Effect of positive end-expiratory pressure on lung mechanics during anaesthesia in dogs

We studied supine anaesthetized dogs to determine whether the use of positive end-expiratory pressure to increase functional residual capacity (FRC) could thereby prevent the small lung compliance values frequently reported during anaesthesia. We first made control measurements of FRC and of both static (CSTAT) and dynamic (CDYN) lung compliance. Continuous positive pressure ventilation (CPPV) with 5 cm H2O positive end-expiratory pressure was then commenced, and FRC and compliance measurements were repeated at 15-minute intervals during one hour of CPPV. We found that the positive end-expiratory pressure served to increase FRC to a level expected for unanaesthetized normally breathing dogs in the same posture (J. Appl. Physiol. 36: 93, 1974), and this FRC was well maintained during the one-hour observation period. Despite the volume preservation, we found a marked decline in lung compliance, equal in both CSTAT and CDYN; however, these changes were completely reversed to control values following hyperinflation of lung (artificial sighting). We conclude that diminution in lung compliance in our anaesthetized dogs was not dependent on low lung volume breathing but must have been due to another mechanism such as suppression of sighing or possibly regional hypoventilation. It might well be important to recognize that reversible reduction of compliance may occur despite increase of FRC in patients managed with positive end-expiratory pressure.