Subjects will be excluded if they: (1) have single ventricle morphology; (2) require post-operative ECMO; (3) have sustained atrial or ventricular arrhythmias that may complicate ventricular pacing; (4) are not able to have functioning epicardial pacemaker leads; (5) are, in the opinion of the intensivist, cardiologist or surgeon, not stable enough medically to participate in the study; or (6) are unwilling to provide informed consent or assent.