In this chapter, we deal with one of the most complex, and difficult to treat surgically, of all congenital cardiac malformations. It is still frequent to find the lesion described as pulmonary atresia with ventricular septal defect. Here, we have chosen to call it tetralogy of Fallot with pulmonary atresia. Why should we consider this anomaly part of the spectrum of tetralogy? It is certainly the case that all the patients to be discussed have pulmonary atresia in the setting of a deficient ventricular septum. But other patients with diverse morphologies also have variants of pulmonary atresia with ventricular septal defect, and they will not be discussed. For example, patients with discordant ventriculo-arterial connections as their primary feature, or those with isomeric atrial appendages, or double inlet ventricle, or atrioventricular valvar atresia, can all have pulmonary atresia associated with a hole between the ventricles. In those patients, almost always, the pulmonary arteries are confluent, and are fed by a patent arterial duct. They do not demonstrate the complexity of pulmonary arterial supply that produces the problems encountered so frequently in the setting of systemic-to-pulmonary collateral arteries. We use the heading of tetralogy with pulmonary atresia for this chapter, therefore, because almost without exception, the intracardiac anatomy is that of tetralogy of Fallot when the pulmonary arterial supply is derived from systemic-to-pulmonary collateral arteries.
PREVALENCE AND AETIOLOGY
Because there is no uniformity in how to classify the patients to be described, it is difficult to provide precise incidence for those having tetralogy with pulmonary atresia. Patients with tetralogy of Fallot, considered as a group, made up almost 4% of those with congenitally malformed hearts in the series reported from Liverpool. 1 It is likely that up to one-tenth of these patients will have had pulmonary atresia rather than pulmonary stenosis. In many, the intracardiac anatomy suggests that the pulmonary outflow tract was initially patent, but became atretic during fetal life. In others, particularly in those with systemic-to-pulmonary collateral arteries, the atresia was probably part and parcel of the initial developmental abnormality. 2 Tetralogy of Fallot with pulmonary atresia is known to be associated with deletions of chromosome 22q11, 3,4 such deletions also now established as the cause of the velo-cardiofacial syndrome, which typically consists of tetralogy of Fallot along with facial and aural anomalies, cleft palate, and developmental delay. 5
MORPHOLOGY
Pulmonary atresia exists when there is either complete obstruction, or absence, of the communication normally present between the cavities of the ventricular mass and the pulmonary arteries. In the setting of tetralogy of Fallot ( Fig. 37-1 ), the obstructive form can sometimes be produced by an imperforate pulmonary valve. More usually, the blockage of the pathway is muscular, either at the entrance to, or at the distal end of, the subpulmonary infundibulum. The connection is lacking when there is absence of the pulmonary trunk, with the extreme form represented by absence of all the intrapericardial pulmonary arteries. Just as tetralogy of Fallot with pulmonary stenosis can be found with marked variability in intracardiac morphology, so can tetralogy with pulmonary atresia. Before discussing the crucial variations in pulmonary arterial anatomy, therefore, we will discuss the variations in morphology within the heart.
Intracardiac Structure
Significant variations are to be found in the morphology of the ventricular outflow tracts, in the morphology of the interventricular communication, and in the precise connection of the aorta to the ventricular mass, as well as in the associated malformations. Apart from the associated malformations, these features are interrelated. To fulfill the basic diagnosis as tetralogy of Fallot, the aorta must be connected to the ventricles in posterior position relative to the atretic pulmonary outflow tract. The aortic valvar orifice then overrides the crest of the muscular ventricular septum, albeit to varying degree. The phenotypic feature of tetralogy is seen in the structure of the subpulmonary outflow tract, with the muscular outlet septum, or its fibrous remnant, being displaced anteriorly and cephalad relative to the limbs of the septomarginal trabeculation ( Fig. 37-2 ).
