www.revportcardiol.org
Revista
Portuguesa
de
Cardiologia
Portuguese
Journal
of
Cardiology
REVIEW
ARTICLE
Insights
into
the
background
of
autonomic
medicine
Sérgio
Laranjo,
Vera
Geraldes,
Mário
Oliveira,
Isabel
Rocha
∗InstitutodeFisiologiadaFaculdadedeMedicinaeCentroCardiovascular,UniversidadedeLisboa,Lisboa,Portugal
Received24May2016;accepted8January2017 Availableonline14October2017
KEYWORDS Autonomic physiology; Cardiovascular regulation; Baroreceptorreflex; Chemoreceptor reflex; Autonomic evaluation; Tilttest
Abstract Knowledgeofthephysiologyunderlyingtheautonomicnervoussystemispivotalfor understandingautonomicdysfunctioninclinicalpractice.Autonomic dysfunctionmay result fromprimarymodificationsoftheautonomicnervoussystemorbesecondarytoawiderangeof diseasesthatcauseseveremorbidityandmortality.Togetherwithadetailedhistoryandphysical examination,laboratoryassessmentofautonomicfunctionisessentialfortheanalysisofvarious clinicalconditions andtheestablishmentofeffective,personalizedandprecisetherapeutic schemes.Thisreviewsummarizesthemainaspectsofautonomicmedicinethatconstitutethe backgroundofcardiovascularautonomicdysfunction.
© 2017SociedadePortuguesade Cardiologia.Publishedby ElsevierEspa˜na,S.L.U.Allrights reserved. PALAVRAS-CHAVE Fisiologia autonómica; Regulac¸ão cardiovascular; Reflexobarorrecetor; Reflexo quimiorrecetor; Avaliac¸ão autonómica; Testedetilt
Introduc¸ãoàmedicinaautonómica
Resumo Oconhecimentosubjacenteàfisiologiadosistemanervosoautónomoéfundamental paraseentenderadisfunc¸ãoautonómicanapráticaclinica.Adisfunc¸ãoautonómicapode resul-tarprimariamentedemodificac¸õesdosistemaou,secundariamente,aumasériedepatologias conducentesamorbilidadeoumortalidade.Juntamentecomacolheitadetalhadadahistória clínicaedoexameobjetivo,aavaliac¸ãoautonómicalaboratorialtorna-seessencialnaanálise dealgumascondic¸õesclínicasenoestabelecimentodeesquemasterapêuticosmaiseficazes, refinadosepersonalizados.Assim,nestarevisãosumarizam-seosaspetosmaisrelevantes da fisiologiaautonómicasubjacenteadisfunc¸ãoautonómicacardiovascular.
©2017SociedadePortuguesa deCardiologia.PublicadoporElsevierEspa˜na,S.L.U.Todosos direitosreservados.
∗Correspondingauthor.
E-mailaddress:isabelrocha0@gmail.com(I.Rocha).
http://dx.doi.org/10.1016/j.repc.2017.01.007
Introduction
Attempts to bridge the gap between basic and clinical science, known as the translational approach to medical knowledge,contributetobetterclinicalpractice,enabling acomprehensiveinterpretationofpathophysiological mech-anisms, more accurate diagnosis and establishing more effective treatment. Advances in autonomic research in recentyearsand thedevelopmentof implantable devices thataffect autonomictone meanthereis agrowing need tounderstandthescientificbasisofautonomicmedicine,in ordertoimprovemanagementofautonomicdysfunctionin cardiology.
This review aimsto provide a basis for understanding autonomicfailureincardiovasculardisease.Thefirstsection dealswiththebasic aspectsof autonomicfunction, while thesecond covers themost important reflexes. The third sectiondescribesthemostcommonmethodsofautonomic evaluation.
The
autonomic
nervous
system
Almostallbodilyfunctionsaredependentontheautonomic nervoussystem(ANS),whichexertsprecisecontrolover vis-ceralfunctions(Figure1).However,themechanismsthrough whichtheANSexertsthiscontrolarenotwellunderstood.
Although the ANS is able to hide its own dysfunction, disautonomy, also known asautonomic failure, can occur duetofunctionalfailure,a physicaldefectin thenervous network,andasaresultoftheagingprocess.Inthese con-ditions, thesystem becomes over-activated,the resulting allostaticoverloadbeingbelievedtocontributetovarious diseases,includinghypertension,atrialfibrillationandother cardiacarrhythmias, ischemic heart disease, obesity, dia-betes, atherosclerosis, sleep apnea, metabolic syndrome andcongestiveheartfailure.1---14
TheANSisoneofthetwomajordivisionsofthe periph-eralnervoussystem (theother being thesomaticnervous system). TheANSfunctions mainlythroughnegative feed-backmechanismsandviareflexarcs,usingspecificneuronal pathways in the periphery and a specific central orga-nization to perform precise and flexible actions. In the present text, we will use Langley’s neuroanatomical ter-minologyandthetermssympathetic,parasympatheticand enteric will only refer to the motor portion of the auto-nomicreflexarc(Figure2).Thisarcalsoincludesintegrative centers located in the central nervoussystem (CNS) (the centralautonomicnetwork)towhichsensoryinformationis conveyedfromperipheralsensorslocatedinspecific reflex-ogenicareas.6,12,14,15
Visceralafferentpathways
Afferent pathways arethe interfacebetween the visceral organsandtheCNS.Mostafferentfibersareunmyelinated, but myelinatedfibers canalsoconduct autonomicsensory information.15Therearetwotypesofvisceralafferents, pri-maryafferentfibersandentericafferentfibers.Thelatter respondtochemicalandmechanical eventsand theircell bodies are located in the gastrointestinal tract walls.15,16 Primaryafferentinputsarecarriedorthodromicallytothe spinalcord,brainstemorprevertebralsympatheticganglia. Thedegreetowhichtheseafferentneuronsare physiolog-ically specific is determined by the responses evoked by chemicalormechanicalstimulation.15---17Mostofthese affer-entstransmitinformationfromthevisceratotheCNS,4,18---20 butsomealsomakecontactwithsympatheticpreganglionic neurons in the prevertebral ganglia.21 These anatomical relationsindicatethatinadditiontotheircentralactions, visceralprimaryafferents(andalsoentericafferentfibers) mayalsoplayaroleinperipheralregulatoryreflexes,mainly those that are active in pathological conditions through positivefeedbackmechanisms.21
Endocrine system Musculoskeletal system Autonomic nervous system Inter nal inputs/inter nal organs External/ environmentalinputs Thalamus/hypothalamus Limbic system Cortex
Figure1 The interactionsbetween theautonomicnervous system,thebrain, thebody andtheenvironment. Adapted from Jänig.16
Autonomic motor fibres Afferent fibres Central nervous system Effector organs Sensor y receptors
