anatomophysiologic basis of tetralogy

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Recebido para publicao: Julho de 2009 Aceite para publicao: Outubro de 2009Received for publication: July 2009 Accepted for publication: October 2009

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ARTIGO DE REVISO

Bases anatomofisiolgicas da tetralogia deFallot e suas implicaes clnicas [44]

RAQUEL PACHECO DURO, CLUDIA MOURA, ADELINO LEITE-MOREIRA

Servio de Fisiologia, Faculdade de Medicina do Porto, Porto, Portugal

Rev Port Cardiol 2010; 29 (04): 591-630

RESUMO

A tetralogia de Fallot a cardiopatia con-gnita ciantica mais frequente. Os meca-

nismos fisopatolgicos desta cardiopatia tmvindo a ser progressivamente esclarecidos,

permitindo uma optimizao da teraputica.As quatro anomalias morfolgicas carac-

tersticas so: comunicao interventricular,estenose subpulmonar, cavalgamento davlvula artica e hipertrofia ventriculardireita, as quais resultam de um desvio

antero-ceflico do septo do tracto de sada ede hipertrofia das trabeculaes

septoparietais.Estas alteraes anatmicas condicionam

uma diminuio do fluxo sanguneopulmonar, resultando em hipxia e cianose.Os principais determinantes da magnitudedo fluxo sanguneo pulmonar so a origem

do fluxo sanguneo para os pulmes, a gravi-dade e o comportamento funcional da

obstruo subpulmonar, as pressesventricular direita e arterial

sistmica e o ducto arterioso.A fisiopatologia das crises de hipxia no

est totalmente esclarecida, tendo sido pro-postos alguns mecanismos, tais como um

aumento da contractilidade do msculoinfundibular subpulmonar, vasodilatao

perifrica e a estimulao de mecanorrecep-tores no ventrculo direito.

Existem duas estratgias cirrgicas pos-sveis em recm-nascidos/crianas com esta

ABSTRACT

Anatomophysiologic basis of tetralogyof Fallot and its clinical implications

Tetralogy of Fallot is the most frequentcyanotic congenital cardiopathy. Itsphysiopathology has been progressivelydescribed which has made better treatmentpossible.The four characteristic morphologicabnormalities are: interventricularcommunication, subpulmonary stenosis,biventricular origin of the aortic valve andright ventricular hypertrophy, which are thedirect result of the antero-cephaladdeviation of the ventricular septal outlet andhypertrophy of the septoparietaltrabeculations.These anatomic abnormalities result indecreased pulmonary blood flow, leading tohypoxia and cyanosis. The maindeterminants of pulmonary blood flow arethe source of the blood flow to the lungs, theseverity and functional behaviour of thesubpulmonary obstruction, the rightventricular and arterial systemic pressuresand the ductus arteriosus.The mechanism of cyanotic spells is notclear. Increases in infundibular contractility,peripheral vasodilatation and rightventricular mechanoreceptor stimulation aresome of the proposed mechanisms.There are two surgical strategies in

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INTRODUO

H mais de um sculo, Arthur Louis

Etienne Fallot demonstrou a co-existn-cia de quatro anomalias morfolgicas na maio-ria das autpsias dos pacientes com la mal-adie blue, ou cianose, como agora designamos.As leses que ele identificou foram: umacomunicao interventricular, uma estenosesubpulmonar, uma vlvula artica com umarelao biventricular e hipertrofia ventriculardireita(1). Sabemos hoje que a combinaodestas anomalias havia sido reconhecidamuito antes de Fallot. De facto, a Neil

Stensen, o monge dinamarqus que tambmdescreveu o ducto parotdeo, que se atribui omrito da descrio da entidade que hoje de-signamos por tetralogia de Fallot(2, 3).

Trata-se da mais comum cardiopatia con-gnita ciantica, correspondendo a cerca de5% de todas as cardiopatias congnitas, e comuma incidncia de cerca de 1 em cada 2400nados vivos(4).

Embora seja uma entidade composta por

quatro anomalias morfolgicas facilmente re-conhecidas, no tem sido fcil chegar a um

INTRODUCTION

It is over a century since Arthur Louis

Etienne Fallot demonstrated the co-exis-tence of four morphological abnormalities inmost post-mortem examinations on patientswith la maladie bleue blue baby syndromeor cyanosis, as we now call it. The lesions thatcharacterised it were: ventricular septaldefect; subpulmonary stenosis; overridingaorta, and right ventricular hypertrophy(1). Weknow today that the combination of theseabnormalities had been recognised longbefore Fallot. In fact Niels Stensen, the

Danish monk who also described the parotidduct, is also held to have described the condi-tion that we now call tetralogy of Fallot(2, 3).

It is the commonest congenital cyanoticcardiopathy and accounts for about 5% of allcongenital cardiopathies, with an incidence ofaround 1 in 2400 live births(4).

Although the condition is composed of fourmorphological abnormalities that are easilyrecognised it has been quite hard to arrive at

a consensus for its anatomical description.There has, in fact, been a great deal of discus-592

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cardiopatia: a estratgia faseada (procedi-mento paliativo inicial com correco total

posterior) ou a correco total precoce.Existem argumentos a favor e contra cada

uma destas estratgias, no se encontrando

o tratamento ideal ainda definido.Conclui-se que a correcta compreenso dosmecanismos fisiopatolgicos desta cardiopa-

tia essencial optimizao do tratamentoda criana. Esta reviso particularmenteactual e relevante uma vez que o conheci-mento da doena de base essencial paraumfollow-up correcto de uma nova popu-

lao de doentes: o adulto com Tetralogia deFallot cirurgicamente corrigida.

Palavras-chave:Tetralogia de Fallot; Defeito cardaco congnito;

Fisiopatologia; Hipxia; Procedimentos cirrgicos.

newborns/children: a staged approach (witha palliative procedure followed by the com-plete repair) or early complete repair. Thereare arguments for and against each of thesestrategies, and the debate about the ideal

treatment continues.In conclusion, the correct understanding ofthis cardiopathys physiopathology is essen-tial to improving the childs treatment. Thisreview is particularly contemporary and rel-evant issue because one must always bearin mind the physiopathology of the originaldisease in order to correctly follow-up a newpatient population: adults with surgicallycorrected Tetralogy of Fallot.

Keywords:Tetralogy of Fallot; Congenital heart defect;Physiopathology; Hypoxia; Surgical procedures.


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consenso na descrio anatmica desta car-diopatia. De facto, muito se debateu sobre aanatomia da tetralogia de Fallot, em grandeparte devido a variaes na nomenclatura quedescreve os feixes musculares(1). Neste traba-lho encontra-se um resumo do que consen-

sual actualmente em termos de nomenclaturaanatmica, a qual essencial para um enten-dimento das implicaes fisiopatolgicas epara a correco cirrgica desta patologiacongnita.

Quanto aos mecanismos fisiopatolgicos,embora existam j muitos estudos, alguns dosquais antigos, os mais recentes tm acrescen-tado novos dados, que originaram novas teo-rias explicativas dos fenmenos caractersti-

cos desta cardiopatia. E se parte destes estu-dos podem parecer de certa forma acadmi-cos, a verdade que na compreenso destesmecanismos fisiopatolgicos que assenta otratamento mdico e cirrgico da tetralogia deFallot.

O tratamento tem evoludo ao longo dosanos, e so muito poucos, comparativamentecom o passado, os pacientes com este diagns-tico que morrem na infncia. No entanto, otratamento ptimo ainda est longe de ser

definido, tanto no que se refere abordagemcirrgica como altura ideal para a sua insti-tuio.(5)

Este trabalho procurar abordar esta car-diopatia congnita de uma forma fisiopato-lgica, iniciando com uma descrio ana-tmica, a qual permitir entender a hemo-dinmica global e as suas implicaes, pila-res essenciais na abordagem teraputica, quese pretende a mais eficaz e a mais fisiolgica

possveis.

CARACTERIZAO MORFOLGICADA TETRALOGIA DE FALLOT

Antes de proceder descrio das alte-raes observadas na tetralogia de Fallot, sernecessrio descrever a estrutura normal dotracto de sada do ventrculo direito, de modoa melhor compreender as alteraes que ca-racterizam esta cardiopatia congnita.

sion about the anatomy of Tetralogy of Fallot,largely because of the variations in nomencla-ture of the muscle bands(1). This article con-tains a summary of what is currently agreedwith respect to the anatomical nomenclature,and this is essential for an understanding of

the physiopathological implications and thesurgical correction of this congenital disorder.

Although many studies have been pub-lished on the physiopathological mechanismsthe more recent ones have contained newinformation and these have generated newtheories to explain the typical features of thiscardiopathy. While some studies may seemsomewhat academic, in it is true to say that themedical and surgical treatment of tetralogy of

Fallot depends on understanding the phys-iopathological mechanisms.Treatment has evolved over the years and

nowadays very few patients with this diagnosisdie in infancy than did in the past. But we area long way from defining the best treatment, inrelation to either the surgical approach or thebest time to start it(5).

This work adopts a physiopathologicalapproach to the tetralogy of Fallot; it startswith an anatomical description that will make

it possible to understand the overall haemody-namics and its implications, essential pillarsof the therapeutic approach which should bethe most effective and most physiological pos-sible.

MORPHOLOGY OF TETRALOGYOF FALLOT

Before describing the changes observed intetralogy of Fallot we shall describe the nor-mal structure of the outflow tract from theright ventricle so as to better understand thechanges found in this congenital cardiopathy.

Normal anatomy of outflow tractsThe main difference between the right and

left ventricle outflow tracts is that whereasthere is fibrous continuity between the mitraland aortic valve rings, in the right ventriclethe pulmonary valve is supported by a com- 593

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Anatomia normal dos tractos de sadaA grande diferena entre os tractos de

sada do ventrculo direito e do ventrculoesquerdo que, enquanto que no ventrculoesquerdo existe uma continuidade fibrosaentre os anis valvulares mitral e artico, no

ventrculo direito a vlvula pulmonar estsuportada por um infundbulo totalmentemuscular(6). Quando se observa do pex doventrculo direito, verifica-se a existncia deuma crista muscular extensa que separa osfolhetos das vlvulas pulmonar e tricspide(1).Esta designada por crista supraventricular(crista supraventricularis)(1, 7).

