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The aim of the present study was to characterize pulmonary surfactant properties in children undergoing cardiovascular surgery with cardiopulmonary bypass.Prospective clinical trial.University hospital pediatric intensive care unit.Fifty pediatric patients with congenital cardiac defects undergoing cardiovascular surgery with (n = 35) and without (n = 15) cardiopulmonary bypass procedure.Tracheal aspirates were collected by saline lavage during routine suctioning before (baseline) and after cardiopulmonary bypass, as well as 4, 8, and 24 hrs after admission to the pediatric intensive care unit.Total protein and phospholipid concentrations were assessed in native tracheal aspirates, in large surfactant aggregates, and in small surfactant aggregates. Phospholipid profiles and phosphatidylcholine fatty acids; surfactant apoproteins SP-A, SP-B, and SP-C (enzyme-linked immunosorbent assay); and surface activity (Pulsating Bubble Surfactometer) were all analyzed in large surfactant aggregates. With cardiopulmonary bypass, an initial increase in total protein content was followed by an increase in phospholipid concentration in tracheal aspirates. Large surfactant aggregates decreased 4 hrs after cardiopulmonary bypass (4 hrs, 22.6 ± 5.6%; mean ± sem;p < .01 compared with baseline, 55.4 ± 9.2%) but recovered within 24 hrs. The phospholipid-protein ratio of large surfactant aggregates 24 hrs after cardiopulmonary bypass (1.2 ± 0.2;p < .01) was significantly decreased compared with baseline (2.9 ± 0.6). The relative amount of phosphatidylglycerol content in the large surfactant aggregates-fraction dropped linearly over time but other phospholipids remained mainly unchanged. Phosphatidylcholine fatty acid profiles remained unaffected by cardiopulmonary bypass. The relative content of SP-B and SP-C in large surfactant aggregates increased approximately three-fold compared with baseline. Altogether, our findings with recovered large surfactant aggregate/small surfactant aggregate ratios and increased phospholipid in tracheal aspirates after 24 hrs represent an approximately ten-fold increase in large surfactant aggregate-associated SP-B and SP-C compared with baseline. Only minor changes were detected in biophysical properties of large surfactant aggregates throughout the observation period.Cardiopulmonary bypass procedure in children induces profound changes in the surfactant system involving both phospholipid and protein components; biophysical function may have been maintained by compensatory increase in SP-B and SP-C.