It might reasonably be thought that those who diagnose and treat patients with congenitally malformed hearts would, by now, have reached consensus concerning the most appropriate way of describing the malformations with which they are confronted. It is certainly the case that nomenclature is far less contentious now than when we produced the first two editions of this book, in 1987 and 1999. It would be a brave person, nonetheless, who stated that the field of description and categorisation was now fully resolved. There are still major differences of opinion as how best to cope with certain topics, such as those patients who have visceral heterotaxy (or so-called splenic syndromes). It is not our intention, in this chapter, to initiate detailed debates on the differences in the approaches to these, and other, contentious issues. Rather, we will describe our own system for description, leaving the readers to decide whether or not this is satisfactory for their needs. By and large, there is no right or wrong way of describing the hearts, simply different ways. 1,2 Even these different ways have been mitigated to considerable extent by the cross-mapping of existing systems, 3 but this should not detract from the need to resolve ongoing differences according to the nature of the abnormal anatomy as it is observed.
It may be asked, however, whether we need a system for nomenclature, since the hearts themselves have not changed since their initial descriptions. The reason that a standardised approach is preferable is that the number of individual lesions that can co-exist within malformed hearts is considerable. Add to this the possibilities for combinations of lesions, and the problem of providing pigeonholes for each entity becomes immense. We all recognise the nature of straightforward lesions, such as septal deficiencies or valvar stenoses. Almost always these entities are encountered in otherwise normally structured hearts. It is when the hearts containing the lesions are themselves built in grossly abnormal fashion that difficulties are produced. We can no longer be satisfied with a wastebasket category for so-called complex lesions. The recognition of apparent complexity does nothing to determine diagnosis or optimal treatment. If these alleged complex lesions are approached in a simple and straightforward fashion, none need be difficult to understand and describe.
The simplicity comes when we recognise that, basically, the heart has three building blocks, namely the atriums, the ventricular mass and the arterial trunks ( Fig. 1-1 ). A system for description and categorisation based on recognition of the limited potential for variation in each of these cardiac segments was developed independently in the 1960s by two groups: one based in the United States of America, and led by Richard Van Praagh, 1 and the other, from Mexico City, headed by Maria Victoria de la Cruz. 4 Both of these systems concentrated on the different topological arrangements of the components within each cardiac segment. When Van Praagh and his colleagues 5,6 introduced the concept of concordance and discordance between atriums and ventricles, they were concerned primarily with the harmony or disharmony to be found between atrial and ventricular situs. At this time, they placed less emphasis for description on the fashion in which the atrial and ventricular chambers were joined together across the atrioventricular junctions. A similar approach, concentrating on arterial relationships, was taken by de la Cruz and colleagues 7 when they formulated their concept of arterio-ventricular concordance and discordance. These approaches were understandable, since it was often difficult, at that time, precisely to determine how the adjacent structures were linked together.
The advent of cross sectional echocardiography changed all that. Since the mid-1970s, it has been possible with precision to determine how atriums are, or are not, joined to ventricles, and similarly to establish the precise morphology found at the ventriculo-arterial junctions. Since we evolved our system concomitantly with the development of echocardiography, our approach has been to concentrate on the variations possible across the atrioventricular and ventriculo-arterial junctions. We call this system sequential segmental analysis (see Fig. 1-1 ). In making such analysis, we do not ignore the segments themselves. Indeed, junctional connections cannot be established without knowledge of segmental topology. In this respect, we have acknowledged our debts to the other schools as our system has evolved. 8–12
Our system, throughout its evolution, has followed the same basic and simple rules. From the outset, we have formulated our categories on the basis of recognisable anatomical facts, avoiding any speculative embryological assumptions. Again, from the start, we have emphasised the features of the morphology of the cardiac components, the way they are joined or not joined together, and the relations between them, as three different facets of the cardiac make-up. It still remains an undisputed fact that any system which separates out these features, does not use one to determine another, and describes them with mutually exclusive terms, must perforce be unambiguous. The clarity of the system then depends upon its design. Some argue that brevity is an important feature and have constructed formidable codifications to achieve this aim. 13 In the final analysis, however, clarity is more important than brevity. We do not shy, therefore, from using words to replace symbols, even if this requires several words. Whenever possible, we strive to use words that are as meaningful in their systematic role as in their everyday usage. In our desire to achieve optimal clarity, we have made changes in our descriptions over the years, most notably in our use of the term univentricular heart. We make no apologies for these changes, since their formulation, in response to valid criticisms, has eradicated initially illogical points from our system to its advantage. It is our belief that the system now advocated is entirely logical, and we hope it is simple. But, should further illogicalities become apparent, we would extirpate them as completely as we removed 14 univentricular heart from our lexicon as an appropriate descriptor for hearts that possess one big and one small ventricle, showing that this can produce a functionally, but not anatomically, univentricular arrangement. 15
BASIC CONCEPTS OF SEQUENTIAL SEGMENTAL ANALYSIS
The system we advocate depends first upon the establishment of the arrangement of the atrial chambers. Thereafter, attention is concentrated on the anatomical nature of the junctions between the atrial myocardium and the ventricular myocardial mass. This feature, which we describe as a type of connection, is separate from the additional feature of the morphology of the valve or valves that guards the junctions. There are two junctions in the normally constructed heart, and usually they are guarded by two separate valves. The two atrioventricular junctions can be guarded, on occasion, by a common valve. If we are to achieve this analysis of the atrioventricular junctions, we must also determine the structure, topology, and relationships of the chambers within the ventricular mass. As with the atrioventricular junctions, the ventriculo-arterial junctions are also analysed in terms of the way the arterial trunks are joined to the ventricular mass, and the morphology of the arterial valves guarding their junctions. Separate attention is directed to the morphology of the outflow tracts, and to the relationships of the arterial trunks. A catalogue is then made of all associated cardiac and, where pertinent, non-cardiac malformations. Included in this final category are such features as the location of the heart, the orientation of its apex, and the arrangement of the other thoracic and abdominal organs.