Uma disseco cuidadosa da crista supra-ventricular revela que esta estrutura tem trs

componentes: a parte maior da crista no mais do que a curvatura interna da paredeparietal do ventrculo direito, que separa acavidade do ventrculo direito da raiz daaorta e da artria coronria direita; a porobasal suporta os folhetos da vlvula pul-monar; por ltimo, a parte muscular que seencontra entre os ramos da trabeculao sep-tomarginal, designada de septo do tracto desada(1, 7). No entanto, esta ltima poro podeser removida de forma a criar uma comuni-cao com o ventrculo esquerdo, o queimplica que nem toda a estrutura de factosupraventricular, mas antes tem um compo-nente interventricular(7). Nesse sentido, parauniformizar a linguagem, foi sugerido que otermo crista supraventricular fosse reservadopara o tecto do ventrculo direito numcorao normal(8). Isto porque as partes quecompem o tracto de sada do ventrculodireito apenas podem ser diferenciadas entre

si nos coraes congenitamente malforma-dos(7).

Assim, os componentes normais do tractode sada do ventrculo direito foram designa-dos da seguinte forma: a curva interna docorao foi denominada de prega ventrculo--infundibular, representando qualquer estru-tura muscular que se interponha entre oslocais de fixao dos folhetos das vlvulasarteriais e aurculo-ventriculares; qualquer

estrutura fibrosa ou muscular que se interpon-ha entre os folhetos das vlvulas arteriais in-

pletely muscular infundibulum(6). The apex ofthe right ventricle reveals an extensive mus-cular crest that separates the leaflets of thepulmonary and tricuspid valves(1). This iscalled the supraventricular crest (cristasupraventricularis)(1, 7).

Careful dissection of the supraventricularcrest reveals that it has three parts: thelargest part of the crest is simply the internalcurvature of the parietal wall of the right ven-tricle which separates the cavity of the rightventricle from the aortic root and the rightcoronary artery; the basal part supports theleaflets of the pulmonary valve; finally, themuscular part which lies between thebranches of the septomarginal trabeculation,

called the outflow septum(1, 7)

. This last por-tion can be removed, however, to create aconnection with the left ventricle, whichimplies that not all the structure is in factsupraventricular, but that there is an inter-ventricular component, too(7). So, to har-monise the terminology it has been suggestedthat the term supraventricular crest only beused for the roof of the right ventricle in thenormal heart(8). This is because the partsmaking up the outflow tract of the right ven-tricle can be distinguished from one anotherin congenitally malformed hearts(7).

The normal components of the right ventri-cle outflow tract have therefore been designat-ed as follows: the inner heart curve has beencalled the ventriculo-infundibular fold, repre-senting any muscular structure interpos ingbetween the attachment of the leaflets of thearterial and atrioventricular valves; any mus-cular or fibrous structure interposing between

the leaflets of the arterial valves themselvesobviously also separates the subarterial out-lets and so is termed the outlet septum; themuscular band that reinforces the septal sur-face, with its antero-cephalad and postero-caudal limbs, and along with a body thatextends into the apical trabecular component,has been called the septomarginal trabecula-tion; the septoparietal trabeculations are themuscular bundles that extend from the cepha-

lad margin of the septomarginal trabeculationto the parietal wall of the outflow tract(1, 7). The594


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terpe-se naturalmente tambm entre os trac-tos de sada subarteriais, e dever ser denom-inada de septo do tracto de sada; o feixe mus-cular que refora a superfcie septal, com osseus ramos antero-ceflico e pstero-caudal, ecom o seu corpo que se extende para o compo-

nente apical, denomina-se de trabeculao se-ptomarginal; as trabeculaes septoparietaiscorrespondem aos feixes musculares que seextendem da margem ceflica da trabeculaoseptomarginal para a parede parietal do tractode sada(1, 8). A disposio dos feixes muscu-lares num corao normal encontra-se retrata-da de um modo global na Figura 1.

layout of the muscular bundles in the normalheart is depicted in Figure 1.

Clinical anatomy of tetralogy of FallotThe essential difference between the

tetralogy de Fallot and the normal heart is that

in the former the components of the supraven-tricular crest are separated and keep theirindividuality. The anterocephalad deviation ofthe insertion of the muscular outlet septum isone of the cardinal manifestations of thetetralogy, but it is not the only change and it isnot enough to produce this congenital abnor-mality. That is because the septum can be

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O desenho esquemtico representa os normais constituintes do tracto de sada do ventrculo direito normal. impossvel distinguir os com-ponentes que representam o septo do tracto de sada e a prega ventrculo-infundibular numa situao normal sem recorrer disseco. Asestrelas mostram o local de origem das trabeculaes septoparietais (adaptado de Anderson e Col.7).

The schematic drawing represents the normal components of the normal right ventricular outflow tract. It is impossible to distinguish thecomponents that represent the outlet septum and the ventricular-infundibular fold in the normal heart without dissection. The stars indicatethe site of origin of the septoparietal trabeculations (adapted from Anderson et al.7).

Artria aortaAortic artery IIII nnnn ffff uuuu nnnn dddd iiii bbbb uuuu llll ooooSSSS uuuu bbbb pppp uuuu llll mmmm oooo nnnn aaaa rrrrSSSS uuuu bbbb pppp uuuu llll mmmm oooo nnnn aaaa rrrr yyyy iiii nnnn ffff uuuu nnnn dddd iiii bbbb uuuu llll uuuu mmmm

SSSS eeee pppp tttt oooo mmmm uuuu ssss cccc uuuu llll aaaa rrrrdddd oooo tttt rrrr aaaa cccc tttt oooo dddd eeee ssss aaaa dddd aaaaMMMM uuuu ssss cccc uuuu llll aaaa rrrr ssss eeee pppp tttt uuuu mmmmoooo ffff tttt hhhh eeee oooo uuuu tttt ffff llll oooo wwww tttt rrrr aaaa cccc ttttTTTT rrrr aaaa bbbb eeee cccc uuuu llll aaaa oooossss eeee pppp tttt oooo mmmm aaaa rrrr gggg iiii nnnn aaaa llllSSSS eeee pppp tttt oooo mmmm aaaa rrrr gggg iiii nnnn aaaa lllltttt rrrr aaaa bbbb eeee cccc uuuu llll aaaa tttt iiii oooo nnnnPPPP rrrr eeee gggg aaaaVVVV eeee nnnn tttt rrrr cccc uuuu llll oooo ---- IIII nnnn ffff uuuu nnnn dddd iiii bbbb uuuu llll aaaa rrrrVVVV eeee nnnn tttt rrrr iiii cccc uuuu llll aaaa rrrr ---- iiii nnnn ffff uuuu nnnn dddd iiii bbbb uuuu llll aaaa rrrr ffff oooo llll dddd

SSSS eeee pppp tttt oooommmm eeee mmmm bbbb rrrr aaaa nnnn oooo ssss ooooMMMM eeee mmmm bbbb rrrr aaaa nnnn oooo uuuu ssss ssss eeee pppp tttt uuuu mmmm


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Anatomia clnica da tetralogia de FallotA diferena essencial entre a tetralogia de

Fallot e o corao normal que, na tetralogiade Fallot, os componentes da crista supraven-tricular se separaram, mantendo a sua indi-vidualidade. O desvio antero-ceflico do septo

do tracto de sada uma das manifestaescardinais da tetralogia, quer ele seja muscularou fibroso, mas no a nica alterao e no suficiente para produzir esta anomalia con-gnita. Isto porque o septo pode sofrer umdesvio no sentido antero-ceflico, com umagrande comunicao interventricular em con-tinuidade com a vlvula artica, sem que hajaestenose pulmonar, como acontece no defeitoseptal ventricular caracterstico do complexo

de Eisenmenger. Assim, para alm deste des-vio antero-ceflico, necessria a existnciade hipertrofia das trabeculaes septoparie-tais para que ocorra estenose pulmonar. Estasso as duas caractersticas comuns a todos oscasos de tetralogia de Fallot, verificando-seum espectro de apresentao clnica muitovarivel entre outras caractersticas.(1, 7)

A obstruo ao tracto de sada do ventr-culo direito produzida na entrada do infun-dbulo pelo aperto provocado pelo septo do

tracto de sada mal alinhado e pelas trabe-culaes septoparietais(7).

O septo do tracto de sada uma estruturaexclusivamente ventricular direita na tetra-logia de Fallot. A comunicao interventricu-lar delimitado inferiormente pelos ramos datrabeculao septomarginal, com a origemventricular direita da vlvula artica suporta-da pela prega ventrculo-infundibular. Estastrs alteraes anatmicas da tetralogia de

Fallot reflectem um denominador comum: alocalizao anormal do septo do tracto desada, o qual, como j foi dito, pode encontrar-se mal alinhado sem no entanto produzirestenose pulmonar. a combinao do desviodo septo do tracto de sada com a anormalrelao entre as trabeculaes septoparietaisque produz a anatomia patognomnica datetralogia de Fallot, observada na Figura 2 (7).

A hipertrofia ventricular direita, que tam-bm faz parte das caractersticas que com-pem esta cardiopatia, uma consequncia

deviated in the antero-cephalad direction, withmajor interventricular communication in conti-nuity with the aortic valve, without there beingpulmonary stenosis, as happens with the ven-tricular septal defect known as theEisenmenger complex. So there must be hyper-

trophy of the septoparietal trabeculations forpulmonary stenosis to occur. These are the twocharacteristics common to all cases of Tetralogyof Fallot, whilst there is a highly variable spec-trum with respect to other characteristics(1, 7).

The obstruction of the right ventricularoutlet tract that occurs at the entrance of theinfundibulum by the squeeze is produced bythe misaligned outlet septum and the sep-toparietal trabeculations(7).

The outlet septum is exclusively a rightventricular structure in tetralogy of Fallot. Theinterventricular defect is bounded on the floorby the limbs of the septomarginal trabecula-tion, with the right ventricular origin of theaortic valve supported by the ventriculo-infundibular fold. These three anatomicalchanges in tetralogy of Fallot have a commondenominator: the abnormal location of the out-let septum which, as mentioned earlier, can bemisaligned but yet does not produce pul-

monary stenosis. It is the combination of thedeviation of the outlet tract and the abnormalrelation between the septoparietal trabecula-tions that produces the pathognomnic anato-my of tetralogy of Fallot shown in Figure 2(7).

