Andersons pediatric cardiology 842
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unrestrictivedefects,obstructionofthepulmonaryoutflowtractinthelargerof
therestrictivedefectswillalsohavealimitingeffectonthemagnitudeofflow.
Therewillbeareversalinthedirectionoftheshuntinallcaseswhereright
ventricularpressureexceedspressureintheleftventricle.Althoughpulmonary
valvarobstructionmaybepresentunequivocallyfrombirth,subvalvar
obstructioncanbeacquired.
ResponseoftheHearttoFlowBetweenthe
Ventricles
Thecardiaceffectsofaventricularseptaldefectdependinitiallyonthe
magnitudeofflowtothelungs.Withfloridpulmonaryflow,usuallyinthe
settingofunrestrictivedefects,leftatrialandleftventricularend-diastolic
volumesareincreasedandleftventricularmusclemassisalsoincreased.Studies
ofpressureandvolumeintheleftventricledemonstratethemarkedincreasein
leftventricularworkimposedbyalargedefect.Thisnecessitatesleftventricular
hypertrophyasacompensatorymechanism.Withextremepulmonaryflow,there
isalsoanincreaseinrightventriculardimensions.Becauseoftheelevatedright
ventricularpressure,therewillbeanevenmoremarkedincreaseinright
ventricularwork.Thisresultsinadditionalrightventricularhypertrophyand
maybeonemechanismforthedevelopmentofsubvalvlarpulmonarystenosis.
Leftventricularworkisincreasedbyrestrictivedefectsinrelationtotheflow
tothelungs,buttherightventricleisrelativelyspared.Consequentlythereisleft
ventricularhypertrophybutlittleincreaseinrightventricularsizeormuscle
mass.Withelevationofpulmonaryvascularresistanceordevelopmentofright
ventricularoutflowobstruction,leftventricularworkisdiminishedbecauseof
thedecreaseinleft-to-rightshuntingandpulmonaryflow.Theelevationofright
ventricularpressureresultsinrightventricularhypertrophy,whichdominatesthe
picture.
Thepathophysiologiceffectsofaventricularseptaldefectcan,inthree
situations,resultincongestivecardiacfailure.Thefirstisanunrestrictivedefect
withhighpulmonaryflowwhenthecompensatorymechanismsareinsufficient
toprovideanadequatesystemicflow.Thesecompensatorymechanismsinclude
recruitmentofsarcomerestooperateatanoptimalend-diastoliclength,muscular
hypertrophyofrightandleftventricles,andincreasedcatecholaminedrivetothe
ventricles.Thosecircumstancesareusuallyreachedbeforetheageof6months.
Thesecondsituationoccursmuchlater.Itisencounteredwhentheright
ventricularmyocardiumhasundergonedegenerativechangesasaconsequence
oflong-termejectionagainsthighresistance.Thisisfoundwithpulmonary
vasculardiseaseandwithprolongedandsevereobstructiontotheright
ventricularoutflowtract.Thethirdsituationiswhenaventricularseptaldefectis
complicatedbyaorticregurgitation,imposingafurthervolumeloadontheleft
ventricleandoutstrippingthecompensatorymechanisms,asdiscussedearlier.It
ismostfrequentlyseeninolderchildrenandadults.
ClinicalFeatures
Presentation
Thetypicalmurmurofaventricularseptaldefectisrarelyheardatbirth,sinceit
takestimeforthepulmonaryvascularresistancetofallfromthehighlevelsof
fetallife—whicharesufficientlyhightolimittheflowthroughevenan
unrestrictivedefect—tolevelsinsufficienttoprovideanaudiblemurmur.The
diagnosiscannowbemadeduringfetallifebymeansofechocardiographic
screening.Evenwhencasesarediagnosedduringfetallife,itisnecessaryto
exercisecaution,sincemanysubsequentlyclosespontaneously.Veryfew,ifany,
ofthosediagnosedduringfetallifewillhavethefullclinicalpictureatbirth.If
diagnosisismadeintheneonatalnursery,itisalmostinvariablyduring
examinationofthebabyjustpriortomaternaldischarge.Attheendofthefirst
weekoflife,whenmanymurmursaredetected,itisrareforthepulmonary
vascularresistancetohavefallensufficientlyforthechildtobesymptomatic.
Symptoms,whentheyoccur,arerelatedtothehighflowofbloodtothelungs,
diminishedpulmonarycompliance,andelevationoftheleftatrialpressureasa
resultofthehighpulmonaryflow.Thisinitiallycausestachypnea,which
progresseswiththeonsetofcongestivecardiacfailuretoproducedyspneaon
effort,mostobviouswhenthebabyfeeds.Inababywithatypicalunrestrictive
orlargerestrictivedefect,theparentsarelikelytocomplainthatfeedstake
progressivelylongeroverthefirstmonthortwooflifeandleadtorapid
exhaustionofthebaby,oftenwithprofusediaphoresis.Notonlyisthefeeding
prolongedbutitdoesnotprovidesufficientcaloricintakeforweightgain.With
thedevelopmentofseverecardiacfailure,thebabywillbecomeobviously
dyspneicevenatrest.Intercostal,subcostal,andsupraclavicularretractionsare
readilyseen.Thisincreasedworkofrespirationimposesanincreased
requirementforenergy,whichisnotsatisfiedbecauseofthedifficultyof
feeding,aviciouscycleresultinginfailuretothrive.Failuretothriveis,indeed,
analternativemodeofpresentationininfancy.Itoccursinthosebabieswith
defectslargeenoughtoallowhighpulmonaryflowbutinwhomthesizeofthe
defect,orthelevelofpulmonaryvascularresistance,preventsthedevelopment
ofintractablecongestivecardiacfailure.Anotherassociationwithhigh
pulmonaryflowisanincreasedsusceptibilitytorespiratoryinfection,suchas
frequentviralupperrespiratoryinfections.Itmaybeduringsuchanepisodethat