In a small number of cases, the atresia is found at the mouth of the muscular infundibulum, and the pulmonary valve itself may then be patent. Alternatively, there can be an imperforate pulmonary valve ( Fig. 37-3 ). In the most common pattern, the muscular outlet septum fuses directly with the parietal musculature of the right ventricle, thus obliterating the ventriculo-pulmonary junction. There is then a muscular wall between the cavities of the right ventricle and the pulmonary trunk. Occasionally, the subpulmonary outflow tract is completely absent, so that the leaflets of the aortic valve are attached directly to the parietal ventricular wall ( Fig. 37-4 ). This arrangement is reminiscent of common arterial trunk. In the example shown in Figure 37-4 , however, the atretic pulmonary trunk takes its origin from the right ventricular musculature, confirming that the patent arterial trunk is an aorta. Should the intrapericardial pulmonary arteries also be absent, it would be impossible to be sure whether the patent trunk had, initially, been an aorta, and not a common structure. Thus, the arrangement with absence of the pulmonary trunk is best described as a solitary arterial trunk, albeit that clinical presentation is as for tetralogy with pulmonary atresia. In other cases, a fibrous remnant of the outlet septum interposes between the leaflets of the aortic valve and an imperforate pulmonary valve ( Fig. 37-5 ). This arrangement represents tetralogy of Fallot with pulmonary atresia in the setting of a doubly committed and juxta-arterial ventricular septal defect.
The interventricular communication, roofed by the overriding aorta, usually has a fibrous postero-inferior border, made up of continuity between the leaflets of the aortic and tricuspid valves, and often reinforced by a membranous flap. This arrangement makes the defect perimembranous ( Fig. 37-6 ). Cases can also be found, as in tetralogy with pulmonary stenosis, when the postero-inferior limb of the septo-marginal trabeculation fuses with the ventriculo-infundibular fold. In this setting, the defect has exclusively muscular borders when viewed from its right side (see Fig. 37-2 ). This muscular rim, when present, serves to protect the ventricular conduction tissues, separating them from the crest of the septum. As discussed previously, the defect can also extend to become doubly committed and juxta-arterial (see Fig. 37-5 ). Such doubly committed defects can themselves extend to become perimembranous, but more usually have a muscular postero-inferior rim. Rarely, the ventricular septal defect may be restrictive, or even completely blocked, due to tissue tags derived from the leaflets of the tricuspid valve. In this latter setting, the overall anatomy of the heart is more like pulmonary atresia with intact ventricular septum, usually with a thick-walled right ventricle and a reduced cavity.
The precise connection of the leaflets of the aortic valve, as in tetralogy with pulmonary stenosis, can vary markedly. In most instances, the leaflets of the aortic valve are connected largely within the left ventricle. Hearts can also be found with predominant, or even total, commitment of the aorta to the right ventricle. This latter combination produces tetralogy of Fallot with pulmonary atresia, but with the ventriculo-arterial connection of double outlet ventricle.
Morphology of the Intrapericardial Pulmonary Arteries
When the pulmonary valve is imperforate, the pulmonary trunk is present, and patent, to the level of the ventriculo-pulmonary junction (see Fig. 37-3 ). Even in this setting, the trunk itself may supply only one pulmonary artery, the other either having no connection with the pulmonary trunk, or else being completely absent. In many other cases, the pulmonary trunk itself is atretic (see Fig. 37-5 ). In extreme cases, it is recognisable only as a fibrous strand running between the ventricular outflow tract and the pulmonary arterial confluence, or else joining to one or other of the pulmonary arteries. When the right and left pulmonary arteries are present, usually they are confluent. The confluence itself, usually tethered by either a patent or atretic pulmonary trunk to the ventricular mass, has the characteristic angiographic appearance of a flying seagull. It can vary markedly in size, usually dependent on its source of arterial supply.