The right ventricular hypertrophy that isalso a feature of this cardiopathy is a haemo-dynamic consequence of the anatomic lesionsdescribed(9). Sanchez-Quintana et al.(10) foundthat the right ventricle in the hearts of patients

with tetralogy of Fallot had 3 layers of muscle(superficial, middle and deep), as opposed tothe hearts of normal patients in whom there isno medial layer. Therefore, and given that thislayer was also present in the heart of new-borns, the authors suggest that this new mus-cle layer may not simply be a haemodynamicadaptation, but it could be an alteration in theventricular myoarchitecture linked to thetetralogy of Fallot.

It is the change in length of the infundibu-lum and in the limits of ventricular septal596


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hemodinmica das leses anatmicas des-critas(9). No entanto, Sanchez-Quintana eCol.(10) verificaram que o ventrculo direito de

pacientes com tetralogia de Fallot possuatrs camadas musculares (superficial, mdiae profunda), contrariamente aos coraes depacientes normais, nos quais estava ausentea camada mdia. Assim, e atendendo a queesta camada estava presente tambm nocorao de um recm-nascido, os autoressugerem que esta nova camada muscularpossa no ser apenas uma adaptao hemo-dinmica, mas sim uma alterao na mioar-quitectura ventricular associada tetralogiade Fallot.

defect and the degree of dextraposition of theaortic valve, which are thus also related to theright ventricle, that determines the anatomic

and clinical variability of this disorder(7)

.

Ventricular septal defectVentricular septal defect through misalign-

ment occurs below the aortic valve and deter-mines a variable degree of aortic override andcontinuity between the leaflets of the aorticvalve and the right ventricle(1, 7). The outletseptum is usually well-formed although badlyaligned in relation to the remaining interven-tricular septum. Part of the pathognomonicanatomy of tetralogy of Fallot is the fact that 597


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O desenho esquemtico evidencia como os constituintes normais do tracto de sada do ventrculo direito se separam na Tetralogia de Fallot.A obstruo subpulmonar produzida por um aperto entre o septo muscular do tracto de sada e as trabeculaes septoparietais hipertrofi-adas. Notar que a comunicao interventricular (estrela maior) est delimitada pelos ramos da trabeculao septomarginal (adaptado de

Anderson e Col.7).

The schematic drawing shows how the normal components of the right ventricular outflow tract are separated in Tetralogy of Fallot. Thesubpulmonary obstruction is produced by a squeeze between the muscular septum of the outflow tract and the hypertrophied septoparietaltrabeculations. Not that ventricular septal defect communication (large star) is delimited by the limbs of the septomarginal trabeculation(adapted from Anderson et al.7).

Artria aortaAortic artery

Vlvula articaAortic valve

InfundibuloSubpulmonar

Subpulmonary infundibulum

Septo musculardo tracto de sada

Muscular septum of theoutflow tract

PregaVentrculo-Infundibular

Ventricular-infundibular fold

TTTT rrrr aaaa bbbb eeee cccc uuuu llll aaaa oooossss eeee pppp tttt oooo mmmm aaaa rrrr gggg iiii nnnn aaaa llllSSSS eeee pppp tttt oooo mmmm aaaa rrrr gggg iiii nnnn aaaa llll tttt rrrr aaaa bbbb eeee cccc uuuu llll aaaa tttt iiii oooo nnnn


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a variao no comprimento do infun-dbulo, assim como nos limites da comuni-cao interventricular e no grau de dextropo-sio da vlvula artica, que desta forma tam-bm se relaciona com o ventrculo direito, quedetermina a variabilidade anatmica e clnica

desta patologia(7).

Comunicao interventricularA comunicao interventricular, por mala-

linhamento, est posicionada por debaixo davlvula artica, determinando cavalgamentoartico varivel e continuidade entre os fo-lhetos da vlvula artica e o ventrculo direi-to(1, 7). O septo do tracto de sada encontra-senormalmente bem formado, embora mal alin-

hado relativamente ao restante septo interven-tricular. Parte da anatomia patognomnica datetralogia de Fallot o facto de o septo seruma estrutura exclusivamente do ventrculodireito e no interventricular(1, 7).

Para podermos definir os limites do defeitodo septo necessrio definir qual o plano peloqual vamos analisar este defeito. Existem v-rios planos de interesse clnico. No entanto, omais importante sem dvida aquele que seextende da margem direita dos folhetos davlvula artica ao topo do septo interventricu-lar, uma vez que este plano que o cirurgioir corrigir de modo a realinhar a aorta com oventrculo esquerdo.(1, 7) So estes os limitesque nos interessa definir, os quais apresentamalguma variabilidade anatmica dependendodas relaes especficas entre os diferentesconstituintes do tracto de sada(1, 7).

Em cerca de 80% dos pacientes cauca-sianos, a prega ventrculo-infundibular termi-

na prximo do ramo pstero-inferior da tra-beculao septomarginal, permitindo con-tinuidade fibrosa entre os folhetos das vlvu-las artica e tricspide(7); superiormente odefeito ser delimitado pelo septo do tracto desada(1). Em numerosos casos, a rea de con-tinuidade fibrosa reforada por uma pregaque representa o componente ventricular dosepto membranoso(7). Este tipo de defeito di-rectamente comparvel aos defeitos perimem-

branosos tpicos

(11)

, o que significa que as refe-rncias para a localizao do eixo de con-

the septum is exclusively a right ventricularstructure and not interventricular(1, 7).

We have to establish the level for which weare going to analyse this defect so that we candefine the limits of the septum defects. Thereare several levels of clinical interest, but the

most important is undoubtedly the one thatextends from the right margin of the leaflets ofthe aortic valve to the top of the interventricu-lar septum, since this is the level that the sur-geon will correct in order to realign the aortawith the left ventricle(1, 7). These are the limitsthat we want to define, those which exhibitsome anatomic variability, depending on thespecific relations between the different com-ponents of the outflow tract(1). In many cases

the area of fibrous continuity is reinforced bya fold that represents the ventricular compo-nent of the membranous septum(7).

In about 80% of caucasian patients theventricular-infundibular fold ends next to thepostero-inferior limb of the septomarginal tra-beculation, thus permitting fibrous continuitybetween the leaflets of the aortic and tricuspidvalves(7); superiorly the defect will be limitedby the outlet septum(7). In many cases the areaof fibrous continuity is reinforced by a foldthat represents the ventricular component ofthe membranous septum(7). This type of defectis directly comparable to the typical perimem-branous defects(11), which means that the refer-ences for locating the atrioventricular conduc-tion axis are the apex of the triangle of Kochand the position of the medial papillary mus-cle(7).

The second commonest variation, whichoccurs in around 20% of caucasians, is char-

acterised by muscular continuity on the rightmargin of the ventricular septal defect and iscaused by the fusion of the ventricular-infundibular fold and the postero-caudal limbof the septomarginal trabeculation(1, 7). Theatrioventricular conduction tissue is posi-tioned postero-inferiorly in relation to thismuscular septum and is separated from thetop of the ventricular septum by the muscularfold and by the muscular septum itself, and so

it is protected by muscle tissue

(1)

.There is a third variant of the defect, char-598


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duo aurculo-ventricular so o pice dotringulo de Koch e a posio do msculo pa-pilar medial(7).

A segunda variao mais comum, queocorre em cerca de 20% dos caucasianos, ca-racteriza-se por continuidade muscular na

margem direita da comunicao interventricu-lar, e deve-se fuso entre a prega ventrculo--infundibular e o ramo pstero-caudal da tra-beculao septomarginal(11, 7). O tecido de con-duo aurculo-ventricular situa-se postero-inferiormente em relao a este septo muscu-lar, encontrando-se separado do topo do septoventricular pela prega muscular e pelo prpriosepto muscular, pelo que est protegido portecido muscular(1).

Existe ainda uma terceira variante do de-feito, caracterizada pela presena de umsepto do tracto de sada fibroso e no muscu-lar(7). Assim, ao contrrio dos outros doistipos, aqui a diferena reside na margemsuperior, que normalmente revestida pelotecido muscular do septo do tracto de sada,mas neste caso apenas constituda por teci-do fibroso pois o septo do tracto de sada nose formou correctamente(1). Trata-se de umdefeito mais comum no Oriente e na Amricado Sul(1, 7). O defeito tanto sub-artico comosub-pulmonar, como consequncia de umafalha na formao de um infundbulo sub-pul-monar muscular completo(7). No entanto, talcomo todos os outros casos de tetralogia deFallot, estes pacientes tambm tm obstruodo tracto de sada do ventrculo direito (9), oque vem reforar a ideia de que a obstruono se deve apenas ao mau alinhamento dosepto. Estes defeitos tanto podem existir com

continuidade fibrosa entre os folhetos dasvlvulas artica e tricspide, sendo assimperimembranosos, ou com um resqucio mus-cular pstero-inferior, sendo este ltimo tipoo mais frequente. Neste caso, o resquciomuscular ir proteger o feixe de conduoaurculo-ventricular, que se encontra emrisco no primeiro caso(7).

O tamanho da comunicao interventricularpode variar, mas na maioria das situaes a co-municao no restritiva, permitindo passa-gem de sangue em sentido bidireccional(9).

acterised by the presence of a fibrous, non-muscular, outlet septum(7). So, contrary to theother two types, the difference here lies in thesuperior margin, which is normally lined bythe muscular tissue of the outlet septum, butin this case it only consists of fibrous tissue

since the outlet septum was not formed cor-rectly(1). This defect is more common in theEast and South America(1, 7). The defect is asmuch sub-aortic as sub-pulmonary, as a resultof a fault in the formation of a complete mus-cular sub-pulmonary infundibulum(7). None-theless, as in all other cases of tetralogy ofFallot, these patients also have obstruction ofthe right ventricular outflow tract(9), whichreinforces the idea that the obstruction is not

caused just by the misalignment of the sep-tum. These defects can exist either with fi-brous continuity between the leaflets of theaortic and tricuspid valves, and are thus pe-rimembranous, or with a postero-inferior mus-cular remnant. This latter type is more com-mon. In this case the muscular remnant willprotect the atrioventricular bundle, which is atrisk in the first example(7).

