 RANDOMIZATION. Patients will be randomly allocated to one of the test gas study groups inhaled 80 parts per million ppm nitric oxide in nitrogen or the placebo group inhaled N2. The intervention will consist of giving the test gas both via the CPB machine and after CPB via the anesthetic circuit, and thereafter via the mechanical ventilator andor with face masknasal prongs in the ICUward. Test gas administration will commence at the onset of CPB and last for 24 hours. At the end of 24 hours, iNO will be weaned and discontinued while carefully monitoring hemodynamics for a period of 2-4 hours. Local guidelines for iNO discontinuation will be adopted. Using an Inovent Ikaria Inc, N.J., USA or volumetrically-calibrated flowmeters, pure nitrogen placebo or 800 ppm NO gas in N2 is mixed with pure O2 or air to obtain a final concentration of 80 ppm NO. During CPB the test gas is delivered through the extracorporeal oxygenator, after CPB the NO is delivered through the inspiratory limb of the anesthetic or ventilator circuit. NO, NO2 and O2 and methemoglobin levels are monitored by an unblinded observer. Patients in the placebo group will receive nitrogen test gas during the same 24 hour-period. When patients are extubated they will breathe test gas via a face mask or nasal prongs. The inspired oxygen levels will be maintained at the usual levels required for routine post-operative care. The test gas tank in the OR and at the bedside will be covered and blinded from the investigators and clinicians treating the patient. Only the perfusionist and the person changing the tanks and monitoring the respiratory levels of NONO2 and methemoglobin will know the treatment group of the patient. No changes to the usual and customary standards of care for any intraoperative or postoperative treatments will be made during the study period. 