SyncopePanic DisorderTime FactorsSyncope, VasovagalMigraine DisordersIschemic Attack, TransientBradycardiaElectrocardiographyMidodrineRecurrencePacemaker, ArtificialElectrocardiography, AmbulatoryPostureHeart BlockComplement Membrane Attack ComplexCarotid SinusArrhythmias, CardiacCardiac Pacing, ArtificialUnconsciousnessFludrocortisoneHeart RateFollow-Up StudiesLong QT SyndromeDeath, Sudden, CardiacProspective StudiesAutonomic DenervationDefibrillators, ImplantableTachycardia, VentricularDizzinessCalciumSick Sinus SyndromeSeptember 11 Terrorist AttacksEtilefrineAtrioventricular BlockHeart ArrestBlood PressureTachycardiaSupine PositionAutonomic Nervous SystemOrthostatic IntoleranceReflex, AbnormalElectrophysiologic Techniques, CardiacAdrenergic beta-AntagonistsHead-Down TiltTerrorismTreatment Outcome