The size of the ventricular septal defectcommunication can vary, but in most cases thecommunication is not restrictive and allowsbi-directional blood flow(9).

All the patients with tetralogy exhibit anventricular septal defect with the characteris-tics of one of the variants just described. Butthese defects of the right ventricular outflowtract can extend to the mouth of the ventri-cle(7).

Subpulmonary infundibulum

Subpulmonary stenosis, which is the inte-grating part of tetralogy of Fallot, occursthrough a narrowing of the zone between theanterocephalad deviated outlet septum andthe abnormally located septoparietal trabecu-lations(1, 7), which makes this obstruction of theright ventricular outflow tract very particular,since it is s dynamic obstruction(9, 12). The sep-toparietal trabeculations, which are normallyhypertrophied, too, though this is not a univer-sal feature, extend to the free wall of the ven-tricle and can easily be removed surgically 599


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Todos os pacientes com tetralogia apresen-tam uma comunicao interventricular com ascaractersticas de uma das variantes descritas.No entanto, estes defeitos do tracto de sadado ventrculo direito podem extender-se aocomponente de entrada do ventrculo, seja

como componente ventricular de um defeitoaurculo-ventricular do septo ou em associa-o a um mau alinhamento entre as estruturasauriculares e ventriculares, por exemplo.(7)

Infundbulo subpulmonarA estenose subpulmonar, que parte inte-

grante da tetralogia de Fallot, ocorre, como jfoi referido, por um estreitamento da zonalocalizada entre o septo do tracto de sada

mal-alinhado anterocefalicamente e as trabe-culaes septoparietais anormalmente situa-das(1, 7), o que torna esta obstruo do tracto desada do ventrculo direito muito particularuma vez que uma obstruo dinmica(9, 12). Astrabeculaes septoparietais, que normal-mente se encontram tambm hipertrofiadas,embora tal no seja um facto universal, exten-dem-se para a parede livre do ventrculo epodem facilmente ser removidas cirurgica-mente sem o risco de lesar estruturas impor-tantes (embora este tipo de resseco tenhacado em desuso actualmente)(7).

A rea mxima de estenose, quando obser-vada pelo pice do ventrculo direito, encon-tra-se entrada do infundbulo subpulmonar,sendo denominada como os infundibulum.Algum grau de estenose adicional pode serencontrado mais proximalmente no ventrculo,produzida por hipertrofia das trabeculaesseptoparietais ou por trabeculaes apicais

proeminentes(7).O infundbulo subpulmonar na tetralogia

de Fallot tem menor volume quando compara-do com o normal e sem dvida mais estreito,embora na maioria dos casos seja curiosa-mente mais longo, (podendo, no entanto, sermais curto do que o normal)(1, 7). No entanto, natetralogia de Fallot, a entrada do infundbulo apenas um dos muitos locais onde podemexistir estenoses. Estas podem tambm estarpresentes quer ao nvel da vlvula pulmonar,que na maioria dos casos apenas tem dois fo-

without risk of harming important structures(though this kind of resection is currently lit-tle used)(7).

The maximum area of stenosis, whenobserved from the right ventricle apex, is atthe mouth of the subpulmonary infundibulum

and is known as the os infundibulum. Somedegree of additional stenosis can be foundmore proximally in the ventricle and this isproduced by hypertrophy of the septoparietaltrabeculations or by prominent apical trabec-ulations(7).

The subpulmonary infundibulum in tetral-ogy of Fallot has less volume than normal andis without doubt narrower, though in mostcases it is, interestingly, longer (but it can be

shorter than normal)(1, 7)

. In tetralogy of Fallot,however, the mouth of the infundibulum is justone of the many sites where these stenoses canexist. They can also occur at the level of thepulmonary valve, which in most cases onlyhas two leaflets, and even in the pulmonaryarteries themselves(1, 7).

Overriding aortic valveIn the normal heart, even though the right

aortic sinus of the aortic valve is immediatelyabove the top of the interventricular septum,the leaflets of the valve are inserted exclusive-ly within the left ventricle. In any circum-stance in which the interventricular septumhas a defect, part of the circumference of theaortic valve orifice remains inserted in or sup-ported by right ventricular structures. Thisoverride of the aorta is more evident when theoutlet septum is deviated so as to be an exclu-sively right ventricle structure, as it is in

tetralogy of Fallot. The degree of override, thatis, the proportion of the aortic valve circum-ference that is supported by right, and not left,ventricular structures can range from 5% to100%. This aspect is obviously relevant tosurgery. The greater the proportion the greaterthe anatomical deviation and the graft neededto reconnect the aorta to the left ventricle(7).

This concept is also important for nomen-clature. If more than half the circumference ofboth arterial valves is supported by the sameventricle, then we have a double-outlet ventric-600


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lhetos, quer mesmo nas prprias artrias pul-monares(1, 7).

Cavalgamento da vlvula articaNo corao normal, embora o seio artico

direito da vlvula artica fique imediatamente

acima do topo do septo interventricular, os fo-lhetos da vlvula inserem-se exclusivamentedentro do ventrculo esquerdo. Em qualquercircunstncia em que o septo interventriculartenha um defeito, parte da circunferncia doorifcio valvular artico fica inserida em, ousuportada por, estruturas ventriculares di-reitas. Este cavalgamento da aorta mais evi-dente quando o septo do tracto de sada estdesviado de modo a ser uma estrutura exclusi-

vamente do ventrculo direito, como o casoda tetralogia de Fallot. O grau de cavalgamen-to, ou seja, a proporo da circunfernciavalvular artica que suportada por estruturasventriculares direitas e no esquerdas, podevariar entre 5% a 100%. Este um aspectocom bvia relevncia cirrgica. Quanto maiora proporo, maior ser o desvio anatmico eo enxerto necessrio para reconectar a aortaao ventrculo esquerdo(7).

Este conceito tambm importante em ter-mos de nomenclatura. Se mais do que metadeda circunferncia de ambas as vlvulas arte-riais se encontram suportadas pelo mesmoventrculo, ento estamos perante uma cone-xo ventrculo-arterial de dupla sada. Assim,no contexto da tetralogia de Fallot, se mais demetade da vlvula artica se encontra supor-tada por estruturas ventriculares direitas,ento a malformao dever designar-se tam-bm de ventrculo direito com dupla cmara

de sada(1, 7, 9).

Outras leses da circulao pulmonarEmbora o infundbulo subpulmonar seja

normalmente a parte mais estreita do tracto desada do ventrculo direito, tal como j foi refe-rido, podem encontrar-se outras leses tanto notracto de sada como nas artrias pulmonares(7).

A estenose da vlvula pulmonar frequen-te, sendo a causa mais frequente a estenose porvlvula bicspide, embora raramente seja estaa leso principal(7).

ular-arterial connection. In the tetralogy ofFallot context, then, if more than half the aorticvalve is supported by right ventricular struc-tures the malformation should also be called adouble - outlet right ventricle(1, 7, 9).

Other pulmonary circulation lesionsAlthough the subpulmonary infundibulum

is usually the narrowest part of the right ven-tricle outflow tract, as mentioned above, otherlesions may be found in the outflow tract orthe pulmonary arteries(7).

Stenosis of the pulmonary valve is commonand stenosis of the bicuspid valve is the mostfrequent cause, though it is only rarely themain lesion(7).

The pulmonary trunk may also be atretic.In about half of these patients the right andleft pulmonary arteries are confluent, withblood flowing to the arteries through the arte-rial duct that remains open. In the other halfof cases the pulmonary supply is multifocaland normally originates only from major aorto-pulmonary collateral arteries(9).

The absence of pulmonary valve leaflets isanother important lesion that can be observed.This gives rise to the variant known as tetra-logy of Fallot with absent pulmonary valve(9, 13).This is a rare variant found in around 3 to 6%of patients with tetralogy of Fallot(14, 5). Here wemost often see a ring of rudimentary fibroustissue in the ventricular arterial junction(resulting from the inadequate formation of thepulmonary leaflets), associated with thedilatation of the pulmonary trunk and itsbranches(7, 15), due to free pulmonary regurgita-tion throughout foetal life(9, 13). The arterial

duct (ductus arteriosus) is normally absent atbirth, and so it is thought that these two abnor-malities are related, even though the mecha-nism involved is not well known. There aretwo theories that are advance to explain thisphenomenon. One argues that the primarylesion will be the absence of pulmonary valveleaflets through failure of migration of thecells from the neural crest, with the atreticarterial duct being a secondary modification.The other theory suggests that the absence ofthe arterial duct is the main lesion and this is 601


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O tronco pulmonar pode tambm estaratrtico. Em cerca de metade destes doentes,as artrias pulmonares direita e esquerda soconfluentes, com o sangue a fluir para asartrias pelo canal arterial persistente. Naoutra metade dos casos, o suprimento arterial

multifocal, tendo normalmente origem emartrias colaterais aorto-pulmonares major(9).

A ausncia dos folhetos da vlvula pul-monar outra leso importante que pode serobservada, e que condiciona a variante desig-nada por tetralogia de Fallot com agenesia davlvula pulmonar(9, 13). Trata-se de uma vari-ante rara, identificada em cerca de 3 a 6% dospacientes com tetralogia de Fallot(14, 15). Nestecaso observa-se, mais frequentemente, um

anel de tecido fibroso rudimentar na junoventrculo-arterial (resultante da inadequadaformao dos folhetos pulmonares), associadaa dilatao do tronco pulmonar e dos seusramos(7, 15), provocada pela regurgitao pul-monar livre durante a vida fetal(9, 13). O ductoarterioso est normalmente ausente ao nasci-mento, pelo que se pensa que estas duas ano-malias esto relacionadas, embora ainda nose saiba bem qual o mecanismo envolvido.Existem duas teorias que procuram explicareste fenmeno. Uma defende que a leso pri-mria ser a ausncia de folhetos da vlvulapulmonar por falha na migrao das clulas dacrista neural, sendo o ducto arterioso atrticouma alterao secundria. A outra teoria pos-tula que a ausncia de ducto arterioso aleso principal, a qual condicionar alte-raes hemodinmicas que propiciam o nodesenvolvimento dos folhetos da vlvula pul-monar.(13) Independentemente do mecanismo

da leso, esta alterao resulta em regurgi-tao pulmonar importante na vida fetal, aqual vai condicionar dilatao exuberante dasartrias pulmonares.(12)

As estenoses observadas nas prpriasartrias pulmonares so leses de grande im-portncia cirrgica e ocorrem sobretudo noslocais de ramificao(7).