The right and left pulmonary arteries can be non-confluent, but one of them usually retains its connection to the remnant of the pulmonary trunk. Non-confluent pulmonary arteries can rarely be found in the absence the pulmonary trunk. Each can then either be supplied by one of bilateral arterial ducts, or one lung can be supplied by systemic-to-pulmonary collateral arteries, with the other fed by a duct through the persisting extrapericardial pulmonary artery. In the most severe examples, the entire intrapericardial arterial tree can be lacking, with supply to the lungs exclusively through systemic-to-pulmonary collateral arteries.
Morphology of Pulmonary Arterial Supply
The final common pathway of pulmonary supply is the capillaries supplying the air sacks of the lungs. These capillaries are connected to an intrapulmonary plexus of arteries, which ramifies within the bronchopulmonary segments. Different parts of the plexus can be supplied with blood from different systemic sources. If all intrapulmonary arteries are connected to unobstructed and confluent intrapericardial pulmonary arteries, the confluence typically supplies all of both lungs, and pulmonary arterial supply is said to be unifocal. When different parts of one lung are supplied from more than one source, the supply is said to be multi-focal.
Unifocal Pulmonary Blood Supply
It is usually the persistently patent arterial duct that provides unifocal pulmonary arterial supply ( Fig. 37-7 ). It is exceedingly rare for confluent pulmonary arteries feeding all of both lungs to be supplied by a solitary systemic-to-pulmonary collateral artery. In rare cases, however, the confluent pulmonary arteries can be fed in unifocal fashion through an aortopulmonary window, or via a fistula from the coronary arteries. In the past, it has also been suggested that the pulmonary arteries can be fed unifocally through a persistent fifth aortic arch. 6 A fifth arch has never been identified during embryological development, so this explanation seems unlikely. Almost certainly the structures described as the fifth arch represent a malpositioned arterial duct ( Fig. 37-8 ).
Multifocal Pulmonary Blood Supply
It is the presence of multifocal pulmonary arterial supply that creates the clinical complexity in tetralogy with pulmonary atresia. The multiple vessels feeding the pulmonary parenchyma are systemic-to-pulmonary collateral arteries ( Fig. 37-9 ). Such arteries hardly ever feed a lung that also receives supply via the arterial duct. It is a useful working rule, therefore, to assume that an arterial duct will not be present when a lung is supplied by systemic-to-pulmonary collateral arteries. Although the systemic-to-pulmonary collateral arteries hardly ever co-exist in the same lung with an arterial duct, they do usually co-exist with confluent intrapericardial pulmonary arteries ( Fig. 37-10 ). In such circumstances, the confluent pulmonary arteries are hardly, if ever, connected to all of the bronchopulmonary segments of both lungs. Instead, it is the rule for different arteries to supply different segments of the two lungs. The confluence of intrapericardial pulmonary arteries, itself fed by one or more major systemic-to-pulmonary collateral arteries, is connected to only part of the lungs, while the remainder of the pulmonary parenchyma is supplied directly by a variable number of major systemic-to-pulmonary collateral arteries. These individual collateral arteries feed individual intrapulmonary segments, or groups of segments ( Fig. 37-11 ). They can also communicate with the confluent intrapericardial pulmonary arterial tree (see Fig. 37-10 ). When both intrapericardial and systemic-to-pulmonary collateral arteries feed different parts of the pulmonary parenchyma, it is essential to determine the proportions supplied by each of the pathways, remembering that in the extreme form of the anomaly, the entirety of both lungs is fed exclusively by systemic-to-pulmonary arteries.
Another variety of multifocal supply is found when the pulmonary arteries are present but non-confluent. The different parts of the lungs may then be supplied by systemic-to-pulmonary collateral arteries, by a duct, by a coronary arterial fistula or aorto-pulmonary window, or by a combination of these. Alternatively, the intrapulmonary arteries may not be supplied either via an arterial duct or by major systemic-to-pulmonary collateral arteries. Blood can then reach them only at precapillary level through acquired collateral arteries. These may either enter the lungs centrifugally through the bronchial arteries, or centripetally via the intercostal or coronary arteries. These acquired collateral arteries can co-exist with the other varieties of arterial supply.
