CARDIAC MALPOSITIONS INCLUDING HETEROTAXY SYNDROMES

P. SYAMASUNDAR RAO, MD

Introduction

Dextrocardia and other cardiac malpositions are usually associated with complex congenital heart defects (CHDs). Furthermore, most physicians are flabbergasted when it comes to evaluation of a baby with cardiac malposition. In this chapter, an orderly, methodical, and systematic approach is presented, hoping to make the evaluating physician more comfortable in approaching an infant with cardiac malposition.

Cardiac Malposition

Definitions – Terminology

Normally, the heart is in the left side of the chest (Figure 36.1) and is described as levocardia. If the heart is in the right side of the chest (Figure 36.2), it is called dextrocardia; or it may be in the middle of the chest (Figure 36.3) and is named mesocardia.¹ Apicality of the heart cannot always be determined with certainty and is not necessary for the diagnostic approach, which will be described in this chapter.

The heart may be pushed (Figure 36.4 and 36.5) or pulled (Figure 36.6) to the right, in which case it is described as dextroposition.

• Situs solitus: Normal position of the viscera with the liver on the right side and the stomach on the left (see Figure 36.1).
  
• Situs inversus: Left-to-right reversal of the viscera with the stomach on the right and liver on the left (see Figures 36.2B and 36.7).
  
• Situs inversus totalis: Dextrocardia with situs inversus (see Figures 36.2B and 36.7).
  
• Isolated dextrocardia: Dextrocardia with situs solitus (Figure 36.8).
  
• Isolated levocardia: Levocardia with situs inversus (Figure 36.9).

Since the babies with dexroposition do not have a higher incidence of heart disease than normal babies, and because it is important to address the condition causing shift of the cardiac position, it is best to identify dextroposition first. The heart may be pushed to the right by thoracic cage abnormality, empyema, pneumothorax, lobar emphysema or diaphragmatic hernia on the left side or pulled to the right by collapse or hypoplasia of the right lung, including scimitar syndrome. Some of these are shown in Figures 36.4, 36.5 and 36.6. A careful review of the chest x-ray may indentify the abnormality causing dextroposition. In addition, deviation of the trachea to the right (see Figures 36.4, 36.5 and 36.6) will help in the diagnosis.

Prevalence

As mentioned above, heart is usually normal in dextroposition. In dextrocardia the prevalence of CHD is higher; this varies with the associated visceroatrial situs. In situs inversus totalis, CHD is present in 3% to 10% of the babies while in isolated dextrocardia and isolated levocardia, CHD is present in almost 100% of the babies. This is in contradistinction to less than 1% CHD prevalence in normal population.^2,3

Segmental Analysis

The cardiac malposition is best evaluated by systematic segmental analysis.¹ The approach is same whether the baby has levocardia, dextrocardia or mesocardia. Once dextroposition is excluded and it is determined that the location of heart in the chest is intrinsic or primary (ie, inherent cardiac malposition), the following questions should be addressed: (1) What is the visceroatrial situs? (2) Ventricular relationship: Are there 2 ventricles or 1? If 2, where are the ventricles located in relation to each other? (3) What is the status of atrioventricular (AV) connections? (4) How are the great arteries related to each other and with ventricles? (5) What is the conotruncal relationship?

What is the visceroatrial situs?     It may be situs solitus, situs inversus or situs ambiguous (symmetricus or indeterminatus). There are number of methods by which the situs may be determined.

P waves in ECG.     Normally the cardiac impulse originates in the sinoatrial node, located at the superior vena cava (SVC)–right atrial (RA) junction and traverses inferiorly and to the left. The resultant atrial depolarization produces P waves in the ECG. The P-wave axis is +45° (Figure 36.10) with positive P waves in leads I and AVF (Figure 35.11). This is indicative of situs solitus. In situs inversus, the atria are inverted and the P-wave axis will be around +135° (see Figure 36.10) giving negative P wave in lead I and positive P wave in lead AVF (Figure 35.12). If the P-wave axis is −45° (Figure 36.13) giving positive P waves in lead I and negative P in lead AVF, the so-called coronary sinus (CS) rhythm, the P axis may not be of much help in situs localization. However, as reviewed latter, coronary sinus rhythm is frequently seen in persistent left SVC and infrahepatic interruption of inferior vena cava (IVC), which in turn are associated with asplenia/polysplenia syndromes.

Visceroatrial concordance.     The general rule of visceroatrial concordance suggests that right-sided liver and left-sided stomach (see Figures 36.1, 36.3B and 36.8) are indicative that morphologic right atrium (RA) is on the right side and morphologic left atrium (LA) on the left side, that is, situs solitus. On the contrary, left-sided liver and right-sided stomach (see Figures 36.2, 36.7 and 36.9) are associated with morphologic RA on the left side and morphologic LA on the right side; that is, situs inversus. If the hepatic lobes are similar in size or midline liver (Figure 36.14) is present, irrespective of the position of the stomach, situs ambiguous may be present and asplenia/polysplenia syndrome are likely.⁴ The principle of visceroatrial concordance is more reliable than P-wave vector in ECG in correctly identifying atrial situs.

Tracheobronchial tree pattern.     In normal babies with situs solitus, the right bronchus (RB) is short and wide and descends rather steeply while the left bronchus (LB) is long and narrow and descends more horizontally (Figures 36.15 and 36.16). In situs inversus, the tracheobronchial tree pattern is reversed (see Figures 36.15 and 36.17). This method appears to be more accurate than the 2 preceding methods.^5,6 If both bronchi appear as morphologic right bronchi (see Figure 36.15), asplenia syndrome is likely while morphologic left bronchi on both sides (see Figure 36.14) are suggestive of polysplenia syndrome.^1,4–6 Exception to these rules do occur but rare.^1,4

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