To characterize the effect of changes in pulmonary hemodynamics on airway and tissue mechanics, forced oscillatory input impedance of the respiratory system (Zrs) was measured between 0.4-12 Hz in two groups of children undergoing surgical repair of congenital heart disease (CHD) immediately before sternotomy and after chest closure during short apneic intervals. Children with lesions associated with high pulmonary blood flow and/or pressure (septal defects; HP group, n = 12) and children with hypoperfused lungs (tetralogy of Fallot; LP group, n = 12) were included in the study. Airway resistance (Raw), and coefficients of respiratory tissue damping (G) and elastance (H), were estimated from Zrs by model-fitting. A postoperative reduction in pulmonary blood flow and/or pressure in the HP group resulted in an immediate decrease in Raw of 29 +/- 9 (SE)% (P < 0.05), whereas children in the LP group had increases in Raw (24 +/- 17%, no significance) after surgery. No significant change was observed in G in either the HP (6.4 +/- 13%) or LP (27 +/- 23%) group, while H increased in children of both the HP (23 +/- 8%, P < 0.05) and LP (36 +/- 7%, P < 0.01) groups. These results suggest that the preoperative pulmonary hemodynamic condition determines changes in airway mechanics: surgical repair of CHD leads to an improvement in airway function only in children with congested lungs. The adverse effects of surgery, mechanical ventilation, and/or cardiopulmonary bypass may be responsible for the increased stiffness of the respiratory system observed in both groups of children.