Major Systemic-to-Pulmonary Collateral Arteries
These arteries are characteristic for the so-called complex variant of tetralogy of Fallot with pulmonary atresia. Their relationship to the bronchial arteries has yet to be fully established. Some of the major collateral arteries have no independent course within the lung parenchyma, extending only from a systemic artery, usually the aorta, to the origin of the intrapulmonary arteries at or near the hilum ( Fig. 37-12 ). Arteries with this morphology are simple conduits. In other circumstances, the collateral arteries extend into the lung along the bronchial tree, branching in the pattern of a bronchial artery, and supplying also the bronchial wall ( Fig. 37-13 ).
A common embryological origin of these vessels with the bronchial arteries cannot be excluded. 7 It is still recommended, nonetheless, to describe these vessels as systemic-to-pulmonary collateral arteries. The arteries, typically two to six in number, usually arise from the anterior wall of the aorta opposite the origin of the intercostal arteries (see Fig. 37-9 ). Individual collateral arteries can also take origin from the brachiocephalic arteries, or even from the coronary arteries. When arising from the aorta, the arteries frequently run a retro-oesophageal course (see Fig. 37-11 ). Usually they can be distinguished from a duct by their histological structure. They can also be distinguished anatomically in most cases, since the arterial duct originates only from a given point within the aortic arch, albeit masquerading sometimes as a purported fifth aortic arch (see Fig. 37-10 ). Typically, even when branching from a non-dominant aortic arch, the duct originates more or less opposite the take-off of a brachiocephalic or subclavian artery.
Characteristic Patterns of Pulmonary Arterial Supply
The potentially complex situation can be simplified by recognition of three major patterns of pulmonary arterial supply. The most favourable arrangement is that in which the right and left pulmonary arteries are confluent, and are supplied by an arterial duct (see Fig. 37-7 ). With this pattern, the pulmonary arteries themselves are usually distributed in normal fashion to all the bronchopulmonary segments. Such pulmonary arterial supply is unifocal.
In the second major pattern, the intrapericardial pulmonary arteries are confluent, but co-exist with systemic-to-pulmonary collateral arteries ( Fig. 37-14 ). The distribution of the confluent pulmonary arteries themselves is then variable, but hardly ever supplies all the bronchopulmonary segments, with blood passing through the confluence often supplying two-thirds or less of the pulmonary parenchyma. Even in this setting, the ultimate supply to the pulmonary arteries is via the collateral arteries, with anastomoses with the intrapericardial network being possible at hilar, lobar, or segmental levels ( Fig. 37-15 ). The confluence of the pulmonary arteries itself also varies markedly in size, reflecting the number of the bronchopulmonary segments supplied. In this setting, those parts of the lung not supplied by the intrapericardial pulmonary arteries are fed directly by systemic-to-pulmonary collateral arteries, with further variation in the number of arteries present, and the amount of lung supplied by each artery. In most cases, the peripheral supplies of central pulmonary arteries and the collateral arteries do not overlap, but in a proportion of segments two sets of arterial ramifications intermingle ( Fig. 37-16 ). 7,8
The third typical pattern of arterial supply ( Fig. 37-17 ) is encountered when there is absence of the intrapericardial pulmonary arteries. In such circumstances, all the bronchopulmonary segments are supplied by multiple systemic-to-pulmonary collateral arteries. In the presence of systemic-to-pulmonary collateral arteries, therefore, the key to complete clinical diagnosis is to establish the course of each artery, to establish whether it runs directly into the lung or makes connections with intrapericardial and central pulmonary arteries, and to identify with precision the sites of these anastomoses.
