To examine the effects of cardiac surgery and cardiopulmonary bypass (CPB) on the lung, functional residual capacity (FRC) and lung-thorax compliance were measured at four stages during open heart surgery in 15 children. The patients were anesthetized with fentanyl/droperidol and N2O/O2, paralyzed, and ventilated with volume-controlled mechanical ventilation at 20-30 breaths/min. FRC was measured by tracer gas washout. Static lung-thorax compliance (CLT) was calculated as tidal volume divided by the airway pressure difference between the end of the postinspiratory pause and the end of the expiration, and also from the increase in FRC caused by adding 5 cmH2O of PEEP (CLT[FRC]). Before skin incision, both FRC and compliance were closely correlated with weight and length. During this stage, FRC was 21 +/- 5 ml/kg, CLT 0.90 +/- 0.21, and CLT(FRC) 1.28 +/- 0.35 ml X cmH2O-1 X kg-1 X PEEP 5 increased FRC by 34 +/- 9%. In patients with intact pleural cavities throughout the operation (n = 10), FRC increased by 4 +/- 2 ml/kg when the sternum was retracted (P less than 0.01). During CPB, FRC decreased by 4 +/- 3 ml/kg (P less than 0.01), and FRC at the end of surgery was 5 +/- 4 ml/kg less than before skin incision (P less than 0.01). In these ten children, there was a 13% and 6% decrease in mean CLT and CLT(FRC), respectively, during the operation (P less than 0.05) and mean CLT(FRC) was at least 40% greater than CLT during all four stages (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)