Congenital Heart Disease: General Principles and Imaging

Step 1—Atrial Situs

Atrial situs is determined by an assessment of the morphology of each atrial appendage. Correct identification of the atrium allows the subsequent determination of the AV connection. The atrial appendages are the most consistent feature of the atrial mass; indeed, the venous attachments to each atrial chamber can form a variety of combinations. The right atrial appendage is a triangular shape, with a broad base and prominent pectinate muscles that extend around the right AV valve, whilst the left atrial appendage is a more elongated, tubular structure and has less extensive pectinate muscles that are confined within the appendage.

The most common lesions involve inversion of situs, or isomerism of the left or right atrial appendages. The non-cardiac thoracic and abdominal organs usually (but not always) demonstrate a similar ‘sidedness’ to that of the atrial chambers.

In the normal heart the morphological right atrium is located to the right of the morphological left atrium (situs solitus). The right lung is trilobed, with a shorter, early-branching bronchus and the left lung is bilobed. In addition, the inferior vena cava (IVC) is to the right of the abdominal aorta, with a right-sided liver and left-sided spleen.

In situs inversus the mirror image of the normal anatomy is present.

Isomerism of the left atrial appendages is usually associated with bilateral bilobed lungs, polysplenia and IVC interruption. Isomerism of the right atrial appendages is usually associated with bilateral triilobed lungs, asplenia and a midline liver. In isomeric lesions there is often a common AV junction (instead of two separate and offset left and right junctions) with varying degrees of AV septal defect (AVSD). Gut malrotation is associated with both right and left-sided isomerism.

Step 2—Ventricular Morphology

Determination of ventricular morphology allows analysis of AV and ventriculo-arterial connections. An AV connection is described as ‘concordant’ when the atria are connected to the expected ventricle (i.e. left atrium with left ventricle and right atrium with right ventricle); ‘discordant’ if the left atrium is connected to the right ventricle and right atrium to the left ventricle; ‘ambiguous’ if there is isomerism of the atrial appendages (e.g. two morphologically right atria connected to a left and right ventricle, respectively (one connection is concordant, the other discordant)); and, finally, ‘univentricular’ if both atria predominately connect to a single ventricle. Irrespective of AV concordance, the AV valve is always concordant with the ventricle—that is the tricuspid valve connects to the morphological right ventricle and the mitral valve connects to the morphological left ventricle.

The most distinguishing feature of the tricuspid valve is the direct attachments to the septum of cords from the septal leaflet. Unlike the tricuspid valve, the mitral valve has no direct septal attachments. The septal insertion of the tricuspid valve is more apical (apically ‘offset’) than that of the mitral valve and these features aid determination of the ventricular morphology. The muscular structure of the ventricles also differs, with the RV being more trabeculated than the LV, with a muscular infundibulum and mid-ventricular ‘moderator band’. Although they are different in normal subjects, the size, shape and degree of trabeculation of the ventricles are not good indicators of ventricular origin, as all are dependent on load effects.

Step 3—Ventriculo-Arterial Connection

Description of ventriculo-arterial connections represents the final element of sequential segmental analysis. This entails describing how each great vessel (aorta, pulmonary artery [PA], or common trunk) is connected to its respective ventricle. A ventriculo-arterial connection may be concordant (RV-PA, LV-aorta), discordant (RV-aorta, LV-PA), double outlet (e.g. RV-PA and aorta) or single outlet (e.g. LV and RV to common arterial trunk). The aorta and pulmonary arteries are defined by their typical branching patterns. Three-dimensional balanced steady-state free-precession (b-SSFP) and contrast-enhanced magnetic resonance angiography (MRA) techniques are particularly useful in determining the arrangement of the great vessels and the connections with their respective ventricles.

Step 4—Identification of Other Abnormalities

Other abnormalities to be considered include abnormal systemic and pulmonary venous connections, intracardiac shunts, valvar abnormalities and vascular abnormalities (PDA, right/left aortic arch, coarctation/interruption or pulmonary arterial abnormalities).

In general, most congenital cardiac lesions are single abnormalities that are easily described; however, almost any combination of abnormalities and connections can occur, and using the sequential segmental analysis method, the description of all conceivable combinations and diagnoses is possible. For more advanced reading, the reader is referred to the textbook Paediatric Cardiology by Anderson and colleagues (see Further Reading).

Diagnostic Features

The ubiquity of the CXR in clinical practice warrants discussion of the diagnostic features that should prompt suspicion of CHD. It is suggested that when reporting images, the reader avoid such terms as ‘boot-shaped’ or ‘snowman’ typically associated with specific lesions because they can be misleading and often erroneous. More appropriate is a descriptive consideration of the cardiomediastinal contour and lungs, attempting to evaluate the predominant physiological profile discussed above. The reader may find it helpful to read this section in conjunction with the section on clinical presentation.

