In this chapter, clinical methods of locating cardiac chambers using chest radiography, electrocardiography (ECG), and echocardiography are discussed. This is followed by application of a principle that may aid in the anatomic diagnosis of the heart in the right chest (dextrocardia) or in the midline (mesocardia). Although these methods are valid, many false-positive and false-negative results are possible. Two-dimensional echocardiography usually reveals the correct diagnosis, but occasionally additional studies such as magnetic resonance imaging (MRI) or angiography may be needed.
The heart and great arteries can be viewed as three separate segments: the atria, the ventricles, and the great arteries. These three segments can vary from their normal positions either independently or together, resulting in many possible sets of abnormalities. The segmental approach of Van Praagh is useful in determining the relationship at each segment. This approach also simplifies the description of complex cardiac defects and abnormal positions of the heart (e.g., dextrocardia, levocardia, mesocardia).
Localization of the Atria
The atria can be localized accurately by three noninvasive methods: chest radiography, ECG, and echocardiography. The radiographic method relies on the fact that the atrial situs is almost always the same as the type of visceral situs; the right atrium (RA) is on the same side as the liver or on the opposite side of the stomach bubble. The ECG method is based on the principle that the sinus node is always located in the RA and that the site of the sinus node can be determined by the P axis. Echocardiography clarifies the relationship between systemic and pulmonary veins and the atria.
Chest radiography. The clinician should locate the liver shadow and the stomach bubble.
1. A right-sided liver shadow and left-sided stomach bubble (situs solitus) indicate situs solitus of the atria (with the RA on the right of the left atrium [LA], as in normal) (Fig. 17-1, A). A left-sided liver shadow and right-sided stomach bubble (situs inversus) indicate situs inversus of the atria (with the RA on the left of the LA) (Fig. 17-1, B).
2. A midline (symmetrical) liver shadow with a variable location of stomach bubble suggests heterotaxia (or splenic syndromes) in which either two RAs or two LAs (situs ambiguus) and other associated complex cardiac anomalies are present (Fig. 17-1, C; see also sections on heterotaxia in Chapter 14).
Electrocardiography. The sinus node is always located in the anatomic RA. Therefore, the P axis of the ECG can be used to locate the atria; the RA is located on the opposite side of the P axis.
1. When the P axis is in the lower left quadrant of the hexaxial reference system (0 to +90 degrees), the RA is on the right side (with the RA to the right of the LA, or situs solitus of the atria) (Fig. 17-2).
FIGURE 17-1 The visceroatrial relationship. A, Situs solitus. B, Situs inversus. C, Situs ambiguus. The right atrium (RA) is either on the same side as the liver or on the opposite side of the stomach (ST). The sinoatrial node (star) is always in the RA. LA, left atrium; S, spleen.
FIGURE 17-2 Locating the atria by the use of the P axis. When the right atrium (RA) is on the right side, the P axis is in the left lower quadrant (0 to +90 degrees). When the RA is on the left side, the P axis is in the right lower quadrant (+90 to +180 degrees). LA, left atrium. (From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 2006.)
2. When the P axis is in the lower right quadrant (+90 to +180 degrees), the RA is on the left side (with the RA on the left of the LA, or situs inversus of the atria) (see Fig. 17-2).
3. With heterotaxia, the P axis may be superiorly directed (as seen with polysplenia syndrome) or may change between the lower left quadrant and lower right quadrant from time to time (as seen with asplenia syndrome, in which two sinus nodes are present).
Two-dimensional echocardiography and other methods. Two-dimensional echocardiography identifies the inferior vena cava (IVC), pulmonary veins, or both. The atrial chamber that is connected to the IVC is the RA, and the atrium that receives the pulmonary veins is the LA. Cardiac MRI, angiocardiography, surgical inspection, and autopsy findings aid further in the diagnosis of atrial situs.
Localization of the Ventricles
Ventricles can be localized noninvasively by the ECG and two-dimensional echocardiography (or by MRI or angiocardiography).
Electrocardiography. The ECG method of localizing the ventricle is based on the fact that the depolarization of the ventricular septum moves from the embryonic left ventricle (LV) to the right ventricle (RV). This produces Q waves in the precordial leads that lie over the anatomic LV.
1. If Q waves are present in V5 and V6 but not in V1, D-loop of the ventricle, as in the normal person, is likely (Fig. 17-3, A).
2. If Q waves are present in V4R, V1, and V2 but not in V5 and V6, L-loop of the ventricles is likely (ventricular inversion) (see Fig. 17-3, B).
