Cardiopulmonary bypass (CPB) may induce serious side effects, potentially leading to myocardial failure. The Na+-K+-ATPase is a key component for myocardial function. Due to its developmental regulation, results from adult studies cannot be adopted to the situation in childhood. Right atrial myocardium from patients with left-to-right shunts at atrial level (VO, n = 8) and those without (NO, n = 8) was excised during heart surgery before and after CPB. Na+-K+-ATPase isoforms ATP1A1 (p = 0.008) and ATP1A3 (p = 0.038) decreased during CPB, which decrease was restricted to the VO group. This study highlights the importance of the underlying heart defect for susceptibility to the effects of CPB, showing a reduced Na+-K+-ATPase mRNA expression only in patients with left-to-right shunts on the atrial level. This seemed to be an early molecular event, as apart from one, none of the patients showed heart failure before or after surgery. (C) 2005 Elsevier Inc. All rights reserved.