Tambm no infrequente a observao deartrias pulmonares no confluentes, verifi-

cando-se mais frequentemente que a artriapulmonar esquerda no tem origem no tronco

what influences the haemodynamic changesthat favour the non-development of the pul-monary valve leaflets(13). Regardless of themechanism of the lesion, this change resultsin important regurgitation during foetal life,which will condition massive dilatation of the

pulmonary arteries(12).The stenoses observed in the pulmonary

arteries themselves are lesions of great inter-est surgically and occur mostly at the site ofbranching of the pulmonary arteries(7).

It is also quite usual to find non-confluentpulmonary arteries, and most frequently it isthe left pulmonary artery that does not origi-nate in the pulmonary trunk (normally beingconnected by the arterial duct to the aortic

arch)(7)

.

Other associated abnormalitiesAny lesions that could potentially be asso-

ciated with tetralogy of Fallot should be con-sidered in advance since some are commonenough to require diagnostic tests to be car-ried out in order to exclude it.

Up to one-sixth of patients have anomalousorigins of the coronary arteries, which shouldbe documented prior to surgery(9). The com-monest and most significant abnormality isanterior descending artery originating in theright coronary artery (5% of cases)(1), and thencoursing anterior toe the subpulmonary out-flow tract, which could be a site of surgicalincision(9).

An atrioventricular defect of the septumcombined with a common atrioventricularjunction is found in around 2% of patientswith tetralogy of Fallot, which complicates

corrective surgery(14).Approximately a third of patients have a

right aortic arch(1) which is of no haemodynam-ic consequence(9). Other associated lesionsmay include atrial and additional ventricularseptum defects(9).

Another change associated with tetralogyof Fallot, in childhood and later life, is thedilatation of the descending aorta, even inpatients who have undergone early surgery.

The latest studies seem to indicate that, inaddition to the initial haemodynamic stress602


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pulmonar (encontrando-se normalmente co-nectada pelo canal arterial ao arco artico)(7).

Outras anomalias associadasQuaisquer leses que possam, potencial-

mente, encontrar-se associadas tetralogia de

Fallot devem ser tidas em conta antecipada-mente, sendo que algumas so suficiente-mente frequentes para requerer a realizaode exames complementares de diagnsticoque permitam a sua excluso(1).

At um sexto dos pacientes tm origemanmala das artrias coronrias, o que deverser documentado antes da cirurgia(9). A ano-malia mais comum e com mais significado aartria descendente anterior ter origem na

artria coronria direita (5% dos casos)(1)

, cur-sando depois anteriormente ao tracto de sadasubpulmonar, local onde poder ser realizadaa inciso cirrgica(9).

Um defeito aurculo-ventricular do septocombinado com uma juno aurculo-ventri-cular comum encontrado em cerca de 2%dos pacientes com tetralogia de Fallot, o quetorna a correco cirrgica mais complexa(14).

Cerca de um tero dos pacientes tm arcoartico direito(1), o que no tem qualquer impli-cao hemodinmica(9). Podem associar-se tam-bm defeitos do septo auricular e defeitos adi-cionais no septo ventricular(9).

Outra alterao associada Tetralogia deFallot, no tanto na infncia mas mais tardia-mente, a dilatao da aorta ascendente, mes-mo em pacientes que foram intervencionadosprecocemente. Os estudos mais recentes pare-cem indicar que, para alm do stress hemo-dinmico inicial por volume artico excessivo,

alteraes histolgicas intrnsecas na camadamdia artica sero importantes para o apare-cimento e desenvolvimento destas leses(16, 18).

FISIOPATOLOGIA DA TETRALOGIADE FALLOT

Aps a caracterizao da anatomia destaanomalia congnita, segue-se a anlise das

consequncias fisiolgicas destas alteraesestruturais, tanto na circulao fetal como

due to excessive aortic volume, intrinsic histo-logical changes in the medial aortic layer willbe important for the appearance and develop-ment of these lesions(16, 18).

PHYSIOPATHOLOGY OFTETRALOGY OF FALLOT

Having characterised the anatomy of thiscongenital abnormality we now analyse thephysiological consequences of the structuralchanges, in both foetal and postnatal circula-tion, in an attempt to link the physiopathologywith the signs and symptoms.

Foetal circulationFoetuses with tetralogy of Fallot are typical-ly those which have developed normally: thepresence of the lesions does not seem to affectfoetal circulation, nor to interfere with theumbilical-placental circulation. It can thus beassumed that there are no alterations in oxy-gen saturation and the flow in the oval fora-men and the tricuspid valve is presumablynormal(19).

The volume and direction of the flow in thepulmonary trunk will essentially depend onthe degree of obstruction of the right ventricleoutflow tract: if obstruction is slight or moder-ate there will be blood flow in the pulmonarytrunk, though less, and it will be sufficient tosupply the lungs and provide some blood tothe descending aorta through the arterial duct;if obstruction is severe or complete, the bloodsupply to the lungs will come from the bloodthat arrives in the pulmonary arteries from the

arterial duct (in the opposite direction to nor-mal)(19).

Regardless of the direction of blood flow inthe arterial duct, it will be substantiallyreduced in foetuses with tetralogy of Fallotcompared with normal foetuses. Reduction inthe volume of blood shunting in the arterialduct probably interferes with the developmentof its muscular wall, and if the direction offlow is from the aorta to the pulmonary artery,

the actual orientation of the arterial duct willbe different (it normally forms an obtuse infe- 603


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ps-natal, tentando fazer uma ligao da fisio-patologia aos sinais e sintomas.

Circulao fetalOs fetos com tetralogia de Fallot so carac-

teristicamente fetos normalmente desenvolvi-

dos: a presena das leses no parece afectara circulao fetal nem interferir com a circu-lao umbilico-placentria. Assim, podeassumir-se que no h alteraes ao nvel dassaturaes de oxignio, sendo o fluxo no fora-men oval e na vlvula tricspide presumivel-mente normal.(19)

O volume e a direco do fluxo no troncopulmonar dependero essencialmente do graude obstruo do tracto de sada do ventrculo

direito: se a obstruo for leve a moderada,haver fluxo de sangue no tronco pulmonar,ainda que menor, e que ser suficiente parasuprir os pulmes e ainda fornecer algumsangue aorta descendente atravs do ductoarterioso; no caso de obstruo grave ou com-pleta, o suprimento sanguneo para os pul-mes ter origem no sangue que chega sartrias pulmonares proveniente do ductoarterioso (em sentido contrrio ao normal)(19).

Independentemente da direco do fluxosanguneo no ducto arterioso, este encontra-sesubstancialmente reduzido nos fetos comTetralogia de Fallot em comparao com osfetos normais. A reduo no volume de sangueque passa no ducto arterioso provavelmenteinterfere com o desenvolvimento da suaparede muscular, e caso a direco do fluxoseja da aorta para a artria pulmonar, aprpria orientao do ducto arterioso serdiferente (normalmente faz um ngulo obtuso

inferior com a aorta, mas em crianas comTetralogia de Fallot o ngulo frequentementeagudo)(19).

A obstruo do tracto de sada do ventrcu-lo direito tender a desviar sangue do troncopulmonar para a aorta via comunicao inter-ventricular (Figura 3); assim, a aorta compor-tar um volume de sangue superior ao normal(ou no caso de atrsia pulmonar, a totalidadedo volume de sangue ejectado pelos dois ven-

trculos). Tal explica o facto da artria aortaser normalmente mais larga do que o normal

rior angle with the aorta, but in children withtetralogy of Fallot the angle is often acute(19).

The obstruction of the outflow tract of theright ventricle will tend to divert blood fromthe pulmonary trunk to the aorta via ventricu-lar septal defect (Figure 3); the aorta will thus

contain a greater volume of blood than usual(or, in the case of pulmonary atresia, the totalvolume of blood ejected by the two ventricles).This explains why the aortic artery is usuallywider than normal in children with tetralogy ofFallot(19), although, as mentioned earlier, thereis also evidence indicating intrinsic histologi-cal changes that are probably already mani-fested at this stage(16, 18).

Since a large amount of blood shunts from

the right ventricle to the aorta through theventricular septal defect, the normally-exist-ing difference in oxygen saturation betweenthe descending aorta (higher) and descendingaorta will tend to decrease or even disappear,and foetuses with Tetralogy of Fallot willexhibit slightly lower oxygen saturation thannormal in the ascending aorta(19).

Oxygen saturation in the descending aortais slightly higher than normal since less bloodshunts from the pulmonary trunk to thedescending aorta (or there is the same flow inthe opposite direction)(19).

The changes described can have adverseeffects in that the reduced saturation of oxy-gen in the ascending aorta implies the arrivalof less well-oxygenated blood in the cerebraland coronary circulations(19). According to astudy by Zeltser et al.(20), at one year of agemost patients with corrected tetralogy of Fallotshowed neurological development within nor-

mal limits, while genetic factors explain thedifferences between individuals (i.e. in thegenetic syndromes often associated with tetra-logy of Fallot there would likely be develop-mental delay). In foetuses with congenital car-diac disease, Donofrio et al.(21) suggest theexistence of a cerebral self-regulation mecha-nism (brain sparing) that lowers the resist-ance of the cerebral vascular bed and therebyincreases blood flow. This offsets any anomaly

in the distribution of oxygen/nutrients arisingfrom the structural abnormality in question.604


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em crianas com Tetralogia de Fallot (19), emb-ora, como j foi referido, existam tambmevidncias que apontem para alteraes his-tolgicas intrnsecas que provavelmente j se

manifestam nesta fase(16, 18)

.Uma vez que h passagem de grande quan-tidade de sangue do ventrculo direito para aaorta atravs da comunicao interventricular,a diferena de saturao de oxignio quenormalmente existe entre a aorta ascendente(mais elevada) e descendente tender a redu-zir-se ou mesmo a desaparecer, apresentandoos fetos com Tetralogia de Fallot uma satu-rao de oxignio ligeiramente mais baixa do

que o normal na aorta ascendente(19)

.A saturao de oxignio na aorta descen-dente estar ligeiramente superior ao normal,uma vez que h menor passagem de sangue dotronco pulmonar para a aorta descendente (ouh mesmo fluxo em direco contrria)(19).

