Embolism.docx

Embolism.docx

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Embolism

7.Embolism

Byembolismismeantthetransferenceofabnormalmaterialbythebloodstreamanditsimpactioninavessel.Theimpactedmaterialiscalledanembolus.Inmostcasesitisafragmentofthrombus（thrombo-embolism）althoughfragmentsofmaterialfromulceratingatheromatousplaquesoftheaortaquitecommonlyformemboliindistalarteries.Afragmentofatumourgrowingintoaveinmayalsobreakoffandformanembolus,andtheremaybeembolismofthecapillariesbyfatglobules,bubblesofairornitrogenandevengroupsofparenchymalcells.Thesiteofembolismwill,ofcourse,dependonthesourceoftheembolus.Thusembolismofthepulmonaryarteriesandtheirbranchesissecondarytothrombosisinthesystemicveinsorintherightsideoftheheart.Rarely,wherethereisapatentforamenovale,anembolusmaypassfromthefightsideofthehearttotheleftatriumandthusbecarriedtotheysystemiccirculation;（crossedorparadoxicalembolism）.Withthisrareexception,embolioccurringinthesystemiccirculationarederivedfromthrombiformedintheleftsideoftheheart,e.g.thromboticvegetationsontheaorticandmitralvalves,andfromthrombiordetachedportionsofatheromatousplaquesintheaortaorlargearteries.Embolicarriedfromtributariesoftheportalveinlodge,ofcourse,intheportalbranchesintheliver.

Effectsofembolism

Systemicarterialemboli.Theresultsaresimplythoseofmechanicalpluggingandvaryaccordingtothesieoftheembolus.

Pulmonarythrombo-embolismisaverycommoneventinpatientswithacuteorchronicdebilitatingdesease.Inmostinstancesitresultsfromdetachmentofthrombusformedintheveinsofthelowerlimbsormuchlesscommonlyinthepelvicorothersystemicveinsortherightsideoftheheart.Detachedthrombusiscarriedinthevenousbloodthroughtherightsideoftheheartandimpactsinthepulmonaryarterialbed.

Theeffectsofpulmonaryembolismdependmainlyonthedegreeofocclusionofthepulmonaryarterialbedandonthestateofthepulmonarycirculation.Alargethrombus,extendinguptothefemoraloriliacvein,mabecomedetachedenmasseandblockthemainpulmonarytrunkorbothofitsbranches,causingalmostinstantaneousdeathbyarrestingthecirculation.Lessmassivebutconsiderableembolismcausesacuterightventricularfailurebyincreasingtheresistancetopulmonarybloodflow.Thisusuallyresultsfromocclusiongofmorethanhalfthepulmonaryarterialbedinpreviouslyhealthyindividuals,butisprecipitatedbylesserdegreesofembolisminpatientswithpulmonaryhypertensionorincipientheartlailure.Occluaionofmedium-sizedorsmallpulmonaryarteriesisusualywithouteffectonthepulmonarycirculationunlessmanyvesselsareoccludedbyashowerofsmalleremboli.Multiplesmallembolioveraperiodofmonthsoryearsveryoccasionallycausechronicpulmonaryarterialhypertension.

Theeffectsofpulmonaryembolismonthelungtissuesuppliedbytheoccludedarterydependverylargelyonthegeneralstateofthepulmonarycirculation.Ifthisisnormal,obstructionofmedium-sizedpulmonaryarteriesdoesnotresultininfarction,butifthecirculationisimpaired,e.g.byheartfailure,infarctionmayresult.

Experimentalstudieshaveshownaremarkablecapacityforremovalofpulmonarythromboemboli.Emboliarepartlydigestedandpartlyremovedbyorganisation,leavingapatchoffibrousthickeingoftheveinwall.Inmannalsothereisevidencethatmanypulmonarythromboemboliareremovedwithlittlelossofpulmonaryvascularbed.Apartfromtheraredevelopmentofpulmonaryhypertensionduetorecurrentsmallemboli,themaindangersaretheimmediateobstructiveeffectsoflargeemboliandthetendencyforvenousthrombosis,andthuspulmonaryembolism,torecur.

Theincidenceofpulmonaryembolismatautopsydependsonthecarewithwhichthelungsareexamined.Inmostcasestheemboliaresmall,andwereoftenunsuspectedclinically.Withoutdoubt,mostofthemofriginatefromthrombiwhichhaveformedasaterminalevent,andwouldhaveundergonefibrinolysishadthepatientsurvived.

Septicemboli.Withthewidespreaduseofantibiotics,septicemboli,containingpyogenicbacteria,havebecomerelativelyuncommon.

Largesepticembolioriginateusuallyfromsepticthrombosisinaveininvolvedinasuppuratinginfectionorfromthevalvesoftheheartinbacterialendocarditis.Wheresuchanembolusimpacts,theeffectsareduetoloaclischaemiaandsepsis,andacombinationoftissuenecrosisandsuppurationresult.Thewallofthearteryatthesiteofimpactionisweakenedbysuppurationandmaybecomelocallydilated:

itmaysubsequentlyrupture.Invarioussepticaemicandpyaemicconditions,thepluggingofarteriolesandcapillariesbyminutefragmentsofsepticthrombusoraggregatesofbacteriaproducessmallbaemorrhagiclesionswhichmaysuppurate.