Implicit in the system is the ability to distinguish the morphology of the individual atriums and ventricles, and to recognise the types of arterial trunk taking origin from the ventricles. This is not as straightforward as it may seem, since often, in congenitally malformed hearts, these chambers or arterial trunks may lack some of the morphological features that most obviously characterise them in the normal heart. The most obvious feature of the morphologically left atrium in the normal heart, for example, is the connection to it of the pulmonary veins. In hearts with totally anomalous pulmonary venous connection, these veins connect in extracardiac fashion. In spite of this, it is still possible to identify the left atrium. It is considerations of this type that prompted the concept we use for recognition of the cardiac chambers and great arteries. Dubbed by Van Praagh and his colleagues the morphological method, 16 and based on the initial work of Lev, 17 the principle states that structures should be recognised in terms of their own intrinsic morphology, and that one part of the heart which is itself variable should not be defined on the basis of another variable structure. When this eminently sensible concept is applied to the atrial chambers, then the connections of the great veins are obviously disqualified as markers of morphological rightness or leftness since, as discussed above, the veins do not always connect to their anticipated atriums. Although Lev 17 placed great stress on septal morphology as a distinguishing feature, this morphology is of little help when the septum itself is absent, as occurs in hearts with a common atrium. Similarly, the atrial vestibule is ruled out as a marker, since it is usually lacking in hearts with atrioventricular valvar atresia. Fortunately, there is another component of the atrial chambers that, in our experience, has been almost universally present and which, on the basis of the morphology of its junction with the remainder of the chambers, has enabled us always to distinguish between morphologically right and left atriums. This is the appendage. The morphologically right appendage has the shape of a blunt triangle, and its cavity has a broad junction with the remainder of the atrium. The junction is marked externally by the terminal groove, and internally by the terminal crest. Its most significant feature is that the pectinate muscles lining the appendage extend all round the parietal atrioventricular junction ( Fig. 1-2 ).
The morphologically left appendage, in contrast, is much narrower and tubular. It has a narrow junction with the remainder of the atrium, and one that is marked by neither terminal groove nor muscular crest. The pectinate muscles are confined within the morphologically left appendage, with the posterior aspect of the morphologically left vestibule, also containing the coronary sinus, being smooth walled as it merges with the pulmonary venous component ( Fig. 1-3 ).
The morphological method also shows its value when applied to the ventricular mass, which extends from the atrioventricular to the ventriculo-arterial junctions. Within the ventricular mass as thus defined, there are almost always two ventricles. Description of ventricles, no matter how malformed they may be, is facilitated if they are analysed as possessing three components. These are, first, the inlet, extending from the atrioventricular junction to the distal attachment of the atrioventricular valvar tension apparatus. The second part is the apical trabecular component. The third is the outlet component, supporting the leaflets of the arterial valve ( Figs. 1-4 and 1-5 ).
Of these three components, it is the apical trabecular component that is most universally present in normal as well as in malformed and incomplete ventricles. Furthermore, it is the pattern of the apical trabeculations that differentiates morphologically right from left ventricles (see Figs. 1-4 and 1-5 ). This is the case even when the apical components exist as incomplete ventricles, lacking either inlet or outlet components, or sometimes both of these components ( Fig. 1-6 ).
When the morphology of individual ventricles is identified in this fashion, all hearts with two ventricles can then be analysed according to the way in which the inlet and outlet components are shared between the apical trabecular components. In order fully to describe any ventricle, account must also be taken of its size. It is then necessary further to describe the way in which the two ventricles themselves are related within the ventricular mass. This feature is described in terms of ventricular topology, since two basic patterns are found that cannot be changed without physically taking apart the ventricular components and reassembling them. The two patterns are mirror images of each other. They can be conceptualised in terms of the way in which, figuratively speaking, the palmar surface of the hands can be placed upon the septal surface of the morphologically right ventricle. In the morphologically right ventricle of the normal heart, irrespective of its position in space, only the palmar surface of the right hand can be placed on the septal surface such that the thumb occupies the inlet and the fingers fit into the outlet ( Fig. 1-7 ).
