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questionableifangiographyisnowrequiredfordiagnosis.Interpretationofthe
images,considereddifficultinthepast,shouldnowbeverymucheasier.Evenin
potentiallysuitablecases,survivorsofattemptedseparationhavethusfarbeen
rare.Aswithexteriorizationoftheheart,thesituationmustbeanticipated
rapidlytoimprove,dependingofcourseonthedegreeoffusionofthecardiac
structures,andtheseverityoftheintracardiaclesionsineachtwin.
AbnormalPositioningoftheHeart
Theheartisnormallylocatedinthemediastinum,withone-thirdofitsbulkto
theright,andtwo-thirdstotheleftofthemidline.Withthisarrangement,the
apexusuallypointsinferiorlyandtotheleft.Thiscombinedpatternis
traditionallydescribedas“levocardia.”Theheartcanbedeviatedfromthis
position,oritsapexbepointedinanunexpecteddirection,forvariousreasons,
andnotalwaysbecauseofacongenitalcardiacmalformation.Forexample,the
heartcanbepushedtotherightbyaspace-occupyinglesionintheleftlungor
leftpleuralcavity,suchashyperinflationoftheleftlung,orpleuraleffusionand
pneumothoraxinvolvingtheleftpleuralcavity.Theheartcanalsobepulledto
therightwhentherightlungisunderdevelopedorcollapsed.Anabnormal
rightwardpositionoftheheartisalsotypicallyseeninassociationwithright
pulmonaryhypoplasiainscimitarsyndrome,ligamentousorductaloriginof
rightpulmonaryartery,oratresiaofstenosisoftherightpulmonaryvein.
Nonetheless,theabnormalpositioninitselfdoesnotnecessarilyimplyan
abnormalityoftheheart.Normalindividualswithmirror-imagedarrangementof
theorgansusuallyhavearight-sidedheart.Thereforewhenassessingthe
significanceofaright-sidedheartoraheartwithitsapexpointingtotheright,it
isnecessarytotakeaccountofallthesevariousfeatures.Severalquestions
shouldbeasked.Whatistheoverallarrangementoftheorgans?Istherean
abnormalityofthelungsorthethoraciccontents?Ifpresent,isitofcongenitalor
acquiredetiology?Istheheartitselfabnormallystructured,orareitschambers
grotesquelyenlarged?Onlywhenthesequestionshavebeenposed,and