The Pulmonary Vasculature

Radiologically normal pulmonary vascularity is present in CHD if the patient is not in heart failure, if no large shunt is present and if there is no significant reduction in pulmonary blood flow: for example, mild PS. The pulmonary vasculature may, however, look normal on the conventional radiograph even in the presence of significant CHD.

Increased pulmonary perfusion (pulmonary plethora) is recognised by enlarged central and peripheral pulmonary arteries and veins in all zones ( Fig. 13.3A ), as occurs in situations with increased pulmonary blood flow: ASD, VSD and PDA with large left-to-right shunts ( Table 13.1 ).

Decreased pulmonary perfusion (oligaemia) ( Fig. 13.4 ) is caused by a reduction in pulmonary blood flow and is typically a phenomenon of cyanotic CHD. Dark lungs and sparse pulmonary vascular markings suggest the diagnosis. Image acquisition must be optimal because overexposure will significantly confound correct interpretation. Pulmonary blood flow may be impaired by obstruction to normal flow through the right heart: for example, tricuspid atresia, TOF and PS ( Table 13.2 ).

Pulmonary venous congestion and oedema (see Fig. 13.3B ) in CHD is caused by functional or anatomical obstruction to pulmonary venous return. In addition to oedema formation caused by increased transvascular pressure gradients, consideration should be given to other pathological processes such as increased vessel leakiness caused by acute lung injuries, for example ( Table 13.3 ). The usual adult pattern of basal oedema, resulting in alveolar hypoxia and constriction of lower pulmonary vasculature and redirection to the apices does not apply to the supine infant. As pulmonary venous pressure increases, there is progressive accumulation of radiological signs, beginning with redistribution (in older children/adults), progressing to interstitial oedema (perivascular haziness, peribronchial cuffing, Kerley B lines, subpleural effusions) and, finally, migration of extravasated fluid centrally, resulting in perihilar alveolar consolidation.

Systemic to pulmonary collateral vessels . Abnormal systemic arterial connections to the pulmonary vasculature may occur as an adaptive mechanism to inadequate pulmonary blood flow. This usually occurs in the setting of pulmonary atresia associated with VSD, in which the RV and pulmonary arteries are not in continuity; instead, discrete MAPCAs (major aortopulmonary collateral arteries) and non-discrete networks of bronchial arteries are the source of pulmonary blood flow. It may also occur during staged management of the single ventricle. They may be recognisable by a nodular lung pattern in the central third of the lung parenchyma, with many small, rounded, opacities representing enlarged bronchial arteries seen end-on.

Pulmonary arterial hypertension may complicate unrepaired CHD. Increased pulmonary blood flow caused by left-right shunting in unrepaired ASD, VSD or PDA gradually causes changes in the pulmonary vasculature, which, over time, leads to increased PVR and overt hypertension. The central pulmonary arteries enlarge and the peripheral pulmonary arteries become smaller than normal. In cases where pulmonary pressure exceeds systemic pressure, shunt reversal occurs, resulting in cyanosis—as occurs in Eisenmenger syndrome.

Heart Size, Shape and Position

Abnormalities of the position of the cardiac apex, aortic arch, liver and stomach may be determined from examination of the CXR. The presence of situs inversus and left aortic arch may be discerned; however, this may or may not be associated with underlying CHD. Some assessment of global and regional heart size is possible (see Fig. 13.3B ) and should be described; however, the limitations of CXR in this regard should be considered. In a study comparing echocardiographical assessment of cardiac enlargement in 95 consecutive paediatric outpatients, the sensitivity of the CXR to identify cardiomegaly was only 58.8% (95% confidence index (CI): 32.9 to 81.6), specificity was 92.3% (95% CI: 84.0 to 97.1).

Atrial Septal Defects

ASDs are the most common congenital heart defect detected in adults. Irrespective of their type and location, isolated ASDs cause left-to-right shunting at the atrial level. This leads to atrial dilation, predisposing to tachyarrhythmias, and RV volume overload. The degree and direction of atrial shunting can be modified by AV valve function and ventricular compliance. The presence of an ASD is an independent risk factor for thromboembolic stroke. This is caused by the ability of thromboemboli, originating either in the right atrium or venous vasculature, to pass through the ASD into the systemic circulation.

ASDs are, anatomically and developmentally, a heterogeneous group of lesions ( Fig. 13.5A ). The specific nature of the ASD influences the natural history and management of this disease. Ostium secundum defects make up 80% of ASDs and are located in the fossa ovalis (see Fig. 13.5B ). These defects are caused by failure of the septum secundum to form closure of the ostium secundum. Other forms of ASD are more properly termed interatrial communications because they do not occur in the true morphological atrial septum. The ostium primum defect is actually a component of a common AV junction, also known as an AVSD. This defect usually occurs together with some degree of AV valve abnormality. The sinus venosus defect is found at the junction of the right atrium and either one of the caval veins (see Fig. 13.5C ). This type of ASD is less common and is always associated with partial anomalous pulmonary venous drainage. The least common type of ASD occurs in the coronary sinus and is termed an unroofed coronary sinus. In this case, there is deficiency of the coronary sinus wall as it passes behind the left atrium, allowing shunting from left to right through the coronary sinus itself.

