ClonidineAdrenergic alpha-AgonistsAdrenergic alpha-2 Receptor AgonistsYohimbinePreanesthetic MedicationReceptors, Adrenergic, alpha-2SympatholyticsAdrenergic alpha-AntagonistsImidazoline ReceptorsReceptors, Adrenergic, alphaGuanfacineInjections, SpinalAnalgesicsIdazoxanAdjuvants, AnesthesiaAdrenergic alpha-2 Receptor AntagonistsOxymetazolineNorepinephrineBlood PressurePrazosinHeart RatePremedicationAnalgesia, EpiduralPhentolamineReceptors, AdrenergicDouble-Blind MethodDioxanesDose-Response Relationship, DrugMorphineHemodynamicsAnesthetics, LocalHypnotics and SedativesTime FactorsGuanabenzPain, PostoperativeNaloxoneNeostigmineDexmedetomidineDrug InteractionsMedetomidineEpinephrineMethoxamineSympathetic Nervous SystemMethyldopaBupivacaineInjections, IntravenousInjections, EpiduralDesipramineAntihypertensive AgentsImidazoles