Changes in Lung Volume and Lung‐Thorax Compliance during Cardiac Surgery in Children 11 Days to 4 Years of Age

&NA; 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 · cmH2O‐1 · kg‐1. PEEP5 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patients with intact pleural cavities throughout the operation (n = 10), FRC increased by 4 ± 2 ml/kg when the sternum was retracted (P < 0.01). During CPB, FRC decreased by 4 ± 3 ml/ kg (P < 0.01), and FRC at the end of surgery was 5 ± 4 ml/kg less than before skin incision (P < 0.01). In these ten children, there was a 13% and 6% decrease in mean CLT and CLT(FRC), respectively, during the operation (P < 0.05) and mean CLT(FRC) was at least 40% greater than CLT during all four stages (P < 0.01). The changes in FRC and compliance were less uniform in children in whom one or both pleural cavities had been opened (n = 5). The decrease in FRC during the operation, however, was similar to that found in patients with intact pleurae. Mean FRC after sternal closure (all children) was 17 ± 5 ml/kg. In conclusion, FRC increased during sternal opening in children with intact pleural cavities. After surgery, FRC was reduced in all children. The findings indicate that PEEP is not neceassary before CPB in children with intact pleurae but should be used after CPB if it can be instituted without causing untoward circulatory effects.