Heart FailureVentricular Dysfunction, LeftHeartHeart RateHeart DiseasesHeart TransplantationHeart Defects, CongenitalFetal HeartHeart ValvesHeart AtriaHeart BlockMyocardiumHeart Arrest, InducedHeart Function TestsMyocardial ContractionMyocardial IschemiaMyocytes, CardiacCoronary DiseaseHemodynamicsCardiac Output, LowVentricular Function, LeftTime FactorsHeart, ArtificialHeart Failure, SystolicPerfusionCardiomyopathy, DilatedCardiomegalyMyocardial InfarctionHeart InjuriesHeart Rate, FetalHeart SeptumCardiotonic AgentsRheumatic Heart DiseaseStroke VolumeCardiomyopathiesEchocardiographyMyocardial Reperfusion InjuryHeart ArrestRisk FactorsHeart Valve ProsthesisAdrenergic beta-AntagonistsHeart Failure, DiastolicAmerican Heart AssociationVentricular RemodelingDogsProspective StudiesOxygen ConsumptionFollow-Up StudiesAutonomic Nervous SystemCardiac Pacing, Artificial