Figure 2 The autonomic reflex arc. The morphological relations between its different components are shown. The autonomicmotorfibersincludesympathetic,parasympathetic andentericfibers.AdaptedfromRocha.106
Afferent neurons are involved in two main functions: regulation of visceral actions, including organ-protective reflexes; and the transport of pain information, includ-ing pain from deep somatic tissues and regulation of hyperalgesia,deeppainandinflammation.15,16,22This dual functionmakesthemfundamentallydifferentfromsomatic afferents,inwhichsensoryandregulatorypropertiescannot beseparated; afferentimpulses fromskin or muscle trig-gerreflexes andbehavioralregulation simultaneouslywith sensoryexperience,whichisnotthecasewithvisceral affer-ents,assomestimulifromthelatterneverreachthelevel ofconsciousness,suchaschangesinbloodpressure(BP)or gutdistension.15---17,23Themajorityoftheviscerashowdual afferentinnervation,withmostafferentfiberstravelingin mixedparasympatheticnerves,suchasthevagusandpelvic nerves.24 The physiological significanceof thisdual inner-vation --- afferent fibers being carried in sympathetic and parasympatheticnerves---isnotfullyunderstood,butdata suggestthatreflexandregulatoryfunctions evokedby vis-ceralstimulationaremainlytriggeredbyactivityinafferent fibers in the vagus and pelvic nerves, while visceral sen-sation,andinparticularvisceralpain,togetherwithsome visceralreflexesoriginatinginthemesenterium,are medi-atedbyafferentfibersinsympatheticnerves.15,16
Efferentpathways
Efferentpathways oftheperipheralANShave threemajor subdivisions: sympathetic, parasympathetic, and enteric.
These systems are the building blocks of the motor part of the autonomic reflex arc and establish the final auto-nomicpathway,25aseachofthemconsistsofaseriesof pre-andpostganglionicneuronsthataresynapticallyconnected in autonomic ganglia, which function as the connection betweenthebraincentersandthetargetorgan.
EachautonomicnervepathwayextendingfromtheCNS toaninnervatedorganisatwo-neuronchain(excepttothe adrenalmedulla that ineffectfunctions asa sympathetic ganglion).Thecellbodyofthefirstneuron,locatedinthe CNS,synapseswithasecond-orderneuron,thecellbodyof which lieswithin an autonomic ganglion.26 It is generally acceptedthat,exceptfor theentericnervoussystem,the organizationof parasympathetic nervouspathways is sim-plerthanthatofthesympatheticnervoussystem.However, whilethismaybetrueforsomepathwaysandtargetorgans suchasthepupillae andciliarymuscles, itseemsunlikely tobethecaseforothertargetorgansliketheheartorthe urinarybladder.13,27---29
Sympatheticefferentpathways
Sympathetic preganglionic neurons are a heterogeneous population. Morphologically, they vary in somal shape, size anddendritic arborization, giving rise to either non-myelinatedormyelinatedaxons,whicharenotselectivein relationtoasingletarget.Inthespinalcord,sympathetic preganglionicneurons are located in four nuclei: the lat-eralfunicular,intermediolateral,intercalated,andcentral autonomicnuclei, ofwhichthemostimportantfor cardio-vascularregulationistheintermediolateralnucleus.
Independently of how preganglionic sympathetic neu-ronsarepositionedwithinthedifferentspinalnuclei,they aresegmentallyorganized,thisarrangementprovidingthe anatomicalsubstrateforamoregeneralrostrocaudal func-tionaltopography.15,30
Sympathetic preganglionic neurons exhibit a low level of tonic activity. This may reflect the influence of both intrinsicmembranepropertiesandtheintegrationof excit-atoryandinhibitorypostsynapticpotentials.Theiractivity isregulatedbysegmentalinputsfromvisceralandsomatic afferents and supra-spinal pathways.31 According to their biophysical properties and functional properties, sympa-thetic preganglionic neurons can be divided into phasic (rapidlyadapting),tonic(slowlyadapting),andthosewith alongafter-depolarization.
Parasympatheticefferentpathways
Comparedtothequantityofresearchonsympatheticreflex responses, therehave been relatively few studies onthe parasympatheticsystem.Therearevariousreasonsforthis buttheyareallduetothefactthatmostparasympathetic gangliaarelocatedclosetoorwithinthetargetorganwalls, andthereforethepostganglionicparasympatheticneuronis veryshort.These lesseasily morphologically-defined neu-ronalstructures,togetherwithlesspronouncedtargetorgan parasympatheticinnervation,hamperneuralrecordingand peripheralmodulationofparasympatheticcircuits.32,33
In neuroanatomicalterms, brain stem parasympathetic preganglionicnucleiincludetheEdinger-Westphalnucleus, thesuperiorandinferiorsalivatorynuclei,thedorsalmotor vagal nucleus and the nucleus ambiguus. Neurons of the
ventrolateralnucleusambiguus providethemain parasym-patheticinnervationofthecardiacganglia,whichinnervate theheart,esophagus,andrespiratoryairways.34
Theheartisinnervatedbyatleasttwoparasympathetic pathways.One,which actsdirectly onthesinus nodeand otherpacemakercells, isinvolved inheartbeatregulation and atrial inotropism. These myelinated neurons emerge from the nucleus ambiguus and are activated by barore-ceptor stimulation. They can show spontaneous rhythmic activity,theabsenceofactivitycoincidingwithinspiration andactivationbeingsimultaneouswithexpiration.This cou-plingtocentralrespiratoryactivityisthebasisofrespiratory sinusarrhythmia.Thesecondpathwayisformedmainlyof unmyelinated neurons that originate in the dorsal motor nucleusofthevagus.Someoftheseneuronscanalsoshow spontaneousactivitythat isnot modulatedby thecentral respiratorydrive orbybaroreceptoractivity;intheheart, theirmainfunctionappearstobetoinducecoronary vasodi-lationwhenactivated.35---38
Dualautonomicinnervation
