The Effect of Unilateral PEEP on Gas Exchange and Pulmonary Perfusion in Canine Lobar Pneumonia

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Abstract
Positive end-expiratory pressure (PEEP) has little beneficial effect in improving gas exchange in canine left lower lobe (LLL) [Streptococcus pneumonial] pneumonia. This is true because while PEEP improves lobar gas exchange, it also increases relative perfusion (QL) to the diseased lobe. To determine if PEEP administered to only the diseased lung would avoid the increased QLLL, 6 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% O2. Measurements of arterial O2 tension (PaO2), shunt (.ovrhdot.Qs/.ovrhdot.Qt) and lobar distribution of pulmonary perfusion were made before, during and after 12 cm H2O PEEP. Changes in QLLL expressed as percent of cardiac output were determined using radiolabeled microspheres. PEEP improved PaO2 from 310 .+-. 86 to 532 .+-. 58 torr and .ovrhdot.Qs/.ovrhdot.Qt from 29 .+-. 5% to 12 .+-. 5%, returning after PEEP to 337 .+-. 84 torr and 26 .+-. 5% respectively. QLLL per cent did not increase during PEEP. Apparently unilateral PEEP improves regional gas exchange within the pneumonia lobe, probably by ventilating units which were previously perfused but not ventilated. 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.

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