Clinical+Diagnosis+4Word文件下载.docx

Clinical+Diagnosis+4Word文件下载.docx

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Clinical+Diagnosis+4@#@CARDIACCONDUCTIVITYANDAUTOMATICITY@#@Thespeedwithwhichtheelectricalimpulsesareconductedthroughdifferentpartsoftheheartvaries.Forexample,conductionspeedorslowestthroughtheAVnodeandfastestthroughthePurkinjefibers.TherelativelyslowconductionspeedthroughtheAVnodeisoffunctionalimportancebecauseitallowstheventriclestimetofillwithbloodbeforethesignalforcardiaccontractionarrives.@#@Inadditiontoconductivitytheothermajorelectricalfeatureoftheheartisautomaticity.Automaticityreferstothecapacityofcertainmyocardialcellstofunctionaspacemakers,tospontaneouslygenerateelectricalimpulsesthatspreadthroughouttheheart.Normally,asmentionedearlier,thesinusnodeisthepacemakeroftheheartbecauseofitsinherentautomaticity.Underspecialconditionsothercellsoutsidethesinusnode（intheatria,theAVjunction.Ortheventricles）canalsoactasindependentpacemakers.Forexample,asmentionedbefore,iftheautomaticityofthesinusnodeiddepressed,theAVjunctionmayfunctionasanescapepacemaker.Inotherconditionstheautomaticityofpacemakersoutsidethesinusnodemaybecomeabnormallyincreased,andtheseectopic（non-sinus）pacemakersmaycompetewiththesinusnodeforcontroloftheheartbeat.EctopyisdiscussedindetailinPartIIofthisbook（inthesectiononcardiacarrhythmias）.@#@Ifyouunderstandthenormalphysiologicstimulationoftheheart,youhavethebasisforunderstandingtheabnormalitiesofheartrhythm（arrhythmias）andconductionthatproducedistinctiveECGpatterns.Forexample,failureofthesinusnodetostimulatetheheartproperlymayresultinvariousrhythmdisturbances,suchassinoatrialblock（SAblock）...Similarly,blockageofthespreadofthestimulusthroughtheAVjunctionproducesvariousdegreesofAVheartblock.Diseaseofthebundlebranchesmayproduceleftorrightbundlebranchblock..Finally,anydiseaseprocessthatinvolvestheventricularmuscleitself（forexample,destruction）alsoproducesmarkedchangesinthenormalECGpatterns.@#@11.BasicECGWaves@#@DEPOLARIZATIONANDREPOLARIZATION@#@Wehave.usedthegeneralterm“electricalstimulation”torefertothespreadofelectricalstimulithroughtheatriaandventricles.Thetechnicaltermforthiscardiacelectricalstimulationisdepolarization.Thereturnofheartmusclecellstotheirrestingstatefollowingstimulation（depolarization）iscalledrepolarization.Thesetermsarederivedfromthefactthatthenormalmyocardialcells（atrialandventricular）arepolarized;@#@thatis,theycarryelectricalchargesontheirsurface.Therestingpolarizedstateofanormalheartmusclecell.Noticethattheoutsideoftherestingcellispositiveandtheinsideoftherestingcellispositiveandtheinsideofthecellisnegative（about–90mV）.@#@Whenaheartmusclecellisstimulated,itdepolarizes.Asaresult,theoutsideofthecell,intheareawherethestimulationhasoccurred,becomesnegative,whiletheinsideofthecellbecomespositive.Thisproducesadifferenceinelectricalvoltageontheoutsideofthecellbetweenthestimulateddepolarizedareaandtheunstimulatedpolarizedarea.Asaresult,asmallelectricalcurrentisformed.Thiselectricalcurrentspreadsalongthelengthofthecellasstimulationanddepolarized.Thepathofdepolarizationcanberepresentedbyanarrow.Fforindividualmyocardialcells（fibers）depolarizationandrepolarizationproceedinthesamedirection.However,fortheentiremyocardiumdepolarizationproceedsfrominnermostlayer（endocardium）tooutermostlayer（epicardium）whilerepolarizationproceedsintheoppositedirection.Themechanismofthisdifferenceismotwellunderstood.@#@ThisdepolarizingelectricalcurrentisrecordedbytheECGasaPwave（whentheatriaarestimulatedanddepolarize）andasaQRScomplex（whentheventriclesarestimulatedanddepolarize）.@#@Afteraperiodoftime,thefullystimulatedanddepolarizedcellbeginstoreturntotherestingstate.Thisisknownasrepolarization.Asmallareaontheoutsideofthecellbecomespositiveagain.Therepolarizationspreadsalongthelengthofthecelluntiltheentirecellisonceagainfullyrepolarized.VentricularrepolarizarionisrecordedbytheECGastheSTsegment,Twave,andUwave.（Atrialrepolarizationisusuallyobscuredbyventricularpotentials）@#@TheECGrecordstheelectricalactivityofalargemassofatrialandventricularcells,notjusttheelectricalactivityofasinglecell.Sincecardiacdepolarizationandrepolarizationnormallyoccurinasynchronizedfashion,theECGisabletorecordtheseelectricalcurrentsasspecificwaves（Pwave,QRScomplex,STsegment,Twave,andUwave）.@#@Tosummarize,regardlessofwhethertheECGisnormalorabnormal,itmerelyrecordstwobasicevents:

@#@

（1）depolarization,thespread