The two divisions of the ANS rarely operate indepen-dently, and autonomic responses generally represent the regulatedinterplayof bothdivisions (Table1).The heart, glands and smooth muscles are innervated by both sym-pathetic and parasympathetic fibers (dual innervation). Moreover,theyareusuallyactivatedreciprocally,i.e.when the activity of one divisionincreases, the activity of the other decreases. Dual innervation by nerve fibers that causeoppositeresponsesprovidesafinedegreeofcontrol overtheeffectororgan.Thesympatheticsystempromotes responsesthatpreparethebodyforstrenuousphysical activ-ity in emergency or stressful situations, the sympathetic responsebeingcharacterizedbytachycardia,hypertension andincreasedbloodflowtotheskeletalmuscles,heart,and brain,releaseofglucosebytheliveranddilatationofthe pupils.26,30Theparasympatheticsystemdominatesinquiet, relaxedsituations;undernonthreateningcircumstances,the bodycanperformitsowngeneralhousekeepingactivities.26 Thereareseveralexceptionstothegeneralruleofdual reciprocal innervation by the two branches of the ANS. Innervatedbloodvessels(mostarteriolesandveins)receive onlysympathetic nervefibers. Regulation is accomplished byincreasingor decreasingthefiring rateaboveorbelow thetoniclevelinthesesympatheticfibers.Theonlyblood vesselsto receive both sympathetic and parasympathetic fibersarethosesupplyingthepenisandclitoris.Mostsweat glandsareinnervatedonlybysympatheticnerves.Salivary glandsareinnervatedbybothautonomicdivisions,but sym-patheticandparasympatheticactivityarenotantagonistic; both stimulate salivary secretion, but saliva volume and compositiondiffer depending on which autonomic branch isdominant.26
Centralautonomicnetwork
Central autonomic pathways are organized at two levels, someforreflexadjustmentsoftheendorgan,whileothers areorganizedinamorecomplexfashionbyconnectingto higherneural centers,formingacentralautonomiccircuit
capable of producing wide-ranging autonomic, endocrine, andbehavioralresponses(Figure3).15,31
Central control of autonomic function involves sev-eral interconnected areas distributed throughout the neuraxis.39 This central autonomic network has a critical role in moment-to-moment control of visceral func-tion, homeostasis, and adaptation tointernal or external challenges.14,15,31,39
The network functions on four closely interconnected hierarchical levels: spinal, bulbopontine, pontomesen-cephalicandforebrain(Figure4).Ofthese,the bulbopon-tine level is involved in reflex control of circulation and respiration,andisthelocationofthenucleustractus soli-tarii(NTS),thefirstrelaystationforreceptionofperipheral visceral information, and the rostroventrolateral medulla (RVLM),whichcontainsbulbospinalneuronsthatare funda-mentalforthecontrolofvasomotor,cardiacandrespiratory function and for the coordination of various cardiovascu-larreflexes.TheseRVLMneuronsalsocontrolhypothalamic function and neurons of the ventral respiratory group involved in respiratory rhythmogenesis.14,15,31,39 Located more rostrally, the parabrachial nucleus is a major relay centerfortheconvergenceofvarioustypesofsensory infor-mation (visceral, nociceptive and thermoreceptive), and containsseparatesubnucleilinkedtogastrointestinal, car-diovascularandrespiratoryregulationtogetherwithclusters ofneuronsinvolvedinosmo-andthermoregulation.14,15,31,39 The midbrain periaqueductal gray integrates autonomic, somaticandantinociceptiveresponsestostressfulstimuli. Morphologically, it is divided intocolumns, which control cardiorespiratoryandurinaryfunctionaswellaspain, ther-moregulationandreproductivefunction.14,15,31,39
The forebrain level includes the hypothalamus and components of the anterior limbic circuit, including the insularcortex,anteriorcingulatecortexandamygdala. Play-ing a central role in neuroendocrine integration critical for homeostasis and integrative adaptive responses, the hypothalamus and periaqueductal gray are also involved in the defense reaction, an acute but active adaptation to stressfulstimuli leading to sympathetic activation and tachycardia, hypertension, positive inotropism, increased stroke volumeand cardiacoutput, redistribution ofblood flow,tachypnea,baroreflexinhibitionandfacilitationofthe chemoreceptorreflex.40Inthisreaction,theparaventricular hypothalamicnucleus coordinatesneuroendocrine integra-tion,includingsympathoexcitation,secretionofvasopressin (VP)andactivationoftheadrenomedullaryand adrenocor-ticalsystems.
Autonomic
cardiovascular
reflexes
Cardiovascular autonomic reflexes include short-term, moment-to-moment, mechanismsthat regulateheart rate (HR) and BP. They result from activation of peripheral receptors whose afferents link to the CNS via the glos-sopharyngealandvagusnerves.6CNSprocessingofafferent informationisfollowedbyregulationofautonomicefferent pathways and adjustmentof cardiovascular parameters.41 CentralBPcontrolinvolvesbothsympatheticand parasym-patheticnervoussystems.Thesympatheticsystemincreases stroke volume, HR and total peripheral resistance,
Table1 Someeffectsofautonomicnervoussystemactivity.AdaptedfromVanderetal.30
Effectororgan Receptortype Sympatheticeffect Parasympatheticeffect
Eyes
Irismuscle Alpha Contractsradialmuscle(widens
pupil)
Contractssphinctermuscle (makespupilsmaller)
Ciliarymuscle Beta Relaxes(flattenslensforfarvision) Contracts(allowslenstobecome moreconvexfornearvision)
Heart
Sinoatrialnode Beta Increasesheartrate Decreasesheartrate
Atria Beta Increasescontractility Decreasescontractility
Atrioventricularnode Beta Increasesconductionvelocity Decreasesconductionvelocity
Ventricles Beta Increasescontractility Decreasescontractilityslightly
Arterioles
Coronary Alpha Constricts
---Beta Dilates
Skin Alpha Constricts
---Skeletalmuscle Alpha Constricts
---Beta Dilates
Abdominalviscera Alpha Constricts
Beta Dilates
---Salivaryglands Alpha Constricts Dilates
Veins Alpha Constricts
---Beta Dilates
Lungs
Bronchialmuscle Beta Relaxes Contracts
Bronchialglands Alpha Inhibitssecretion Stimulatessecretion
Beta Stimulatessecretion
Salivaryglands Alpha Stimulateswaterysecretion Stimulateswaterysecretion Beta Stimulatesenzymesecretion
Stomach
Motility,tone Alphaandbeta Decreases Increases
Sphincters Alpha Contracts Relaxes
Secretion Inhibits(?) Stimulates
Intestine
Motility Alphaandbeta Decreases Increases
Sphincters Alpha Contracts(usually) Relaxes(usually)
Secretion Alpha Inhibits Stimulates
Gallbladder Beta Relaxes Contracts
