TIDAL VENTILATION AT LOW AIRWAY PRESSURES CAN AUGMENT LUNG INJURY

Intermittent positive pressure ventilation with large tidal volumes an d high peak airway pressures can result in pulmonary barotrauma. In th e present study, we examined the hypothesis that ventilation at very l ow lung volumes can also worsen lung injury by repeated opening and cl osing of airway and alveolar duct units as ventilation occurs from bel ow to above the infection point (P-inf) as determined from the inspira tory pressure-volume curve. We ventilated isolated, nonperfused, lavag ed rat lungs with physiologic tidal volumes (5 to 6 ml/kg) at differen t end-expiratory pressures (above and below P-inf) and studied the eff ect on compliance and lung injury. In the groups ventilated with posit ive end-expiratory pressure(PEEP) below P-inf, compliance fell dramati cally af ter ventilation. It did not change in either the control grou p or the group ventilated with PEEP above P-inf. Lung injury assessed morphologically was significantly greater in the groups ventilated wit h a PEEP below P-inf, and in these groups the site of injury was depen dent on the level of PEEP. The group ventilated without PEEP had signi ficantly greater respiratory and membranous injury to bronchioles, whi le the group ventilated with PEEP of 4 cm H2O had significantly greate r alveolar duct injury. In conclusion, ventilation at lung volumes bel ow those found at P-inf caused a significant decrease in lung complian ce and progression of lung injury. Therefore, in addition to high airw ay pressures, end-expiratory lung volume is an important determinant o f the degree and site of lung injury during positive-pressure ventilat ion.