 An estimated 175 subjects will be enrolled at approximately 40 investigative sites. Subjects will be divided into two treatment groups to receive either high dose or low dose Dexmedetomidine DEX. The loading and maintenance doses in both groups will be stratified according to the presence or absence of cardiopulmonary bypass. The level of sedation will be assessed using the University of Michigan Sedation Scale UMSS. Score 0 awakealert; Score 1 sleepyresponds appropriately; Score 2 somnolentarouses to light stimuli; Score 3 deep sleeparouses to deeper; Score 4 unarousable to stimuli. 