Liver Alphaandbeta Glycogenolysisandgluconeogenesis
---Pancreas
Exocrineglands Alpha Inhibitssecretion Stimulatessecretion
Endocrineglands Alpha Inhibitssecretion
---Beta Stimulatessecretion
Fatcells Alphaandbeta Increasesfatbreakdown
---Kidneys Beta Increasesrenninsecretion
---Urinarybladder
Bladderwall Beta Relaxes Contracts
Sphincter Alpha Contracts Relaxes
Uterus Alpha Contractsinpregnancy Variable
Beta Relaxes
Reproductivetract(male) Alpha Ejaculation Erection
Skin
Musclescausinghairerection Alpha Contracts
---Sweatglands Alpha Localizedsecretion Generalizedsecretion
Premotor neurons Autonomic output Visceral afferents Hormonal outputs Endocrine system Limbic system Behavioral responses Nucleus tractus solitarius Reflexes Central integration
Figure3 Diagramofthetwomaintypesofvisceral informa-tionprocessingbythecentralautonomicnetwork.Information originatingintheperipheryisprocessedandproduceseithera reflexresponseorintegratedautonomic,hormonaland behav-ioraloutput,theclassicexampleofwhichisthermoregulation inthehypothalamus.AdaptedfromLoewyandSpyer.15 increasingBP, while parasympathetic activation decreases HR,cardiac inotropism and stroke volume, reducing BP.42 Several receptor types are involved in the modulation of sympatheticand parasympathetic activity, including arte-rialbaroreceptors,cardiopulmonaryreceptorsandarterial chemoreceptors.6,43---48
Baroreceptorreflex
The baroreceptor reflex is the major mechanism for adjustingBP.Itisinitiatedbystimulationofarterial barore-ceptors, which are nerve endings found in vascular and
cardiac reflexogenicareas thataresensitive tostretching duringthecardiaccycle.Inbloodvessels,themost impor-tantbaroreceptors arelocated inthecarotidsinus,aortic archandmesentericcirculation.49Theaorticarchreceptors andmedullarcardiovascularcenterscommunicatethrough thevagusnerve,whileitisHering’snerve,abranchofthe glossopharyngealnerve,thatconveysinformationfromthe carotidsinusreceptors.50 Arterialbaroreceptorsplayakey roleinshort-termadjustmentsofBP,maintainingitwithina normalrangebyactingoncardiacoutput,peripheral resis-tanceandinotropism.51,52
Baroreceptorsrespondtothedistensionanddeformation thatlocalBPchangeselicitedbyphasesofthecardiaccycle induceinthevesselandthatchangethefrequencyofnerve impulsesthatarecarriedtotheNTS.53,54Changesin barore-ceptoractivityalsoaffectbreathing.Asanexample,invivo studiesonanesthetizedvagotomizeddogsshowedthatthe carotidbodychemoreceptorreflexresponsewaseliminated bysurgicallyexcluding thecarotidbodiesfromthecarotid sinusbaroreceptorarea.55
Baroreceptors are also involved in secretion of VP,56 particularly in response to hypotension, possibly due to neuronalprojectionsfromtheNTStothehypothalamus.57 Whenthebaroreceptorreflexisactivatedbyareductionin BP, anincreasein VPsecretionisobserved,58,59 andintact arterialbaroreceptorsareessentialtomaintainBPandVP secretion.60,61However,inthissituation,theiractionis rein-forced by facilitation of the chemoreceptor reflexin the NTS,duetothenatureofthestimulus.40
Severalstudieshaveexaminedtheroleofthe barorecep-torreflexinthelong-termregulationofsympathetic activ-ity, as central resetting of the baroreceptor-sympathetic reflexmay bean importantcomponentof themechanism causing sustained changes in renal sympathetic activity. However,littleisknownaboutthemechanismsthatcould causesuchresetting.62
Visceral afferents Electrolytes / hormones
NTS RVLM NA/DMV Area prostrema Parabrachial nucleus Periaqueductual gray Hypothalamus Amygdala Cortex pCO 2 and pH
2nd level convergence of visceral information
Autonomic and antinociceptive responses Lat HYP chronobiology (sleep-wake cycle) Medial HYP homeostasis (food, osmolality , etc) PVN neuroendocrine regulation
Autonomic, endocrine & motor (emotions) Cingulus autonomic responses to motivation Insula visceromotor control
Figure4 Centralautonomiccontrolareasandlevelsofinteractionofautonomiccontrol.DMV:dorsalmotornucleusofthevagus; HYP:hypothalamus;LatHYP:lateralhypothalamus; NA:nucleusambiguous;NTS:nucleustractussolitarii;PVN:paraventricular nucleusofhypothalamus;RVLM:rostralventrolateralmedulla.
Chemoreceptorreflex
Arterialchemoreceptorsarehighlyspecializedcellsthatcan detectchangesinthepartialpressureofoxygen(pO2), par-tialpressureofcarbondioxide(pCO2)andpHintheblood. Peripheral chemoreceptors aremore sensitive to changes in pO2 than in pCO2 or pH, while central chemoreceptors respondprimarilytochangesinpCO2andpH.42
Peripheral chemoreceptors are mainly located in the aortic and carotid bodies, but may also be found in the mesentericcirculation.Thecarotidbodiesarelocated bilat-erally at the bifurcation of the common carotid artery, while aortic bodies are located between the pulmonary arteryandtheaorticarch.50 Carotidbodiesaremore sen-sitive to hypoxia andhypercapnia, anddetect changesin blood gas tension, while aortic baroreceptors are more sensitivetoanemia,carboxyhemoglobinemiaandsystemic hypotension.63 Thus, carotid bodies monitor the venti-lation/perfusion ratio and aortic bodies are responsible for reflexcontrol of systemic vascular resistance.Central chemoreceptionwasinitiallylocalizedtoareasonthe ven-tralmedullarysurfacein theareaprostrema,but thereis substantialevidencethatmanysitesparticipateincentral chemoreception,somelocatedatadistancefromthe ven-tralmedulla.64
ChangesinthepartialpressureofgasesorpHdetected byperipheralchemoreceptorsaresenttotheNTSthrough the vagus or the glossopharyngeal nerve.65 Stimulationof chemoreceptorsincreases theactivity ofNTS cells.These cellssimultaneouslyexciteneuronsinthevagalnucleiand RVLM,leadingtoanincreaseinsympatheticand parasym-pathetic tone, which results in ventilatoryadjustments ---increasedair flow volume, respiratory rateand breathing volume --- that play an important role in the reflex con-trol of ventilation.66 In addition to ventilatoryresponses, chemostimulationalsoinduceschangesinthe cardiovascu-lar system such as tachycardia and vasoconstriction that maintainthechemicalcompositionofthebloodandtissue perfusionatoptimallevels.
