The next step in the segmental approach is the determination of the bulboventricular loop, which can be D-loop {–,D,–}, L-loop {–,L,–}, or X-loop {–,X,–}. With D-looping {–,D,–}, the bulboventricular tube bends and shifts to the right, placing the bulbus cordis, the precursor of the right ventricle, to the right of the primitive ventricle which is the precursor of the left ventricle. Therefore, in situs solitus with D-looping {S,D,–}, there is atrioventricular concordance with the right atrium connected to the right ventricle and the left atrium connected to the left ventricle (Fig. 12.1a). In early fetal development, the ventricular portion of the heart, the ventricular mass, is in the right hemithorax. Subsequently, the bulboventricular loop shifts to the left hemithorax, resulting in levoversion. In the case of partial shift of the bulboventricular loop to the midline or failure to shift to the left, the result is situs solitus with mesoversion and situs solitus with dextroversion, respectively.

In L-looping {–,L,–}, the heart tube folds and rotates to the left, which places the bulbus cordis to the left of the primitive ventricle. That means that the ventricular mass is in the left hemithorax, and the bulboventricular loop tends to rotate to the right hemithorax. Therefore, situs solitus with L-looping {S,L,–} results in atrioventricular discordance, with the right atrium connected to the left ventricle and the left atrium connected to the right ventricle (Fig. 12.1b) [10]. As above, the degree of shift of the cardiac mass determines the orientation (version) of the cardiac apex. Complete shift of the apex toward the right results in situs solitus with atrioventricular discordance and dextroversion; partial shift to the midline results in mesoversion and absence of shift results in levoversion. In a similar manner, formation of an L-loop (concordant loop) in cases of situs inversus results in atrioventricular concordance {I,L,–} (Fig. 12.1c), whereas a D-loop (discordant loop) results in atrioventricular discordance {I,D,–} (Fig. 12.1d). Therefore, a complete, partial, or a failure of shift of the cardiac apex to the right further classifies these forms into situs inversus with dextroversion, situs inversus with mesoversion, and situs inversus with levoversion [1, 7, 10].

In X-loop {–,X,–}, a single ventricle is present, and ventricular looping is undefined. Single ventricle or univentricular heart has three major presentations according to the trabecular pattern. If the pattern is similar to either the right or the left ventricle, then it is called single right or single left ventricle, respectively. If the trabecular pattern is indeterminate, then it is called single “undifferentiated” ventricle. The atrial connection is through a common atrioventricular valve (common inlet; AV canal) or through two atrioventricular valves (double inlet; right and left AV valves). Both atrioventricular valves may be patent (double inlet) or one may be often atretic (single inlet). Anatomically, the two valves are different from the tricuspid and mitral valves, and therefore, they should be referred to as right and left atrioventricular valve. The exit of flow of the main ventricular cavity into the great vessels may be direct or through a rudimentary outlet chamber. In single right ventricle, the exit is usually direct, whereas in single left ventricle, the exit is through a rudimentary chamber (outlet chamber). The outlet chamber can be anterior, superior and to the right (normally related), or anterior, superior, and to the left (inverted) of the base of the main cavity. It is uncommon for the outlet chamber to be in front of the main cavity. A bulboventricular foramen (ventricular septal defect; outlet foramen) connects the two chambers. The aorta and the pulmonary artery are almost always transposed (D- or L-malposition), and both great vessels may arise from the rudimentary chamber or from the main cavity. Subvalvular or valvular pulmonary stenosis may be present, and restriction of the bulboventricular foramen may be found. The outlet foramen may straddle the right or left atrioventricular valve depending on its location.

After determining the cardiac situs and the position of the bulboventricular loop, the next step in the segmental approach is to identify the ventriculoarterial relationship [4, 6, 11]. Normally, the heart tube folds to the right {–,D,–}, and the developing ascending aorta is positioned to the right of the developing pulmonary artery in the truncus arteriosus with both structures at the same level. Growth in the subpulmonic free wall of the truncus elevates the pulmonic valve and advances it anteriorly with a rightward rotation; while resorption in the subaortic free wall results in the aortic-mitral fibrous continuity [12]. Therefore, normally the pulmonic valve is positioned anteriorly, superiorly, and to the left of the aortic valve (solitus) {–,–,S}. The mirror-image configuration results in an anteriorly placed pulmonic valve to the right of the aortic valve (inversus) {–,–,I}.

Defining the atrial situs (cardiac situs), the ventricular topology, and the position of the great vessels does not predict the atrioventricular and ventriculoarterial connections, which must be separately evaluated [4, 11]. Similarly, the orientation of the cardiac apex (ventricular mass) is independent of the position of the cardiac atria and depends on the degree of the horizontal shift of the cardiac mass. Defining the position of the cardiac apex after identification of the atrial situs and the cardiac loop determines the “version.” The terms dextroversion, mesoversion, and levoversion indicate the position of the cardiac apex, when this structure is not in the usual location for the corresponding visceroatrial situs. The terms dextrocardia, mesocardia, and levocardia refer to the position of the heart within the thoracic cavity and, in common parlance, are used interchangeably with the version terminology. The cardiac situs is usually concordant with the thoracic situs (tracheobronchial tree) and the position of the ventricular mass (cardiac apex) and must be determined to predict the probability and type of congenital heart disease. There are two forms of determinate situs, situs solitus with levoversion or levocardia, the normal configuration {S,D,S}, and situs inversus with dextroversion or dextrocardia {I,L,I}, the mirror-image configuration.

Interestingly, the degree of shift of the cardiac apex in the horizontal plane (the version) predicts the occurrence and complexity of congenital heart disease. Severe congenital heart disease is present in more than 90 % of people who have situs solitus with dextroversion and situs inversus with levoversion [1, 13]. The severity and occurrence rate is much lower in both situs solitus and situs inversus with mesoversion (Table 12.2).

The thoracic and abdominal organs are usually lateralized on either side of the midline. Situs solitus totalis {S,D,S} refers to the expected configuration of thoracic and abdominal organs with a trilobed right lung and a right-sided liver and inferior vena cava. The cardiac apex is toward the left and the spleen, the stomach and the aorta are also positioned on the left. In the case of normal thoracoabdominal concordance, the rate of congenital heart disease is 0.8 % [13]. The mirror-image configuration of thoracic and abdominal organs is referred to as situs inversus totalis {I,L,I} and has a 5 % incidence of congenital heart disease [13]. Approximately 20 % of the cases of situs inversus totalis are associated with Kartagener syndrome [14] comprised of the classic triad of situs inversus totalis, chronic sinusitis, and bronchiectasis [15]. Conversely, only half of the patients with primary ciliary dyskinesia have situs inversus [12, 13].

In the rare occasion of thoracoabdominal discordance (thoracic situs solitus with abdominal situs inversus or vice versa), there is high incidence of congenital heart disease [1, 16].