The effect of unilateral PEEP on gas exchange and pulmonary perfusion in canine lobar pneumonia.

Positive end-expiratory pressure (PEEP) has little beneficial effect in improving gas exchange in canine left lower lobe (LLL) pneumonia. This is true because while PEEP improves lobar gas exchange, it also increases relative perfusion (QL) to the diseased lobe. The authors hypothesized that PEEP administered to only the diseased lung would avoid the increased QLLL. Six dogs with LLL pneumonia in which PEEP was applied only to the left lung were observed. The dogs were studied supine and each lung was ventilated separately with 100 per cent O2. Measurements of arterial oxygen tension (PaO2), shunt (Qs/Qt) and lobar distribution of pulmonary perfusion were made before, during, and after 12 cm H2O PEEP. Changes in QLLL expressed as per cent of cardiac output were determined using radiolabeled microspheres. PEEP improved PaO2 from 310 +/- 86 to 532 +/- 58 torr and Qs/Qt from 29 +/- 5 per cent to 12 +/- 5 per cent, returning after PEEP to 337 +/- 84 torr and 26 +/- 5 per cent, respectively. QLLL per cent did not increase during PEEP. These results suggest that unilateral PEEP improves regional gas exchange within the pneumonia lobe, probably by ventilating units which were previously perfused but not ventilated. Further, this improvement in regional gas exchange occurred without the diversion of blood flow towards consolidated lung that occurs with whole-lung PEEP, and so resulted in a substantial net improvement in overall gas exchange.