It has been suggested that peripheral chemoreceptors haveatonicexcitatoryinfluenceoncardiovascularcontrol duetosympatheticactivation,thuscontributingto mainte-nanceofBPlevels.67
Cardiacreflexes
Therearealsovolumereceptorslocatedintherightatrium andvenaecavaethatrespondtobloodvolumedecreasesby reducingtheirfiringrates.Theafferentsofthesereceptors join thevagus nerve andterminate inthe NTS,synapsing withneuronsprojectingtothePVN.68Thispathwayis acti-vatedbychangesinbloodvolumeofaslittleas8-10%.69,70 Overall,activationofatrialreceptorsinducesinhibitionof sympatheticvasomotortoneandanincreaseinVPsecretion, affectingrenalfunction.
Other cardiac reflexes that regulate BP are the Bain-bridgeandBezold-Jarischreflexes.IntheBainbridgereflex, BPis indirectlyregulatedthroughHRchanges.Whenright atrial volume increases, low-pressure stretch receptors initiate a reflex that increases HR through sympathetic nerve activation.63 The Bainbridge reflex is not always
Table2 Summaryoftheprovocativemaneuversfor auto-nomicevaluationmostcommonlyusedinclinicalpractice. Cardiovascular Mixedmaneuvers:head-uptilt
(60◦/70◦);activestanding;Valsalva maneuver
Addressingmainlysympathetic branch:handgrip(isometric contraction),coldpressortest, mentalarithmetictest
Addressingmainlyparasympathetic branch:deepbreathing
Liquidmealchallenge Carotidsinusmassage Biochemical Plasmanoradrenaline,urinary
catecholamines;plasmarenin activityandaldosterone Pharmacological Noradrenaline---␣-adrenoceptors,
isoprenaline----adrenoceptors, atropine
Sudomotor Thermoregulatorysweattestfor centralregulation
Quantitativesudomotoraxonreflex testforquantitativelocalsweat evaluation
Siliconeimprintwithatropine iontophoresisforsemi-quantitative localsweatevaluation
Eye Schirmer’stest
Pupillaryfunctiontests Intraocularpressuretests
active,itseffectdependingonHR,beingstrongeratlower than at higher HR values. In this way, the Bainbridge reflexacts inoppositiontothebaroreceptorreflex,which increasesHRwhen stretchingis decreased in hypotension or hypovolemia.71 The Bezold-Jarisch reflex is a cardiac reflexthat issensitive tochemicals, evokingastrong car-diovasculardepressor responseleadingtobradycardiaand hypotensionasa directconsequence of chemical stimula-tionof receptorsintheventricles orcoronarycirculation. Thefall inBPis duetoboth bradycardia andvasodilation causedbyinhibitionofsympatheticvasomotoractivity,and isalsomodulatedbyreninreleaseandVPsecretion.72 Con-versely,decreasesintheactivityoftheseinhibitorysensory receptorsincreasesympatheticactivity,vascularresistance, plasmareninactivityandVPsecretion.72
Evaluation
of
the
autonomic
nervous
system
Therearevariousstandardprovocativeautonomic maneu-vers designed to test the ANS with stimuli ranging from suprathresholdto maximum intensity,in orderto observe evokedresponses intargetorgansin termsofpresence or absence,duration and magnitude (Table 2 and Figure5). Thesemaneuversshouldbeperformedinadedicated auto-nomiclaboratory,whichshouldhavecontrolledtemperature and humidity (20-23◦C and 25-35%, respectively) and an areaof∼20m2.Dependingonthetypeofassessment,each patientmayundergotwoor moredifferenttests.Alltests
Carotid sinus massage 1mV Time (s) -15 0 10 20 30 40 Pneumogram(V)` 125 25 5 4 0.5 0.4 ECG (bpm) mBP (mmHg) MSNA (V)
Figure5 Carotidsinusmassage.Thisfigureshowstherelationbetweencardiovascularvariables,ventilationandmuscle sym-patheticnerveactivityonsinusmassageinahealthysubject.ECG:electrocardiogram;mBP:meanbloodpressure;MSNA:muscle sympatheticnerveactivity.RochaandLaranjo,unpublisheddata.
shouldbeperformed undermedicalsupervisionby experi-encedtechnicians,whosetrainingiscriticaltothesuccess ofanyautonomictest battery.11,14 These techniciansmust befamiliarwithsudomotorfunction, electrocardiography, beat-to-beatBP,bloodflowrecordingsandcomputers,and mustbeabletoidentifyandsolvetechnicalproblemsand recognizethemain electrocardiographicabnormalities, as wellasbeknowledgeableinelectricalsafetyandbetrained incardiopulmonaryresuscitation.11,14
Patients undergoing autonomic evaluation should not consumefoodor tobaccoat least fourhours,and alcohol at least 12 hours, before the test, which should be per-formedduringthemorning,andshouldwearlightclothing. Medications,particularlythosethatdirectlyaffecttheANS, shouldbediscontinuedaccordingtothedrugs’half-lifeand thepatient’s condition.In viewofthe considerable intra-andinter-individual variability in data,normal values are set by each laboratory and shouldbe grouped by gender, ageanddecade of life.There aredifferentwaysof cate-gorizingautonomicteststhattakeintoaccountthetarget system,the typeof variables recordedand thedegree of invasion.Usually,duetothenatureoftherecordingdevices, mostmaneuverstarget thecardiovascular systemand are non-invasiveinnature.
AutonomicmaneuversandtheEwingtestbattery Theevaluationanddataanalysisprotocolsshouldbe appro-priate to the study. A standard and the most common evaluation protocol is the Ewing test battery,14 which
assesses HR response to deep metronomic breathing, BP responsetosustainedhandgripandBPandHRresponseto the Valsalva maneuver andactive standing.11,14,73,74 Other non-invasive maneuvers, including the cold pressor test, mentalstresstestandtilttable,arealsousedinautonomic evaluation.
