<qa>  What is the primary objective of this prospective observational study?The primary objective of this prospective observational study in 150 patients undergoing cardiac surgery with an intraoperative measured cardiac output 2Lminm2 is to monitor NT-proBNP for 7 days. Cardiac index and NT-proBNP will be correlated with morbidity and mortality 8. A critical threshold value for intraoperative cardiac index and NT-proBNP will be calculated. 