Aspiration pneumonia: beneficial and harmful effects of positive end-expiratory pressure.

With an ex vivo, isolated, ventilated, perfused canine pulmonary lobe, the effects of various levels of positive end-expiratory pressure (PEEP) were evaluated following acid injury. Following intrabronchial instillation of hydrochloric acid, eight lobes were ventilated with 5 cm of H2O of PEEP, 12 lobes with 10 cm of PEEP, and eight lobes with 15 cm of PEEP during a 4 hour perfusion period. Blood flow was kept constant in all preparations. Lobes with 5 cm of PEEP developed a 39% intrapulmonary shunt and increased their weight by 220%. When PEEP was increased to 10 cm, weight gain was similar (184%), but shunting decreased markedly, to 7%. When PEEP was increased further to 15 cm, shunting remained low (13%), but weight gain increased markedly, to 411% of the initial lobe weight. This study demonstrates the beneficial effects of PEEP in aspiration pneumonia, but it also points out that increasing levels of PEEP can magnify acid-pulmonary injury by causing a further increase in interstitial and intralveolar edema.