Heart Conduction SystemHeart RateElectric StimulationHeartImpulse Control DisordersHeart Rate, FetalHeart FailureAutonomic Nervous SystemBlood PressureHeart DiseasesHeart TransplantationHemodynamicsHeart Defects, CongenitalFetal HeartSympathetic Nervous SystemTime FactorsHeart AtriaElectrocardiographyBaroreflexElectrocardiography, AmbulatoryHeart BlockAction PotentialsAdrenergic beta-AntagonistsDogsMyocardiumMyocardial ContractionHeart ValvesExercise TestParasympathetic Nervous SystemOxygen ConsumptionHeart Function TestsRespirationPropranololTelemetryBradycardiaPressoreceptorsTachycardiaSinoatrial NodeCatsNeural ConductionAtropineStroke VolumeReflexMyocardial IschemiaVentricular Function, LeftNorepinephrineCardiovascular Physiological PhenomenaRestMechanoreceptorsPhysical ExertionCoronary DiseaseExerciseCardiovascular SystemPropanolaminesCardiac OutputCardiac Pacing, ArtificialCardiotonic AgentsModels, CardiovascularHeart Arrest, InducedNeurons, AfferentMyocytes, CardiacArrhythmia, SinusPostureSignal Processing, Computer-AssistedEchocardiographyRats, Sprague-DawleyProspective StudiesArrhythmias, CardiacMyocardial InfarctionAutonomic Nervous System DiseasesDose-Response Relationship, DrugCardiac Output, LowIsoproterenolRisk FactorsEpinephrineRats, WistarMetoprololReference ValuesElectrophysiologySupine PositionElasticity Imaging TechniquesRegional Blood FlowDouble-Blind MethodSystoleCardiomyopathy, DilatedAnti-Arrhythmia AgentsBody TemperatureAtrioventricular NodeDiastoleVentricular Dysfunction, LeftCatecholaminesFollow-Up StudiesRabbitsPerfusionAnalysis of VarianceCircadian RhythmAnesthesiaTilt-Table TestAtenololHypotension