Congenital heart malformations are often associated with altered pulmonary hemodynamics. Lesions associated with increased pulmonary blood flow (PBF) or increased mean pulmonary artery pressure (MPAP) may in turn alter respiratory mechanics. Surgical correction of these cardiac defects frequently involves the use of cardiopulmonary bypass (CPB), during which the lung may be partially or completely atelectatic for lengthy periods, further compromising lung mechanics. The aims of this study were to document the effect of PBF on respiratory mechanics in children and to determine whether the detrimental effects of CPB were outweighed by the potentially positive effects of the corrective surgery. Twenty-three children (2-120 mo) undergoing surgery were studied while anesthetized, paralyzed, and mechanically ventilated. Pulmonary to systemic blood flow ratio was used as an index of PBF. Seventeen children had lesions associated with increased PBF (group 1), while six had decreased or normal PBF (group 2). Respiratory mechanics were measured just before the commencement of CPB and within approximately 2 h after the cessation of CPB, with the chest closed. Dynamic elastance (Ers,dyn) and resistance (RRS) were calculated from flow, volume (V), and pressure (Pao) measurements, using multiple linear regression with a volume-dependent single compartment model. Static elastance (ERS,st) was calculated from Pao and V measurements obtained when deflating the lung in steps from a maximal Pao of 30 cm H2O. ERS,dyn, ERS,st, and RRS increased significantly with increasing PBF to 220-330% predicted. There was no correlation between MPAP and respiratory mechanics. After CPB, ERS, dyn and RRS fell to normal levels in group 1.(ABSTRACT TRUNCATED AT 250 WORDS)