ClonidineAdrenergic alpha-AgonistsAdrenergic alpha-2 Receptor AgonistsYohimbinePreanesthetic MedicationReceptors, Adrenergic, alpha-2SympatholyticsAdrenergic alpha-AntagonistsImidazoline ReceptorsReceptors, Adrenergic, alphaGuanfacineInjections, SpinalAnalgesicsIdazoxanAdrenergic alpha-2 Receptor AntagonistsAdjuvants, AnesthesiaOxymetazolinePrazosinNorepinephrineBlood PressurePhentolamineAnalgesia, EpiduralReceptors, AdrenergicPremedicationHeart RateDioxanesMorphineDouble-Blind MethodGuanabenzDose-Response Relationship, DrugAnesthetics, LocalHypnotics and SedativesNaloxoneHemodynamicsNeostigminePain, PostoperativeDexmedetomidineMedetomidineMethoxamineMethyldopaDrug InteractionsSympathetic Nervous SystemEpinephrineBupivacaineDesipramineInjections, Epidural