MORPHOGENESIS
Much has been written about the morphogenesis of both the ventricular and pulmonary arterial features of tetralogy of Fallot with pulmonary atresia, albeit derived from speculative embryological concepts, and arguably not improving our understanding. From the stance of ventricular morphology, nonetheless, the anomaly is readily explained in terms of end-stage tetralogy of Fallot, with variation depending upon the specific morphology of the subarterial outlets. Some cases, in contrast, can be interpreted as representing common arterial trunk with absence of the intrapericardial pulmonary arteries, such as those in which a solitary trunk is connected to the ventricular mass in absence of central pulmonary arteries. This anomaly was initially classified along with other variants of common arterial trunk. 9 On re-examination of these specimens, doubt was raised as to whether the central pulmonary arteries were indeed absent, or instead were severely hypoplastic. 10 Examples do exist, nonetheless, with unequivocal absence of the intrapericardial pulmonary arteries absent, and with no evidence within the right ventricle of the subpulmonary infundibulum. The argument of common trunk versus tetralogy then depends upon whether the absent pulmonary arteries, had they been present, would have taken origin from an arterial trunk or directly from the right ventricle. The argument is no longer hypothetical, since hearts have now been found with an atretic pulmonary trunk arising from an arterial trunk, showing that the trunk itself had initially been a common structure. 11 Experimental studies of rats dosed with bisdiamine have also shown that some fetuses develop classical tetralogy of Fallot, while others exhibit common arterial trunk with pulmonary atresia. 2 From the standpoint of anatomy, this conundrum is easily resolved simply by describing the ascending great artery found in absence of the pulmonary trunk as a solitary arterial trunk rather than an aorta. Summarising the overall morphogenesis of tetralogy of Fallot with pulmonary atresia, the anatomical prototypes can readily be interpreted, on the basis of morphology, as developing in the setting of typical tetralogy, in the setting of tetralogy with doubly committed ventricular septal defects or, very rarely, in the setting of common arterial trunk.
Embryology has also been invoked to account for the typical patterns of pulmonary arterial supply. 12 Thus, the lungs in tetralogy with pulmonary atresia are supplied either through the confluence of the pulmonary arteries fed by the arterial duct, itself derived from the embryological sixth aortic arch, or else through systemic-to-pulmonary collateral arteries. Initially, it is known that the developing intrapulmonary arterial plexus is connected to the primitive intersegmental arteries, which in turn are connected to the aortic arch system, eventually via the fourth arch. 13 The concept advanced to explain the arrangement seen in the abnormal hearts is that, when the intrapulmonary plexus eventually achieves its connection to the sixth arch, it loses its connections with the fourth arch and the systemic arterial system. The systemic-to-pulmonary collateral arteries are then explained on the basis of persistence of the primitive intersegmental arteries, some of which also become bronchial arteries. It is argued that these collateral arteries persist only in absence of the duct, which is the critical connection between the structures derived from the sixth arch and the aortic sac. This concept accounts adequately for the majority of cases, and offers an excellent working hypothesis. It is undermined by those occasional instances when systemic-to-pulmonary collateral arteries co-exist with the duct, and both supply the intrapulmonary plexus in the same lung.
In the majority of cases, nonetheless, embryology aids greatly in understanding the complexity of the pulmonary arterial supply. In essence, the intrapulmonary arteries develop along with the lung. They are the final common pathway supplying arterial blood to the pulmonary air sacs. This common pathway can be supplied at the hilum, either by the intrapericardial pulmonary arteries fed through the arterial duct, the derivative of the artery to the sixth pharyngeal arch, by the rarer sources of unifocal supply, or else by systemic-to-pulmonary collateral arteries, which are primitive intersegmental arteries. These sources of supply can anastomose with different parts of the lungs in the same patient, although usually all the arteries in one lung are supplied either by the duct, or else by the systemic-to-pulmonary collateral arteries. The common pathway can subsequently be further enhanced by acquired collateral arteries, which then reinforce the acinar supply at precapillary level.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
