The hypothesis that the changes in the respiratory system pressure- volume (PV) curve during pulmonary edema mainly reflect distal airway obstruction was investigated in rats. Normal rats had a well-defined upper inflection point (UIP) at low airway pressure. Airway occlusion by liquid instillation decreased compliance (Crs) and the volume (Vuip) of the UIP, and increased end-inspiratory pressure. The same changes were observed during the progression of edema produced by high volume ventilation (HV). Changes in Vuip and in Crs produced by HV were correlated with edema severity in normal rats or rats with lungs preinjured with alpha-naphthylthiourea. Vuip and Crs changes were proportional, reflecting compression of the PV curve on the volume axis and suggesting reduction of the amount of ventilatable lung at low airway pressure. In keeping with this explanation, the lower Vuip and Crs were before HV, the more severe HV-induced edema was in alpha-naphthylthiourea-injected rats. When edema was profuse, PV curves displayed a marked lower inflection point (LIP), the UIP at low pressure disappeared but another was seen at high volume above the LIP, and the correlation between Vuip changes and edema severity was lost. These observations may have clinical relevance in the context of the "open lung" strategy.
Keywords: ventilator-induced lung injury; respiratory mechanics; acute respiratory distress syndrome