Effects of sustained pressure application on compliance and blood gases inhealthy porcine lungs

Background: Short periods of sustained increase in airway pressures (Press( up)) are believed to re-open lung areas that collapsed upon induction of an aesthesia. Recruitment of alveolar surface is usually assessed in terms of changes in the pressure-volume (PV) curve. The purpose of this study was to analyse PV-curves before and after a Press(up) and to ascertain whether su ch changes are compatible with the concept of recruitment of lung volume. Methods: During ketamine anaesthesia, 12 healthy piglets were subjected to a Press(up) with end-expiratory pressure (PEEP) of 12 cmH(2)O and end-inspi ratory pressure of 40 cmH(2)O. Before and after Press(up), PV-curves were o btained from a slow insufflation of 630 ml at zero PEEP (ZEEP). Results: Compliance was non-linear both before and after Press(up) increasi ng up to 300 ml and sharply decreasing thereafter. After Press(up), the ent ire compliance curve was, shifted to a higher absolute level. Up to 100 ml and a pressure level corresponding to the lower inflection point on the PV- curve (LIP), compliance was higher before Press(up). No effects on blood ga ses could be observed. Conclusion: If the similar shape of the compliance curve corresponds to a s imilar chain of re-opening and overdistension events, this would imply that all volume gained by Press(up) is lost within 10 min, without explaining t he higher absolute compliance following Press(up). We speculate that a) re- opening of rapidly collapsing small airways determines the initial complian ce increase; b) the lower compliance after Press(up) until LIP indicates re duced intratidal re-opening of lung regions; and c) changes in bronchomotor tone induced by Press(up) raise the absolute compliance, with a similar sc enario of alveolar and small airway recruitment now taking place but at dif ferent degrees of airway stiffness.