IntheValsalvamaneuver,whichassessesthesympathetic andparasympatheticreactiontobaroreflexactivation,the subjectmaintainsan expiratorypressureof40mmHg/15s withanopenglottis.Thetestresponseisdividedintofour phases, two of which are reflexin nature (II and IV) and twomechanical(IandIII).The resultsdependonthe sub-ject’sposition,ageandgender,aswellasthedurationand intensityofexpiratorypressure.Inpatientswithautonomic dysfunction,thereis typicallya lossof bothBP overshoot andreflexbradycardia(Figure6).
The cold pressor test assesses sympathetic activation mediatedbynociceptors,whichisobservedmainlythrough BP changeswhenthehand isimmersedup tothe wristin ice-cold water at 4◦C. This test, which is predominantly sympathetic,differsfromthecold facetest,inwhich the application of a cold stimulus to the face stimulates the trigeminalnerveandelicits bradycardia,andisrelatedto thedivingreflex.The cold pressortest, themental stress test andthehandgriptest aretheprovocativemaneuvers mainlyusedforsympatheticevaluation.
On deep metronomic breathing, autonomic function is assessed with the patient breathing metronomically at a rateofsixcycles/minforthreeminutes,whichmaximizes respiratorysinusarrhythmia(Figure7).ChangesinHRwith deepbreathingareaparameterofparasympatheticcardiac
sBP (mmHg) HR (bpm) Control VM II I III IV 150 100 50 -15 0 15 30 45 Time (s)
Figure 6 The Valsalva maneuver. Data from anormal sub-ject,inwhichtheValsalvamaneuverhasfourphases:(I)there isabrief decreaseinheartrate(HR)andincreaseinsystolic blood pressure (BP) due to aortic compression; (II) BP first decreases andthen increases; (III) inthis phase, due to the releaseofstrain,consecutivedeclinesinBPandincreasesinHR areobserved,precedingaBPovershootduetopersistent sym-patheticactivity,togetherwithnormalizationofvenousreturn (IV).TheincreasedBP mediatesbaroreflex-induced bradycar-diaandisquantifiedusingtheValsalvaratio,theratioofthe highest HR(II) to thelowest HR(IV). Results dependon the position,ageandgenderofthesubject,aswellastheduration andintensityofexpiratorypressure.Inpatientswithautonomic dysfunction,thereistypicallyalossofbothBPovershootand reflexbradycardia.VM:Valsalvamaneuver.AdaptedfromXavier etal.74
control.11,14Thesubject’spositionandbodyweight,rateand depthof breathing,hypocapnia,sympatheticactivity,and use ofsalicylates andother drugs influence HRvariability duringdeepbreathing.
The Ewing battery uses the 30:15 ratio together with bloodpressureevaluationinordertoassesscardiovascular responsestoanactiveorthostaticchallenge.The30:15ratio iscalculatedastheshortestRRintervalaroundthe15thbeat dividedbythelongestRRintervalaroundthe30thbeatafter standing.SimultaneouslywithchangesinHRthereisa phys-iologicaldecreaseinBP.However,ifthisfallinsystolicBPis atleast20mmHgorindiastolicBPatleast10mmHgwithin 3minofstanding,theBPchangesaredefinedasorthostatic hypotension,asignofcardiovascularautonomicfailure.
Tilting SBP (mmHg) HR (bpm) Time (s) 0 15 30 45 60 75 90 140 120 100 80 100 80 60
Figure8 Normalheartrateandbloodpressureresponsesto head-uptilttesting.In thistestthesubjectliesonatilttest table,which isthen tilted upward at60◦-70◦ after aresting period of at least5 min. Patients with different degrees of cardiovascularautonomicimpairmentmayshowdelayed adap-tationtotheorthostaticchallengeormay evenbeunableto adaptandhaveasyncopalevent.HR:heartrate;SBP:systolic bloodpressure.AdaptedfromDucla-Soaresetal.73
Autonomicevaluationofactivestandingcanbe comple-mentedbythehead-uptilttest.Intheory,thisdetectsthe hemodynamicmodificationselicitedbybaroreceptorreflex activationwithout theinterferenceof themuscular pump ofthelegs.However,inpracticethisrarelyoccurs,as sub-jectsusually develop an alerting reaction when they see the bed beginning to tilt, which is superimposed on the changesinBPandHRcausedbybaroreflexactivation.The hemodynamicchangesassociatedwithhead-uptilt testing are considered to consist of two stages: an initial acute cardiovascular response lasting around 30 s, and a stabi-lizationphase consisting of an adaptation period 1-2 min afterorthostasisfollowedbya lateresponsetoprolonged orthostasislastingmorethan5min11,14(Figure8).
Invasive
and
biochemical
techniques
applied
to
autonomic
evaluation
Among the methods for measuring patients’ sympathetic nervous system activity are tests that assess individual sympathetic nervous outflows, such as microneurogra-phy and measurement of norepinephrine (NE) spillover to plasma from the sympathetic nerves of individual
Deep breathing Time (s) 200 RR (ms) 500 1500 0
Figure7 Heartrateresponsetodeepbreathing.Anormalresponseshowingtheimprintofrespirationintheheartraterecording duringdeepbreathing.AdaptedfromDucla-Soaresetal.73
Table3 Summaryoftime,frequencyandmodelingmethodsofbaroreflexsensitivityassessment.
Method Briefdescription
Timedomain Sequencestechnique108,109 BRSasthemeanoftheslopesbetweenSBPandRRvaluesineach identifiedbaroreflexsequence,consideringSBPwitha1-beatlag Dualsequencemethod110 Equivalenttothesequencestechnique,identifyingbaroreflexsequences
consideringSBPandRRwithashiftofupto3beats
xBRS111 BRSastheslopebetweenSBPandRRvaluesover10-swindowschoosing theshift(upto5beats)thatmaximizesSBPandRRcross-correlation.SBP andRRseriesresampledat1Hz
Eventstechnique112 BRSasoneglobalslopebetweenSBPandRRvaluesinallidentified baroreflexevents,consideringSBPwitha1-beatlagwithrespecttoRR Frequencydomain Transferfunction113 BRSasthemeanmagnitudeofthetransferfunctionbetweenSBPandRRin
theLFband
Alphatechnique114 BRSasthesquarerootoftheratiobetweenRRandSBPpowersintheLF band
Model-based Closed-loopbivariate115 QuantificationoffeedbackandfeedforwardSBPandRRpathways, assumingaclosed-loopSBPandRRsystem
Closed-looptrivariate116 QuantificationoffeedbackandfeedforwardSBPandRRpathways, consideringtwo-waypathwaysbetweenSBP,RRandrespiration xAR117 QuantificationoffeedbackandfeedforwardSBPandRRpathways
consideringrespirationasanexogenousinputintheSBPandRRloopand assumingaclosed-loopSBPandRRsystem
Causalanalysis118 QuantificationofBRSusinganexogenousinputmodeltodivideRR variabilityintoSBP-relatedand-unrelatedparts
BRS:baroreflexsensitivity;LF:low-frequency;SBP:systolicbloodpressure.
