qt), arterial oxygen tension (PaO2), and the relative distribution of pulmonary blood flow were measured before, during, and after PEEP. During the control periods, QLLL when expressed as a percent cardiac output averaged 24 +/- 7%. With PEEP, QLLL increased by more than 50% to a mean value of 37 +/- 5%. Despite this increase, the respective values of Qs/Qt and PaO2 improved slightly from 28% and 288 Torr to 25% and 370 Torr. This result suggested that PEEP also improved regional gas exchange, and this finding was subsequently confirmed in Group O where the shunt fractions of the lower lobes were additionally measured. Accordingly, our results show that PEEP altered regional perfusion and shunt so that, on the mean, there was little change in total pulmonary shunt. Whether PEEP worsened or improved gas exchange in any individual experiment was dependent on the relative magnitude of the changes in each of these regional parameters.