As alteraes referidas podem ter efeitosadversos no sentido em que a diminuio dasaturao de oxignio da aorta ascendenteimplica chegada de sangue menos oxigenado

s circulaes cerebrais e coronria

(19)

. Noentanto, segundo o estudo de Zeltser e Col(20),

With respect to tetralogy of Fallot, the studydid not show significant differences in theresistance of the cerebral vascular bed com-pared normal foetuses, and the author sug-

gests that these foetuses manage to increasethe cardiac output so as to improve oxygendistribution to the central nerve cells.

In the case of tetralogy of Fallot withabsent pulmonary valve agenesis the arterialduct is normally atretic(13). Here pulmonaryregurgitation arising from the absence of thevalve places the right ventricle in a situationof excessive volume at high pressure (sincethe ventricular septal defect determines

equalisation of pressure). So it is common forthese foetuses to exhibit heart failure (mani-fested as ascites, build-up of pericardial fluidaccumulation, subcutaneous oedema or fetalhydrops), which is far less frequent in moresevere obstruction since, although the pres-sure is maintained on one side, on the otherthe excess of volume is considerably less.Important degrees of regurgitation from thepulmonary valve can also compromise the

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Tetralogia de Fallot no feto. Nos crculos esto indicadas as saturaes de oxignio19

Tetralogy of Fallot in the foetus. Saturation of oxygen is indicated in the circles 19


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ao ano de idade a maioria dos pacientes comtetralogia de Fallot corrigida tinham umdesenvolvimento neurolgico dentro dos li-mites da normalidade, sendo que factoresgenticos explicariam as diferenas inter-individuais encontradas (isto , nas sndromes

genticas frequentemente associados tetra-logia de Fallot era mais provvel existir umatraso do desenvolvimento). Nos fetos comdoena cardaca congnita, Donofrio e Col.(21)

sugerem a existncia de um mecanismo deauto-regulao cerebral que permitiria dimi-nuir a resistncia do leito vascular cerebral demodo a aumentar o fluxo sanguneo (mecanis-mo denominado brain sparing), permitindocompensar qualquer anomalia na distribuio.

de oxignio/nutrientes decorrentes da anoma-lia estrutural em questo. Em relao tetra-logia de Fallot, o estudo no mostrou dife-renas significativas na resistncia do leitovascular cerebral em relao a fetos normais,sugerindo o autor que estes fetos consiguemaumentar o dbito cardaco de modo a melho-rar a distribuio de oxignio s clulas ner-vosas centrais.

No caso de tetralogia de Fallot com age-nesia da vlvula pulmonar, o ducto arterioso normalmente atrsico(13). Nestes casos, aregurgitao pulmonar decorrente da ausnciada vlvula coloca o ventrculo direito numasituao de excesso de volume a presso ele-vada (uma vez que a comunicao interven-tricular condiciona uma equalizao depresses). Assim, comum estes fetos apre-sentarem insuficincia cardaca (manifestadacomo ascite, acumulao de lquido pericrdi-co, edema subcutneo ou hidrpsia fetal),

muito menos frequente na presena de grausmais severos de obstruo pois, embora porum lado a presso se mantenha, por outro oexcesso de volume diminui consideravel-mente. Graus importantes de regurgitao davlvula pulmonar podem tambm comprome-ter a oxigenao dos vasos intrapulmonares(diminuindo a presso diastlica na artriapulmonar, que normalmente se encontra igual presso sistmica, e consequentemente di-

minuindo a perfuso, o que pode explicar oanormal desenvolvimento destes vasos nesta

artery, which is normally equal to the systemicpressure, and thus reducing perfusion, whichmay explain the abnormal development ofthese vessels in this pathology)(19). The dilata-tion of the pulmonary trunk that accompaniesthis disorder can compromise the airways,

which often causes cyanosis in theseinfants(12).

Postnatal circulationAfter birth the oxygenation function of the

blood is transferred from the placenta to thelungs and relies on adequate pulmonary ven-tilation and blood flow. Pulmonary blood flowwill tend to increase, usually by 8 to 10times, because of the lowered pulmonary vas-

cular resistance associated with the ventila-tion of the lungs with air. In tetralogy ofFallot the main factors that determine themagnitude of pulmonary blood flow are: thesource of blood flow to the lungs; the severi-ty and functional behaviour of the obstructionof the tract of the right ventricle, the rightventricle and systemic arterial pressure, andthe arterial duct(19).

Source of blood flow in the lungsIn children with tetralogy of Fallot the

source of blood flow to the lungs can be one ofthe following (or a combination of any ofthem): right ventricle; arterial duct; majoraorto-pulmonary collateral arteries, and bron-chopulmonary collateral arteries(19).

Pulmonary blood flow can derive partly oreven fully from the right ventricle outflowtract, in which case it is essentially deter-mined by the latters degree of obstruction. If

there are no major aorto-pulmonary collateralarteries or if the arterial duct is not patent, thismay be the only source of flow. Because of theobstruction this flow is not usually satisfacto-ry. So, with age, collaterals often developbetween the bronchial arteries and the pre-capillary pulmonary arteries. The factorsresponsible for this increased flow in thebronchial arteries have not been fully eluci-dated. It has been shown that bronchopul-

monary arterial flow increases when there isan increase in systolic arterial pressure or a606


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patologia)(19). A dilatao do tronco pulmonarque acompanha esta patologia pode comprimiras vias areas, o que condiciona frequente-mente cianose nestes recm-nascidos(12).

Circulao ps-natal

Aps o nascimento, a funo de oxigenaodo sangue transferida da placenta para ospulmes, dependendo de uma ventilao efluxo sanguneo pulmonares adequados. Ofluxo sanguneo pulmonar tender a aumentar,normalmente entre 8 a 10 vezes, como resul-tado da diminuio da resistncia vascularpulmonar associada ventilao dos pulmescom ar. No caso da tetralogia de Fallot, osprincipais factores que determinam a magni-

tude do fluxo sanguneo pulmonar so: a fontedo fluxo sanguneo para os pulmes, a gravi-dade e o comportamento funcional da obstru-o do tracto de sada do ventrculo direito, aspresses ventricular direita e arterial sistmi-ca e o ducto arterioso(19).

Fonte do fluxo sanguneo para os pulmesNas crianas com tetralogia de Fallot, a

fonte de fluxo sanguneo para os pulmes podeser uma das seguintes (com possibilidade decombinao entre elas): ventrculo direito;ducto arterioso; artrias colaterias aorto-pul-monares e artrias colaterais broncopulmona-res(19).

O fluxo sanguneo pulmonar pode derivarem parte, ou mesmo na sua totalidade, do trac-to de sada do ventrculo direito, sendo nestecaso determinado essencialmente pelo graude obstruo deste. Se no existirem artriascolaterais aorto-pulmonares ou se o ducto ar-

terioso no se encontrar patente, esta pode sera nica fonte de fluxo. Pela obstruo exis-tente, este fluxo no normalmente satis-fatrio. Assim, com a idade, frequente odesenvolvimento de colaterais entre as art-rias brnquicas e as artrias pulmonares pr--capilares. Os factores responsveis por esteaumento no fluxo das artrias brnquicas noesto totalmente esclarecidos. Foi demonstra-do que o fluxo arterial broncopulmonar au-

menta quando h um aumento na presso arte-rial sistlica ou uma diminuio na presso

fall in pulmonary arterial pressure, and so inpatients with tetralogy of Fallot the two mech-anisms are probably involved. These collater-al arteries are not usually developed at birth,but they will develop with age to allow sur-vival until adulthood(19).

When blood flow depends predominantlyon the arterial duct, as happens in severeobstruction or pulmonary atresia, the magni-tude of blood flow is determined by the size ofthe duct and its degree of constriction. Whenthe principal pulmonary trunk is missing andthere is no confluence of the pulmonary arter-ies, the blood flow can originate in a unilater-al or even in a bilateral arterial duct(19).

In cases of more serious obstruction of the

right ventricle outflow tract the major aorto-pulmonary collateral arteries are more rele-vant. These arteries are born directly from theaorta and communicate with the intrapul-monary portions of the pulmonary arteries.They probably represent persisting primitiveembryological connections between the intra-pulmonary vascular system and the dorsalaorta, which normally regress when that vas-cular system develops connections with thepulmonary arteries(19).

Taking the specific case of pulmonary atre-sia, all the systemic and pulmonary venousblood is mixed in the ascending aorta so thatthe oxygen content of the blood in the sys-temic and pulmonary circulations is essential-ly the same. The degree of oxygen saturationis determined by the ratio between pulmonaryand systemic blood flow. Immediately afterbirth the baby may be moderately cyanosed,but as pulmonary arteriolar resistance falls

then pulmonary flow should increase if themajor aorto-pulmonary collateral arteries arewell-developed. The degree of cyanosis mayfall but occasionally the infant develops heartfailure due to excess volume in the left ventri-cle. Since the major aorto-pulmonary collater-al arteries originate in the aorta they are sub-ject to pressures similar to systemic ones, andso they will subject the pulmonary arteriolesto high pressures and there is a risk of pul-

monary vascular obstructive changes develop-ing in the segments supplied by those vessels. 607


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arterial pulmonar, pelo que nos pacientes comtetralogia de Fallot provavelmente os doismecanismos esto envolvidos. Normalmenteestas artrias colaterais no se encontramdesenvolvidas nascena, mas vo-se desen-volvendo com a idade, permitindo a sobre-

vivncia at idade adulta(19).Quando o fluxo sanguneo depende predo-

minantemente do ducto arterioso, como acon-tece nos casos de obstruo grave ou no casode atrsia pulmonar, a magnitude do fluxosanguneo determinada pelo tamanho doducto e pelo seu grau de constrio. Nos casosde ausncia do tronco pulmonar principal e deno confluncia das artrias pulmonares, ofluxo pulmonar pode ter origem num ducto

arterioso unilateral ou mesmo num ducto arte-rioso presente bilateralmente(19).Nos casos mais graves de obstruo do

tracto de sada do ventrculo direito, assumemmaior relevo as designadas artrias colateraisaorto-pulmonares. So artrias que nascem di-rectamente da aorta e que comunicam com aspores intrapulmonares das artrias pulmo-nares, representando provavelmente a persis-tncia de conexes embriolgicas primitivasentre a rede vascular intrapulmonar e a aortadorsal, que normalmente regridem quandoessa rede vascular desenvolve ligaes com asartrias pulmonares(19).