organs.75,76 Alternatively, overall sympatheticactivity can beassessed by analysisof plasma or urine catecholamine concentrations.77,78
Microneurography providesseparate recordingsof sym-patheticnerveactivity (SNA)trafficin muscle(MSNA)and skin(SSNA).MSNAreflectsthevasoconstrictorsignaltothe skeletal muscle vasculature. It is highly sensitive to BP changes and is regulated by both arterial and cardiopul-monaryreflexes. Thesereflexes do notaffect SSNA. SSNA reflectsvasomotorneuraltraffictoskinbloodvesselswith almostnosudomotoractivity.Thetworecordings(MSNAand SSNA)differsignificantlywithregardtomorphology.Studies haveshownthatmeasurementofsympatheticnerveactivity fromperipheralnervesissafe,accurateandreproducible. Furthermore,ithas been shown thatrecordingsfromone limbcanbereliablyassumedtoreflectrecordingsof sympa-theticnerveactivitytothemusclevascularbedthroughout thebody.Themethod’squantitativenatureisalsoa signifi-cantadvantage.79,80
Assessmentof sympatheticactivitybasedonplasma or urineNEconcentrationhassignificantlimitations, asNEis subjecttochangeablereuptakedependentonthedensityof thebasilarplexusandbloodflowvelocityinaspecificorgan. Moreover, circulating NE represents only a small fraction (5-10%) of the quantity of the neurotransmitter secreted fromnerveterminals.80MeasurementofplasmaNEis, how-ever,animprovementoverassessmentofurineepinephrine, NEandtheirprecursorsandmetabolites, whichwas tradi-tionallyusedtoassessANStone.79,80
The NE spillover rate has advantages over the above-mentioned methods, since it assesses NE release from specifictargetorgans.TheNEradiolabeledmethodisbased
on intravenous infusion of small amounts of tritiated NE; tissueclearanceofthis substanceis thensubtractedfrom plasmaNEvalues,andtheremainderisamarkerofspillover of the neurotransmitter from neuroeffector junctions. In steady-stateconditionsthisspilloverreflectsthesecretion ofNEfromsympatheticnerveterminals.79,80Invasive tech-niquesmeasuretotalbodyandregionalNEspilloverinthe heart,splanchnicandrenalcirculations,andthebrain.81
Variousmethodsareusedforexperimentalquantification ofsympatheticactivityinanimals.75,82,83Directrecordingsof SNA(e.g.renalorlumbar)arecommonlyobtainedinanimals bythesurgicalimplantationofrecordingelectrodesintothe appropriatesympatheticfibers.84
Evaluation
of
baroreflex
function
Baroreceptorfunctionisoneofthemostimportant mecha-nismsregulatingmoment-to-momentBP.Itcanbeassessed bybaroreflexsensitivity(BRS)teststhatrelate changesin heart periodto a BP change. BRS can be assessed under dynamic or steady-state conditions,usingphysiological or pharmacological approaches. The most common methods include vasoactive drugs (Oxford method), the Valsalva maneuver, the neck chamber technique and analysis of spontaneous BP and HR fluctuations. The Oxford method usesphenylephrine(analpha-1adrenergicreceptoragonist) to induce a rapid increase in BP (15-40 mmHg) together withHRchanges.ModificationsoftheOxfordmethodassess BRSthroughsequentialinjectionsofdepressorandpressor drugs. Thereis somecontroversy withphenylephrine con-cerningtheselectivityofthereflexarctarget,sinceother
reflex arcs, particularly the arterial chemoreceptor and thepulmonarymechano-andchemoreceptors,canalsobe activated.Applyingnegativeorpositivepressuretotheneck selectively activatescarotidbaroreceptors andcan actas anexcitatory orinhibitory stimulusdependingonwhether positiveornegativepressureisapplied.
Computer-basedtechniques(Table3)assessBRSby cor-relating spontaneous fluctuations of BP with consecutive HRchanges.Thesecomputationalmethodscan bedivided into time domain, frequency domain and model-based approaches.Time(sequence)andspectraltechniqueshave provenreliabilityandhavebecomeastandardtoolinmany autonomictesting devices.BRScan bedeterminedby the sequentialmethod.85ThismethodsearchesrampsofBPand RR. A ramp defines a variation of at least 1 mmHg and 4 ms between adjacent values of BP and RR, respec-tively.This concept canonly beapplied tothreeor more cardiac cycles varying monotonically, either increasing or decreasing.WhenaBPrampoccursatthesametimeasan RRramp,aBRSeventisidentified.BRScanbedetermined bythemeanBRSslope:RRI(ms)/SBP(mmHg),whereRRI is RRinterval.86 Asteeper slope indicateshigh BRS,while a shallower slope indicates lower sensitivity. The barore-flexeffectivenessindex(BEI)istheratiobetweenthetotal numberofBRSeventsandtotalnumberofpressureramps, increasingor decreasing,foragivenperiod.BEIisan indi-catoroftheeffectivenessofbaroreceptor-mediatedcardiac regulation.
