Serial postoperative serum lactate (PoSL) measurements are used to assess the clinical progress in critically ill children following corrective surgery. Elevated PoSL levels after cardiac surgery are also associated with hypoperfusion and tissue hypoxia. To the best of our knowledge there are no reports of Preoperative Serum Lactate (PrSL) estimations in patients undergoing open-heart surgeries. The use of 1H-NMR spectroscopy is demonstrated to examine the role of pre-and-post operative metabolic changes in such patients. Mixed venous and arterial blood samples were drawn in fifty-two patients with congenital heart diseases; before, at the termination of surgery and thereafter at 24 hour intervals till the patient expired or for 5 days. The concentrations of the metabolites were determined using 1H-NMR technique. Eight non-survivors had significantly raised PrSL levels [median(range) 5.3(1.9-6.4)mM] vs. survivors [median(range) 2.8(0.6-11.0)mM] (p<0.02). Two of the non-survivors also had significant amounts of bacterial metabolites (Acetate, Pyruvate, Succinate and Formate) in their preoperative samples. Routine preoperative tests and preclinical assessments in all the patients did not reveal any infection or cardio-renal dysfunction. Forty-four survivors, with low PrSL levels and absence of metabolites indicative of infection had their PoSL levels returned to baseline after an initial rise. In all the eight non-survivors PoSL levels remained significantly higher [median(range) 7.3(5.3-8.6)mM] compared to the forty-four survivors [median(range) 3.9(1.1-10.2)mM] (p<0.01). This study underlines the significance of PrSL and bacterial-metabolite concentrations in controlling sub-clinical infection or cardio-renal dysfunction to achieve better postoperative outcomes.