Analisando o caso particular da atrsiapulmonar, todo o sangue sistmico e venosopulmonar se mistura na aorta ascendente, detal forma que o contedo em oxignio dosangue das circulaes sistmica e pulmonar essencialmente igual. O grau de saturaoem oxignio determinado pela razo entre o

fluxo sanguneo pulmonar e sistmico.Imediatamente aps o nascimento, o bebpode encontrar-se moderadamente cianosado,mas medida que a resistncia arteriolar pul-monar diminui, o fluxo pulmonar poderaumentar se as artrias colaterias aorto-pul-monares major se encontrarem bem desen-volvidas. O grau de cianose pode diminuirmas ocasionalmente o recm-nascido desen-volve insuficincia cardaca por excesso de

volume no ventrculo esquerdo. Como asartrias colaterais aorto-pulmonares major

These collateral arteries tend to progressivelydevelop a degree of stenosis, which protectsthe pulmonary circulation, albeit at the risk ofreducing pulmonary flow and then increasingthe level of cyanosis(19).

Obstruction of the right ventricle out-flow tract

The site, as well as the degree of obstruc-tion of the right ventricle outflow tract, haveimportant haemodynamic consequences. Asalready noted, subpulmonary infundibularstenosis is characteristically found in tetra-logy of Fallot, though we may find stenoses atother levels, too. Resistance at the outflow ofthe right ventricle results from the sum of the

resistance of all these sites and the pulmonaryvascular resistance. With ventricular septaldefect normally being large enough to allowequalisation of pressure in the two ventriclesthe volume ejected by each ventricle willdepend basically on the afterload to which it issubjected. So if total pulmonary resistance isgreater than systemic vascular resistance thisimplies that there will be blood in the rightventricle to be sent to the aorta though theventricular septal defect (right-left shunt),with a drop in the volume ejected to the pul-monary circulation, causing cyanosis. Thegreater the degree of stenosis the greater theshunt and the smaller the pulmonary bloodflow(19).

But if the degree of stenosis is such thatthe total pulmonary resistance is less than thesystemic vascular resistance the blood flow inthe ventricular septal defect will preferential-ly be in the left-right direction, leaving the

heart through the pulmonary aorta. The pul-monary flow will thus exceed the systemicflow and there will be no cyanosis (pink tetral-ogy). In these infants, during the neonatalperiod the infundibular stenosis and the highvascular resistance may combine to limit left-right shunting, and even induce a right-leftshunt, with varying degrees of cyanosis. Aspulmonary vascular resistance falls, so doestotal pulmonary resistance, and a left-right

shunt arises (Figure 4). This shunt can beenough to cause cardiac insufficiency due to608


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tm origem na aorta, esto sujeitas a pressessemelhantes s sistmicas, pelo que irosujeitar as arterolas pulmonares a presseselevadas, existindo o risco de desenvolvimen-to de alteraes obstrutivas vasculares pul-monares nos segmentos supridos por esses

vasos. Estas artrias colaterais tendem adesenvolver progressivamente algum grau deestenose, o que protege a circulao pulmo-nar, embora com o risco de diminuir o fluxopulmonar e logo aumentar o grau de cianose(19).

Obstruo do tracto de sada do ven-trculo direito

O local, bem como o grau de obstruo dotracto de sada do ventrculo direito, apresen-

tam importantes consequncias hemodinmi-cas. Como j foi referido, na Tetralogia deFallot caracteristicamente existe uma este-nose ao nvel do infundbulo subpulmonar,embora possamos encontrar estenoses tam-bm a outros nveis. A resistncia ao fluxo desada do ventrculo direito resulta do soma-trio das resistncias de todos estes locaiscom a resistncia vascular pulmonar. Sendo acomunicao interventricular normalmentesuficientemente grande para permitir equa-lizao de presses nos dois ventrculos, o vo-lume ejectado por cada ventrculo vai depen-der essencialmente da ps-carga a que estsujeito. Assim, se a resistncia pulmonar totalexceder a resistncia vascular sistmica, talimplica que vai existir sangue do ventrculodireito a dirigir-se para a aorta atravs dacomunicao interventricular (shunt direito--esquerdo), com diminuio no volume ejecta-do para a circulao pulmonar, causando cia-

nose. Quanto maior o grau de estenose, maioro shunt e menor o fluxo sanguneo pulmo-nar(19).

Se, no entanto, o grau de estenose for talque a resistncia pulmonar total seja inferior resistncia vascular sistmica, o fluxo de san-gue na comunicao interventricular ter pre-ferencialmente direco esquerdo-direito,saindo do corao pela artria pulmonar. As-sim, o fluxo pulmonar ser superior ao sist-

mico, e no existir cianose (tetralogia cor--de-rosa). Nestas crianas, durante o perodo

an excess of left ventricular volume, althoughthis is not usually important since theinfundibular stenosis always acts to limit theleft-right shunt (which also explains why pul-monary arterial pressure in these children isnever very high, contrary to what happens in

large and isolated, ventricular septal defectdefect). The risk of morphological changes inpulmonary vasculature is therefore relativelylow, especially with increasing age, with thetendency for the infundibular stenosis toincrease (due to progressive hypertrophy ofthe muscle), which limits the left-right shuntand the excessive volume in the left ventricle.This progressive infundibular stenosisthrough muscular hypertrophy can condition

the appearance of cyanosis, initially withphysical exercises and afterwards persisting.The progress of the stenosis varies and it mayappear between 6 and 8 months or only manyyears later(19).

Stenosis in children with cyanosis at birthalso increases due to muscular hypertrophy,with consequent increase in cyanosis and fallin pulmonary blood flow with greater right-leftshunt(19).

The functional behaviour of the subpul-monary infundibulum has been widely stud-ied, especially with a view to determining itsrole in triggering hypoxia episodes. This issuewill thus be looked at later.

Right ventricle pressureAs noted earlier, ventricular septal defect

is usually non-restrictive which implies equalpressure in the two ventricles. The pressure inthe right ventricle follows the same pattern as

that in the left one: rapid rise, with a plateauand a rapid fall(19).

Since the pressure is the same in the twoventricles anything that changes the pressurein one will do so in the other. For instance,systemic venous return interferes with thepressure in the right ventricle, with posturebeing a major factor in venous return.Orthostatic position without motion leads to abuild-up of blood in the lower limbs and

abdomen and a fall in overall venous returnsuch that the pressure in the right ventricle 609


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neonatal, a estenose infundibular e a elevadaresistncia vascular pulmonar podem combi-nar-se de forma a reduzir o shunt esquerdo-direito, e mesmo induzir umshunt direito-es-querdo, com graus de cianose variveis. me-dida que a resistncia vascular pulmonar di-

minui, a resistncia pulmonar total diminuitambm, e surge um shunt esquerdo-direito(Figura 4). Este shunt pode ser suficientepara causar insuficincia cardaca por exces-so de volume ventricular esquerdo, emboraesta no seja normalmente importante umavez que a estenose infundibular actua semprecomo factor limitante do shunt esquerdo-di-reito (o que explica tambm que a pressoarterial pulmonar nestas crianas nunca seja

muito elevada, contrariamente ao que acon-tece no caso dos defeitos interventricularesisolados de grandes dimenses). O risco dealteraes morfolgicas ao nvel da vasculatu-ra pulmonar , assim, relativamente baixo,sobretudo com o avanar da idade, pelatendncia que a estenose infundibular tem emaumentar (por hipertrofia progressiva do ms-culo), o que vem limitar o shunt esquerdo-direito e o excesso de volume no ventrculo

decreases, resulting in a fall in pulmonaryblood flow. This influence of posture onvenous return is important in children withhypoxic crises, in response to physical exer-cise and it can also partly explain the hypox-aemia associated with anaesthesia and seda-

tion (since patients are normally motionless ona flat surface, which leads to accumulation ofblood in the lower limbs). Under the adminis-tration of anaesthetics and sedatives hypox-aemia is also due to the fact that these oftenproduce peripheral vasodilatation, with areduction in peripheral vascular resistance, inthe pressure of the left and right ventriclesand, consequently, a fall in pulmonary flowand a rise in right-left shunt, resulting in

cyanosis(19)

.But an increase in peripheral vascularresistance will elevate aortic and left ventriclepressure, consequently increasing right ven-tricular pressure, pulmonary blood flow(which rises through increased perfusion pres-sure, since the infundibular stenosis is rela-tively fixed) and decreasing right-left shunt-ing. This is the basis for the use of peripheralvasoconstrictors like phenylephrine to treat

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Tetralogia de Fallot com obstruo moderada do tracto de sada do ventrculo direito num recm-nascido. Neste caso, como a obstruono severa, existe um pequeno shunt esquerdo-direito. Nos crculos esto indicadas as saturaes de oxignio 19.

Tetralogy of Fallot with moderate obstruction of the right ventricular outflow tract in a newborn. Here the obstruction is not severe; there isa small left-right shunt. Saturation of oxygen is indicated in the circles19.


21/40

esquerdo. Esta progresso da estenose infundi-bular por hipertrofia muscular pode condicionaro aparecimento de cianose, inicialmente com oexerccio fsico e posteriormente de forma per-sistente. A progresso da estenose varia, poden-do a cianose aparecer entre os 6 e 8 meses ou

mesmo apenas muitos anos depois(19).Tambm nas crianas com cianose ao

nascimento existe aumento da estenose porhipertrofia muscular, com aumento conse-quente da cianose e diminuio do fluxo san-guneo pulmonar com um aumento do shuntdireito-esquerdo(19).