Fatembolism.Entranceofglobulesofneutralfatintothecirculationprobablyoccursafterallbonefractures.Mostofthefatisarrestedinthesmallvesselsinthelungs,wheretheglobulesfusetoformcolumnsoffat.Inmostcasesthisissymptomlessandthefatisremoved,butinasmallproportionofpatientswithmajorfracturesofthelongbonesorlimbgirdles,andparticularlywhenfattytissueislacerated,alargeamountoffatentersthecirculationandthefatembolussyndromemaydevelopduringthefollowingthreedays.Itsmainfeaturesarementalconfusion,fever,dyspnoeaandtachycardia,apetechialrashandsometimescyanosis,haemoptysis,comaanddeath.Thesymptomsareattributablelargelytohypoxiaresultingfrompulmonaryfatembolismwhichiscomplicatedbypulmonaryoedemaandhaemorrhages.Someofthefatpassesthroughthepulmonarycirculationandimpactsincapillareisthroughoutthesystemiccirculation,whereitcauseswidespreadpetechialhaemorrhagesintheskinandvarioustissues.Infatalcases,fatemboliareseeninthecapillariesinmanytissuesandpericapillaryhaemorrhagesandminuteinfarctsareoftenfoundinthebrain,principalyinthewhitematter.Thrombocytopeniadevelopsinsomepatientsandmaycontributetohaemorrhages.However,mostpatientsrecoverwithoutresidualdisability.

Fatembolismmayalsocomplicatetraumaoffattytissueandofagrosslyfattyliver,severeburns,majorsurgery,acutepancreatitis,andindecompressionsickness（seebelow）.

Airembolism.Thepressureinveinssituatedabovetheleveloftherightatrium（e.g.intheheadandneckwhenupright）isbelowthatoftheatmosphereandwhenawoundinvolvesthewallofsuchavein,airmaybesuckedintoitandpassintothecirculation.Thisisparticularlyliabletohappeninneckwoundsinvolvingthemajorveinsandisadangerincardiothoracicsurgery.Itmayhappenalsowherepositivepressureisusedinvenousorarterialcatheterisationandinvenousinfusionofbloodorfluids.Smallvolumesofairenteringthecirculationarerapidlyabsorbedwithoutilleffect,butvolumesofover100mlmaycauseacutedistressand3—moormoremaybefata.Suchlargevolumesbecomechurnedupwithbloodintherightsideoftheheart:

thefrothisnoteasilyexpelledandblocksthepulmonaryarterialcirculation.

Decompressionsickness

Theamountsofinhaledgaseswhichpassintosolutioninthebodyfluidsandtissuesareproportionaltotheatmosphericpressures,andwhenthisisreducedrapidly,someofthedissolvedgasescomeoutofsolution（justasafizzydrinkeffervescesonopeningthebottle）andformbubblesinthebloodandtissues.Thismayoccurwhendiversarebroughttothesurfacetooquickly,whenworkersincompressedair,e.g.tunnelsandcaissons,arerestoredrapidlytoatmosphericpressure,andfollowingrapidascenttohighaltitudesinanunpressurisedaircraftorsuddenlossofpressureinapressurisedsircraftathighaltitude.

Withtheincreasingpopularityofscubadivingasasportandincreaseinnavalandindustrialdiving,decompressionsicknesshasincreasedinimportance.Ifairhasbeenbreathed,thegasbubblesconsistlargelyofnitrogen,whichislessreadilyre-absorbedthanoxygenandcarbondioxide.Bubblesformintheblood,bodyfluidsandtissues,andparticularlyinfattytissue,whichcanabsorbahighconcentrationofnitrogen.Thebubblesofgasenlargeandmaycoalesceand,inadditiontotheirmechanicaleffect,thoseinthebloodmayinduceimportantchangesatthegas/bloodinterface.Theselatterincludedenaturationofplasmaproteinswithconsequentsludgingoftheredcellsandincreasedviscosityoftheblood.Hagemanfactormaybeactivatedattheinterfaceandplateletaggregationandthereleasereactionmayoccur,followenbyactivationoftheclotting,fibrinolytic,kininandcomplementsystems.Inseverecases,thesechangescanleadtodisseminatedintravascularcoagulation,thrombocytopeniaandreleaseofvaso-activeagentswhichincreasevascularpermeabillty,causingleakageoffluidandproteinsfromthesmallvessels,haemoconcentrationandastateofshock.

Symptomsofdecompressionsicknessmaydevelopduringascentofadivertothesurfaceoratanytimeupto36hoursaftercompletingadive.Inmostcases,theonsetiswithin12hours,andthereisageneralinversecorrelationbetweenthelatentperiodandtheseverityofsymptoms,althoughthereareindividualvariationswithinthisrelationship.

Symptomsvarygreatly,dependingonwherethegasbubblesaccumulate.Thecommonest（andmildest）formconsistsofasymmetricalcrampsoflimbmuscles（the‘bends）andpainaroundoneormorelargejointsusuallytheshoulderorknee.Skinmanifestationsincludeitching,patchyerythema,cyaosisandoedema.Accumulationofbubblesinthelungsmayresultin