SyncopeSyncope, VasovagalDeath, SuddenDeath, Sudden, CardiacSudden Infant DeathBradycardiaArrhythmias, CardiacElectrocardiography, AmbulatoryMidodrinePacemaker, ArtificialElectrocardiographyPostureTachycardia, VentricularRecurrenceDefibrillators, ImplantableLong QT SyndromeHeart BlockCardiac Pacing, ArtificialVentricular FibrillationHeart RateHeart ArrestCarotid SinusUnconsciousnessCardiomyopathy, HypertrophicBrugada SyndromeFludrocortisoneFollow-Up StudiesAutonomic DenervationTachycardiaProspective StudiesAutonomic Nervous SystemSupine PositionElectrophysiologic Techniques, CardiacSick Sinus SyndromeAtrioventricular BlockDizzinessAnti-Arrhythmia AgentsBundle-Branch BlockEtilefrineAdrenergic beta-AntagonistsRisk FactorsCardiac ElectrophysiologyReflex, AbnormalOrthostatic IntoleranceHead-Down TiltBlood PressureNAV1.5 Voltage-Gated Sodium ChannelRetrospective Studies