The management of ASDs has changed in recent years, particularly with the increasing use of transcatheter ASD closure devices. Previously, surgical closure was only considered when a large left-to-right shunt led to RV volume overload, atrial dilation and symptoms; however, with the advent of transcatheter techniques, management has become more aggressive. Transcatheter techniques are only viable in patients with small-to-medium-sized ostium secundum defects that have adequate margins with which to anchor the device. Deficiency of the anterior or posteroinferior rim of the defect usually precludes transcatheter closure. Patients with large ostium secundum defects, or defects with deficiency of the anterior or posteroinferior rim. or with sinus venosus lesions, usually require operative repair. The clinical aim is to complete ASD closure before the development of cardiac failure or atrial dilation and timing of intervention depends on the haemodynamic status of the patient; thus, evaluation of ASDs requires definition of type and location of the defect, quantification of the net shunt (pulmonary flow: systemic flow (Q _p :Q _s )), detection of any intra-atrial thrombus, assessment of RV volume and systolic function and visualisation of the pulmonary venous anatomy.

Visualisation of most interatrial communications is possible by transthoracic echocardiography, although sinus venosus or coronary sinus defects are challenging without a high level of suspicion. In addition, detection of pulmonary venous abnormalities is technically difficult using the transthoracic approach. Transoesophageal echocardiography is the main imaging technique used to assess ASDs (particularly at the time of catheter and surgical closure); however, transoesophageal echocardiography cannot be used to accurately quantify the shunt (Q _p :Q _s ) and it can be difficult to delineate pulmonary venous anatomy. CMR has, therefore, a significant role in the diagnosis and pre-interventional assessment of ASDs.

Three-dimensional whole-heart techniques, with isotropic resolution, allow accurate multiplanar reformatting with no loss of resolution. These techniques allow 3D rendering of the atrial anatomy. Multislice 2D gradient-echo techniques can be used to assess the dynamic 3D anatomy of the defect, and phase-contrast through-plane flow techniques can accurately size the cross-sectional dimensions of the defect. Multiple or fenestrated defects may also be diagnosed.

Haemodynamic assessment is also an important part of the evaluation of ASDs. Invasive catheterisation has historically been used to quantify left-to-right shunts. Quantification of left-to-right shunts using velocity-encoded phase-contrast MRI compares well to invasive catheterisation results (see Fig. 13.5D ). It has the benefit of being non-invasive and does not require exposure to ionising radiation. Ventricular overload can also be accurately assessed using multislice b-SSFP short-axis imaging and can give important information influencing the timing of intervention.

Atrioventricular Septal Defects

An atrioventricular septal defect (AVSD) is a lesion caused by a deficiency of the tissues that normally interpose the atrial and ventricular chambers ( Fig. 13.6A and C ). The involved tissues include the atrial primum septum, the AV valves and the inlet portion of the ventricular septum. The feature shared by all AVSDs is a common AV junction guarded by a common AV valve, which may have either one or two orifices (see Fig. 13.6A ).

The common AV junction can be discerned by the loss of the usual ‘offset’ of the tricuspid and mitral valves in the normal heart. The valve, even when it has two orifices, is no longer referred to as a mitral and tricuspid valve; instead they are called left and right AV valves. The common valve typically has five leaflets, referred to as the superior bridging, right anterosuperior, right inferior/mural, inferior bridging and left mural leaflets (see Fig. 13.6AB ).

The relative deficiency of the septal structures and the number of valve orifices give rise to the classification as complete (both ASD and VSDs and single valve orifice), intermediate/incomplete (VSD with two valve orifices) and partial (ASD with two valve orifices, also called an ostium primum ASD). Another clinically useful description is the relative size of the ventricular chambers, allowing for classification as balanced (equal-sized ventricles) or unbalanced (disproportionate ventricles). AVSD can be associated with other cardiac abnormalities, including TOF, subaortic stenosis, atrial isomerism and ventricular hypoplasia, which modify the presentation, prognosis and surgical management.

The diagnosis of AVSD is made in the neonatal period on the basis of a transthoracic ECG. Other imaging techniques are usually not required. Surgical repair is carried out at approximately 3 to 4 months of age and certainly before 6 months of age to prevent the development of pulmonary vascular disease. The repair involves closing the septal defects and creating competent left and right valves from the common AV valve tissue. The association of AVSD with trisomy 21 is well known; repair in this group is associated with lower mortality than non-trisomy 21.

Additional imaging techniques including CMR may be useful in the long-term management of patients with repaired AVSD, including surveillance for important late complications such as AV valve regurgitation (see Fig. 13.6C ).