Analysis
of
variability
of
biological
signals
The fact that the rhythm of physiological signals is not entirely regular has raised the possibility of extract-ing an autonomic signature from these fluctuations using signal-processing techniques. Extremely complex neural mechanismsare responsible for these fluctuations, based mainly on interactions between the sympathetic and parasympatheticnervoussystem.Theythereforerepresent arichsourceofinformationthatcanprovideconsiderable insightintothemechanismsofcardiovascularcontrol.87---93 Cardiovascular signals, in particular HR, are most com-monly used. However, as in any biological assessment in whichthe environmentaffectsthe result,standardization is a problem, mainly because most autonomic evaluation isperformedwithoutaprofoundknowledgeofmethodsor physiology,whichcanleadtoconfoundingdataand misinter-pretationofphysiological phenomena.Nonetheless,signal processingmethods,whencorrectlyused,areanimportant toolfor identifying autonomic markers and for improving treatmentandpatientfollow-up.Signalprocessingcanbeappliedinatleastthreedomains --- time, frequency and time-scale --- which individually or together can identify different pathological response profiles, suchasdelays in adaptive responses to provoca-tivemaneuvers,dyssynergybetweenBPandHRresponses, and/orexaggeratedresponsessuchasorthostatic hypoten-sion,posturalorthostatictachycardiaandsyncope.94
Tilting Tilting Systogram (mmHg) LF HF WT (mmHg2) HHT (mmHg2) 50 s 0 x10 3 x10 3 5 0 5 60 130
Figure9 Signalprocessingofbiologicalsignals.Left:data fromanormalsubject;right:datafromapatientwithparoxysmal atrialfibrillation.Thesamesignal,thesystogramobtainedfromsystolicbloodpressure,hasbeentreatedwithwavelets(Db12) andtheHilbert-Huangtransform.Thedifferencesintheautonomicoutputofthetwoindividualscanbeclearlydistinguished.Note thatthisexampleismerelyillustrative,asthetachogramobtainedfromtheRRintervalforbothpatientsisnotshown.HF:high frequency;HHT:Hilbert-Huangtransform;LF:lowfrequency;WT:wavelets.
Normal profile HF HF HF LF LF LF 1 1 1 0 0 0 F requency (Hz) F requency (Hz) baseline baseline baseline TILT adapt coherence high low TILT TILT
TILT adapt TILT adapt
TILT
300s
300s 300s
Figure10 Autonomicanalysistoolsusedtoshowreverseautonomicmodulationinpatientswithreflexsyncope.Left:changes inwaveletcoherenceevokedbyatiltmaneuverinanormalsubject.Aftertilting(verticalline)thereisafallincoherence,which reachesitsminimumvalue approximately20 saftertilting,recoveringlater toasignificantlylowervalue thanbaseline;right: modificationofcoherenceofheartrateandsystolicbloodpressurevariabilityduringatilttrainingprogram designedtoinduce autonomicremodelinginpatientswithreflexsyncope(A:baselineconditionsbeforetrainingprogram;B,CandD:1st,4thand9th tilt-trainingsessions,respectively).Theincreaseincoherenceoverthecourseofthetrainingsessions,associatedwithincreased baroreceptorremodeling,isreflectedinimprovementinbandorganizationtogetherwithmoreintenseorange/redcolor.These changesaremoreclearlyseenaftertilting(verticalline).AdaptedfromLaranjoetal.107
Inparticular,fastFouriertransform(FFT)and autoregres-sivespectralanalysisappliedtoHRandBPsignalshavemade an importantcontribution to autonomic evaluation.87,94---97 FFT,usingsinusfunctionsofdifferentfrequenciesand ampli-tudes,decomposesthesignalstoproduceapowerspectrum inwhichforhumansubjectsthreemajorfrequencyranges can be recognized: very low frequency (VLF; <0.04 Hz), lowfrequency (LF;0.04-0.15Hz) andhigh frequency (HF; 0.15-0.4Hz).98TheVLFbandisbelievedtoberelatedto non-neural factors,such astemperature and hormones.99 The HFbandisdominatedbytheparasympatheticsystem,98,100 whiletheLFbandisbelievedtobemediatedbybothcardiac sympatheticandparasympatheticnervousoutflows.
Guzzetti et al. reported that patients with essential hypertension are characterized by greater LF power and smallerHFpowerofRRintervalvariabilityduringsupinerest comparedwithnormotensivesubjects.101Theyalsoreported thatthe powersshoweda smallerincrease anddecrease, respectively,duringpassivetilting.Theseobservationswere interpreted as indicating that cardiac sympathetic tone is increased and cardiac vagal tone and modulation are decreasedinessentialhypertension,aconclusionthatisin agreementwithpreviousstudiesinwhichautonomiccardiac modulationwasinvestigatedbydifferenttechniques.102,103 FFT analysis, however, has important limitations, as it requiresastationary signal andalong periodofdata col-lection,ofatleast5min.Also,itcannotlocateandfollow changesinafrequencyovertime.
Toovercomesomeoftheselimitations,likeits applica-tiontononstationaryandnonlinearsignals,awavelet-based methodologyhasbeenproposedtodeterminetheevolution
over time of LF and HF frequencies.73 Wavelet analysis is a linearnon-stationary representation of signals in the timeandfrequency domainsinwhichtheoriginalsignal is decomposedinashiftedversionofabasicfunction,called themotherwavelet,withadifferentbasescale.Amother waveletfunctionisanon-periodic,oscillatoryfunctionthat begins and ends at zero in the timedomain.104 However, althoughagoodalternativetoFFT,waveletslackresolution, particularlyat low frequencies (Figure9).Wavelet coher-encecanalsobeusedtoanalyzethedegree ofautonomic remodelinginpatientsunderspecifictherapeuticschemes (Figure10).
TheHilberttransformisalinearoperatorableto deter-minetheinstantaneousfrequencyofasignal,corresponding tothe convolution of the input signal withthe kernel. In order for amplitude, frequency and phase to have phys-iological application, the signal to be transformed must have an instantaneous null DC component.105 Recently, Huang proposed fulfilling this condition throughempirical mode decomposition(EMD),whichcan beappliedto non-linearandnon-stationaryprocesses.Thecombinationofthe HilberttransformwithEMDresultsinwhatisknownasthe Hilbert-Huangtransform.
Conclusion
The ANS has moved towards center stage in cardiovas-cular medicine. Dysregulation of this system contributes to cardiovascular disease, including hypertension, atrial fibrillation and other cardiac arrhythmias,ischemic heart
disease, obesity, diabetes, atherosclerosis, sleep apnea, metabolic syndrome and congestive heart failure, and is oftenassociatedwithamoreseverediseaseburden. How-ever, there are serious gaps in our understanding of ANS function,andtreatmentoptionstargetingtheANSarestill in their infancy. It is important tobegin by performing a thoroughassessmentoftheANSasitrelatestothe cardio-vascular system. There are asyet no gold standard tests forautonomictestinganddifferencesintestperformances are an obstacle to comparing scientific and clinical data acquiredindifferentlaboratories.Withthisreview,wehope toprovidevaluablehelp byfocusingonstandard testsfor diagnosisofcardiovascularautonomicdysfunction.
Conflicts
of
interest
Theauthorshavenoconflictsofinteresttodeclare.
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