Cardiac ElectrophysiologySyncopeSyncope, VasovagalElectrophysiological ProcessesElectrophysiologic Techniques, CardiacEpicardial MappingArrhythmias, CardiacModels, CardiovascularHeartMyocytes, CardiacHeart Conduction SystemElectrophysiologyVoltage-Sensitive Dye ImagingElectrophysiological PhenomenaElectrocardiographyAction PotentialsAtrioventricular NodeHeart VentriclesLong QT SyndromeCardiac OutputHeart RateComputer SimulationCatheter AblationMyocardiumDeath, Sudden, CardiacCardiac Surgical ProceduresCardiac Pacing, ArtificialPotassium ChannelsAlgorithmsHeart DiseasesHeart ArrestCardiomegalyCardiac CatheterizationPacemaker, ArtificialBradycardiaCardiac TamponadeMyocardial ContractionTime FactorsHeart FailureHemodynamicsEchocardiographyCardiomyopathiesHeart AtriaElectrocardiography, AmbulatoryHeart BlockBlood PressureMyoblasts, CardiacPatch-Clamp TechniquesCardiac MyosinsCardiac Imaging TechniquesTachycardia, VentricularProspective StudiesDogsCardiac GlycosidesDefibrillators, ImplantableMyocardial InfarctionTreatment OutcomeVentricular Function, LeftRats, Sprague-DawleyCalciumCardiac Output, LowHeart Function TestsRecurrenceOut-of-Hospital Cardiac ArrestMembrane PotentialsVentricular FibrillationMidodrineFollow-Up StudiesCardiac VolumeVentricular RemodelingHeart TransplantationAutonomic Nervous SystemVentricular FunctionNAV1.5 Voltage-Gated Sodium ChannelTroponin IAnti-Arrhythmia AgentsPostureHeart NeoplasmsStroke VolumeVagus NerveRetrospective StudiesIsoproterenolDisease Models, AnimalTachycardiaAdrenergic beta-Antagonists