O comportamento funcional do infundbulosubpulmonar objecto de muitos estudos, so-bretudo numa tentativa de esclarecer qual o

seu papel no desencadeamento das crises dehipxia. Este assunto ser por isso abordadoposteriormente.

Presso do ventrculo direitoComo j foi referido previamente, a comuni-

cao interventricular normalmente norestritiva, o que implica uma equalizao depresses nos dois ventrculos. Assim, a pressodo ventrculo direito assume o mesmo perfil quea presso no ventrculo esquerdo: subida rpi-da, com umplateau e uma descida rpida(19).

Uma vez que as presses nos ventrculosso idnticas, tudo que altere a presso numventrculo resultar tambm numa alteraono mesmo sentido da presso no outro ven-trculo. Assim, por exemplo, o retorno venososistmico interfere com a presso no ventrcu-lo direito, sendo a postura um grande determi-nante do retorno venoso. A posio ortostticana ausncia de movimento resulta num ac-

mulo de sangue nas extremidades inferiores eabdmen, com diminuio do retorno venosoglobal, diminuio da presso do ventrculodireito e logo diminuio do fluxo sanguneopulmonar. Esta influncia da postura noretorno venoso importante nas crianas comcrises de hipxia, na resposta ao exercciofsico e tambm pode explicar em parte ahipoxemia associada anestesia e sedao(pois os doentes esto normalmente imveis

numa superfcie plana, o que resulta numaacumulao de sangue ao nvel das extremi-

hypoxic episodes in children. An increase insystemic venous return also raises right ven-tricular pressure and thus pulmonary bloodflow: this explains the beneficial effect of pos-ture in enhancing peripheral blood oxygena-tion in children with hypoxic episodes or after

exercise. A rise in right ventricular pressureand volume can also be beneficial by distend-ing the infundibular area, thereby reducingthe obstruction of the right outflow tract (19).

Although the equalisation of the pressurein the two ventricles is more common, some-times the ventricular septal defect is smalland does not allow this. This occurs veryrarely in childhood and probably occurs froma partial closure of the defect with age. When

the ventricular septal defect is restrictive, thepressure curves of the two ventricles are notnow identical and the pressure curve of theright ventricle can have a contour that is morelike the one typical of pulmonary stenosis withintact septum, that is, a triangular shape(19).

The arterial duct (ductus arteriosus)This duct is important after birth to provide

pulmonary blood flow when the obstruction ofthe right ventricular outflow tract is seriousand there are few if any major aorto-pul-monary collateral arteries. But as it carries areduced blood flow during foetal life it ismuch smaller than normal. With lower pul-monary vascular resistance after birth the flowthrough this duct usually increases, althoughnot sufficiently to allow normal pulmonaryblood flow. Even so, oxygen saturation in theperipheral blood generally rises and reachesbetween 85% and 92% (Figure 5)(19).

The maintenance of oxygen saturation atlevels above foetal ones depends on the paten-cy of the arterial duct. The behaviour of thearterial duct in response to oxygen and otherconstrictors in children with tetralogy of Fallotis not known. As it is usually smaller than nor-mal, its musculature is not likely to be well-developed, and so its behaviour may differfrom the normal. If its response to oxygen werenormal, then an increase in oxygen pressure

(PO2) achieved by the increase in pulmonaryblood flow after birth will tend to constrict the 611


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dades inferiores). No caso particular da admi-nistrao de anestsicos e sedativos, a hipo-xemia advm tambm do facto de estes pro-duzirem frequentemente vasodilatao perifri-ca, com diminuio da resistncia vascularperifrica, das presses dos ventrculos esquer-

do e direito e consequentemente diminuio dofluxo pulmonar e aumento do shunt direito--esquerdo, traduzindo-se em cianose(19).

Pelo contrrio, um aumento da resistnciavascular perifrica ir aumentar as pressesartica e ventricular esquerda, aumentandoconsequentemente a presso ventricular direi-ta, o fluxo sanguneo pulmonar (que aumentapor um aumento da presso de perfuso, umavez que a estenose infundibular relativa-

mente fixa) e diminuindo o shunt direito-es-querdo. esta a base para o uso de agentesvasoconstritores perifricos tais como a fenile-frina no tratamento de crianas com criseshipxicas. Um aumento do retorno venososistmico tambm aumenta a presso ventri-cular direita e logo aumenta o fluxo sanguneopulmonar: isto explica o efeito benfico dapostura no aumento da oxigenao do sangueperifrico em crianas com crises hipxicasou aps o exerccio fsico. Um aumento napresso e no volume ventriculares direitospoderia ter ainda um efeito benfico adicionalpela distenso da rea infundibular, reduzindoassim a obstruo do tracto de sada direito(19).

Embora o mais comum seja uma equaliza-o das presses nos dois ventrculos, porvezes a comunicao interventricular peque-na e no permite que tal acontea. uma situ-ao muito rara na infncia e resultarprovavelmente de um encerramento parcial do

defeito com a idade. Quando a comunicaointerventricular restritiva, as curvas de pres-ses dos dois ventrculos j no so idnticas,e a curva de presso do ventrculo direito podeassumir um contorno mais semelhante ao ca-racterstico das situaes de estenose pul-monar com septo intacto, isto , uma forma trian-gular(19).

O ducto arterioso

Este ducto importante aps o nascimentopara fornecer fluxo sanguneo pulmonar quan-

duct. There could thus be a dynamic relation-ship between the systemic PO2, the degree ofconstriction of the duct and the pulmonaryblood flow. Increased pulmonary blood flowwould lead to higher systemic arterial PO2which in turn will tend to cause the duct to

contract and so interfere with the pulmonaryflow and thereby reduce the systemic PO2,consequently dilating the duct. A changing,unstable situation with intermittent cyanosiscould occur(19).

Systemic arterial pressure is important forits influence on pulmonary blood flow throughthe arterial duct. A fall in pressure thus has anadverse effect on pulmonary blood flow, notonly for its effect of increasing right-left shunt-

ing by reducing the flow through the right ven-tricular outflow tract, but also by decreasing theflow through the arterial duct(19).

In children with tetralogy of Fallot the clo-sure of the arterial duct is usually delayed fora several days or even weeks. This may berelated both to abnormalities in its morpholog-ical development and in its functional devel-opment(19).

Constriction or closure of the arterial ductresults in loss of oxygen saturation of theperipheral blood and a fall in PO2 (Figure 6).The degree and speed of the fall in these fig-ures depends on the extent to which the rightventricle outflow tract is obstructed and howlong it takes the duct to close. If the pul-monary stenosis is severe and the duct closureis swift, peripheral arterial blood saturationwill drop quickly to levels as low as 35% to40%, and PO2 will fall to between 20 and 30mmHg. Hypoxia will then develop. If there is

an adequate opportunity for collateral circula-tion to the lungs to develop the pulmonaryblood flow will be sustained, even after clo-sure of the duct(19).

The arterial duct may not be so importantin supplying blood flow to the pulmonary cir-culation for patients with pulmonary atresiasince major aorto-pulmonary collateral arter-ies usually develop. It may be useful in theearly neonatal period, however, since the fall

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do a obstruo do tracto de sada do ventrcu-lo direito grave e existem poucas ou mesmonenhumas artrias colaterias aorto-pulmona-res. No entanto, como durante a vida fetal con-duz pouco fluxo sanguneo, consideravel-mente mais pequeno do que o normal. Com a

diminuio da resistncia vascular pulmonaraps o nascimento, o fluxo por este ducto nor-malmente aumenta, muito embora no deforma suficiente para permitir um fluxo san-guneo pulmonar normal. Mesmo assim, a sa-turao em oxignio do sangue perifricogeralmente aumenta, atingindo nveis entre85% e 92% (Figura 5)(19).

A manuteno da saturao de oxignio emnveis acima dos fetais est dependente da

patncia do ducto arterioso. O comportamentodo ducto arterioso em resposta ao oxignio e aoutros agentes constritores em crianas comtetralogia de Fallot no conhecido. Comonormalmente mais pequeno que o normal, asua musculatura no estar provavelmentebem desenvolvida, logo o seu comportamentopode diferir do normal. Se a sua resposta aooxignio for a normal, ento um aumento napresso de oxignio (PO2) conseguida peloaumento do fluxo sanguneo pulmonar aps o

arteries can be delayed by exposure to thehigher systemic pressures. Closure of the ductwill remove all the flow from the pulmonaryarteries. The result is that these will not growand they could perhaps become morehypoplastic.

Depending on the severity of the obstruc-tion of the right ventricle outflow tract, aninfusion of prostaglandin E1 may be consid-ered in the immediate neonatal period, with aview to maintaining patency of the arterialduct and thereby keeping a stable source ofpulmonary blood flow(19). Patients who needthis kind of drug treatment will probably needsurgical correction very quickly(9).

Supply of oxygen to the rest of the bodyThe main worry in patients with tetralogy ofFallot is hypoxaemia due to inadequate pul-monary blood flow, which is determined by theright ventricular pressure and the degree ofobstruction. As systolic pressure in the rightventricle is the same as in the left ventricleand the aorta, a drop in systemic arterial pres-sure will result in a reduction in pulmonaryblood flow. In order to monitor any diagnosticor therapeutic procedure in a patient with

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Tetralogia de Fallot com obstruo severa no tracto de sada do ventrculo direito, num recm-nascido que possui um ducto arterioso ainda

patente. Nos crculos esto indicadas as saturaes de oxignio19.

Tetralogy of Fallot with severe obstruction in the right ventricular outflow tract, in a newborn whose arterial duct is still patent. Saturationof oxygen is indicated in the circles19.


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nascimento tenderia a contrair o ducto. Assim,poder existir uma relao dinmica entre aPO2 sistmica, o grau de constrio do ductoe o fluxo sanguneo pulmonar. Um aumento dofluxo sanguneo pulmonar levaria a umaumento na PO2 arterial sistmica a qual, por

sua vez, tenderia a contrair o ducto, inter-ferindo assim com o fluxo pulmonar e logodiminuindo a PO2 sistmica

anatomophysiologic basis of tetralogy

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