Hypotension, OrthostaticOrthostatic IntoleranceDizzinessLower Body Negative PressureTilt-Table TestHead-Down TiltPostureSupine PositionSyncopeHeart RatePostural Orthostatic Tachycardia SyndromeBaroreflexSyncope, VasovagalBlood PressureExercise TestSpace FlightWeightlessness SimulationHypovolemiaSympathetic Nervous SystemBed RestStress, PhysiologicalVascular CapacitancePlasma VolumeAutonomic Nervous SystemHemodynamicsElectrocardiographyVascular ResistanceRestAutonomic Nervous System DiseasesHypocapniaCerebrovascular CirculationCardiovascular Physiological PhenomenaForearmRegional Blood FlowMidodrineBlood Flow VelocityPressoreceptorsShy-Drager SyndromeBlood VolumePure Autonomic FailureDipyridamoleSalivary alpha-AmylasesCardiac OutputEchocardiography, StressDobutamineNorepinephrineStroke VolumeCardiovascular DeconditioningStress, PsychologicalValsalva ManeuverVasoconstrictionTachycardiaTime FactorsEpinephrineHydrocortisoneWeightlessness CountermeasuresMyocardial Perfusion ImagingChest PainUrinary Incontinence, StressSuburethral SlingsMuscle, SkeletalCoronary AngiographyFludrocortisoneCoronary DiseasePredictive Value of TestsPrimary DysautonomiasThallium RadioisotopesAstronautsMyocardial IschemiaGravity SuitsWeightlessnessTomography, Emission-Computed, Single-PhotonCardiotonic AgentsDysautonomia, FamilialEchocardiographyAngina PectorisCoronary Artery DiseaseSalivaHeat Stress DisordersExerciseLigaments, ArticularMicrovascular AnginaGravity, AlteredAtlanto-Occipital JointExercise TolerancePyridostigmine BromideSensitivity and SpecificityJoggingIntracranial HypotensionMultiple System AtrophyHistory, Modern 1601-DroxidopaEarly AmbulationFollow-Up StudiesHeart Function TestsUrologic Surgical ProceduresLegFluid ShiftsCardiovascular SystemProspective Studies