Prior to conducting a safety and efficacy trial however we intend to first establish a dose of Sildenafil that is tolerated by cardiac surgery patients and compare the pharmacokinetics of this dose to the effective dose identified in our preclinical work. We therefore propose to evaluate the pharmacokinetic profile, safety and tolerability of sildenafil in 36 patients undergoing cardiac surgery, in an open label, dose escalation Phase I study. This study will inform the design of a subsequent randomized Phase IIb trial that will determine efficacy and safety