The palmar surface of the left hand then fits in comparable fashion within the morphologically left ventricle, but it is the right hand that is taken as the arbiter for the purposes of categorisation. The usual pattern, therefore, can be described as right hand ventricular topology. 18 The other pattern, the mirror image of the right hand prototype, is then described as left hand ventricular topology. In this left hand pattern, seen typically in the mirror-imaged normal heart, or in the variant of congenitally corrected transposition found with usual atrial arrangement, it is the palmar surface of the left hand that fits on the septal surface of the morphologically right ventricle with the thumb in the inlet and the fingers in the outlet ( Fig. 1-8 ).
These two topological patterns can always be distinguished irrespective of the location occupied in space by the ventricular mass itself. A left hand pattern of topology, therefore, is readily distinguished from a ventricular mass with right hand topology in which the right ventricle has been rotated to occupy a left-sided position. Component make-up, trabecular pattern, topology and size are independent features of the ventricles. On occasion, all may need separate description in order to remove any potential for confusion.
Only rarely will hearts be encountered with a solitary ventricle. Sometimes this may be because a right or left ventricle is so small that it cannot be recognised with usual clinical investigatory techniques. There is, nonetheless, a third pattern of apical ventricular morphology that is found in hearts possessing a truly single ventricle. This is when the apical component is of neither right nor left type, but is very coarsely trabeculated, and crossed by multiple large muscle bundles. Such a solitary ventricle has an indeterminate morphology ( Fig. 1-9 ).
Analysis of ventricles on the basis of their apical trabeculations precludes the need to use illogically the term single ventricle or univentricular heart for description of those hearts with one big and one small ventricle. These hearts may have a functionally univentricular arrangement, but all chambers that possess apical trabecular components can be described as ventricles, be they big or small, and be they incomplete or complete. Any attempt to disqualify such chambers from ventricular state must lead to a system that is artificial. Only hearts with a truly solitary ventricle need be described as univentricular, albeit that the connections of the atrioventricular junctions can be univentricular in many more hearts.
In determining the morphology of the great arteries, there are no intrinsic features which enable an aorta to be distinguished from a pulmonary trunk, or from a common or solitary arterial trunk. The branching pattern of the trunks themselves, nonetheless, is sufficiently characteristic to permit these distinctions ( Fig. 1-10 ).
The aorta gives rise to at least one coronary artery and the bulk of the systemic arteries. The pulmonary trunk gives rise directly to both, or one or the other, of the pulmonary arteries. A common trunk supplies directly the coronary, systemic and pulmonary arteries. A solitary arterial trunk exists in the absence of the proximal portion of the pulmonary trunk. In such circumstances, it is impossible to state with certainty whether the persisting trunk is common or aortic. Even in the rare cases that have transgressed one of these rules, examination of the overall branching pattern has always permitted us to distinguish the nature of the arterial trunk.
ATRIAL ARRANGEMENT
The cornerstone of any system of sequential analysis must be accurate establishment of atrial arrangement, since this is the starting point for subsequent analysis. When arrangement of the atriums is assessed according to the morphology of the junction of the appendages with the rest of the atriums, 19 then since all hearts have two atrial appendages, each of which can only be of morphologically right or left type, there are only four possible patterns of arrangement ( Fig. 1-11 ).
The most common is the usual arrangement, also called situs solitus, in which the morphologically right appendage is right-sided, and the morphologically left appendage is left-sided. The second arrangement, very rare, is the mirror image of the usual. It is often called situs inversus, even though the atrial chambers are not upside down. In these two arrangements, the appendages are lateralised, with the morphologically right appendage being to one side, and the morphologically left appendage to the other. The two other arrangements do not show such lateralisation. Instead, there is isomerism of the atrial appendages. In these patterns, the two appendages are mirror images of each other, with morphological characteristics at their junctions with the rest of the atriums on both sides of either right type or left type.
RECOGNITION OF ATRIAL ARRANGEMENT
The arrangement of the appendages, ideally, is recognised by direct examination of the extent of the pectinate muscles round the vestibules (see Figs. 1-2 and 1-3 ). This feature should now be recognisable using cross sectional echocardiography, particularly from the transoesophageal window. In most clinical situations, however, it is rarely necessary to rely only on direct identification. This is because, almost always, the morphology of the appendages is in harmony with the arrangements of the thoracic and abdominal organs. In patients with lateralised arrangements, that is the usual and mirror imaged patterns, it is exceedingly rare for there to be disharmony between the location of the organs ( Fig. 1-12 ).
When the appendages are isomeric, in contrast, then the abdominal organs are typically jumbled-up ( Fig. 1-13 ).