Two-dimensional echocardiography. The anatomic RV and LV are identified by the facts that the tricuspid valve leaflet usually inserts on the interventricular septum in a more apical position than the mitral septal leaflet and that the LV is invariably attached to the mitral valve and the RV to the tricuspid valve. A ventricular chamber that has two papillary muscles is the LV.
Magnetic resonance imaging or ventriculography. The anatomic RV is coarsely trabeculated and triangular, and the anatomic LV is finely trabeculated and ellipsoid.
FIGURE 17-3 Locating the ventricles from the electrocardiogram (ECG). The left ventricle (LV) is usually located on the same side as the precordial leads that show Q waves. If V6 shows a Q wave, the LV is on the left side (A). If V4R and V1 show a Q wave, the LV is to the right of the anatomic right ventricle (RV) (B). Note that Q waves are also present in V1 in severe right ventricular hypertrophy. (From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 2006.)
FIGURE 17-4 Four types of relationships between the great arteries, viewed in the horizontal section. A, Solitus (S) relationship is present when the aortic valve is posterior and to the right of the pulmonary valve. B, In inversus (I) relationship, the aortic valve is posterior to and left of the pulmonary valve (mirror-image of normal). C, Complete transposition (D) is present when the aortic valve is anterior and to the right of the pulmonary valve. D, Congenitally corrected transposition (L) is present when the aortic valve is anterior and to the left of the pulmonary valve. AO, aorta; PA, pulmonary artery.
Localization of the Great Arteries
One can accurately determine the relationship between the two great arteries and the relationship of the great arteries to the ventricles noninvasively through echocardiography or MRI (and invasively through angiocardiography. The ECG and chest radiography are not very helpful in determining the relationship between the great arteries and the ventricles. In many cases, however, one can deduce the relationship through the loop rule (of Van Praagh). The loop rule states that the D-loop of the ventricle (with the anatomic RV to the right of the LV) usually is associated with normally related great arteries or with complete transposition of the great arteries (D-TGA). The L-loop of the ventricle (with the anatomic RV to the left of the anatomic LV) usually is associated with the mirror image of normally related great arteries or with congenitally corrected transposition of the great arteries (L-TGA).
There are four types of relationships between the two great arteries: (1) solitus, (2) inversus, (3) D-transposition, and (4) L-transposition (Fig. 17-4). One can deduce the relationship of the great artery. For example, when the situs solitus of the atria and the D-loop of the ventricle are confirmed, a situs solitus relationship is present if the patient is not cyanotic; if the patient is cyanotic, a D-TGA is present.
The following symbols are used in describing the segmental relationship of cardiac chambers and great arteries:
1. Visceroatrial relationship: S (solitus), I (inversus), or A (ambiguus)
2. Ventricular loop: D (D-loop), L (L-loop), or X (uncertain or indeterminate)
3. Great arteries: S (solitus), I (inversus), D (D-transposition), or L (L-transposition)
With these symbols, the segmental relationship of the heart can be expressed by three letters. The first letter signifies the visceroatrial relationship; the second letter, the ventricular loop; and the third letter, the relationship of the great arteries. The segmental approach to the diagnosis of cardiac malposition is independent of the location of the cardiac apex. Consequently, this approach applies to normally located hearts (levocardia in situs solitus) as well as to abnormally located hearts, such as dextrocardia and mesocardia. A few examples of normal and well-known abnormal segmental relationships can be expressed as follows:
Normal heart with situs solitus: S, D, S
Normal heart with situs inversus (mirror image of normal): I, L, I
D-TGA: S, D, D
D-TGA with situs inversus: I, L, L
L-TGA with situs solitus: S, L, L
A normally formed heart that is displaced to the right side of the chest secondary to hypoplasia of the right lung (“dextroversion”): S, D, S
Dextrocardia and Mesocardia
Dextrocardia refers to a condition in which the heart is located on the right side of the chest. Mesocardia indicates that the heart is located approximately on the midline of the thorax; that is, the heart lies predominantly neither to the right nor to the left on the posteroanterior chest radiograph. The terms dextrocardia and mesocardia express the position of the heart as a whole but do not specify the segmental relationship of the heart.
The four common types of dextrocardias are classic mirror-image dextrocardia (Fig. 17-5, A), normal heart displaced to the right side of the chest (Fig. 16-5, B), L-TGA (Fig. 17-5, C), and single ventricle. Less commonly, asplenia and polysplenia syndromes with situs ambiguus and complicated cardiac defects cause dextrocardia (Fig. 17-5, D). All of these abnormalities may result in mesocardia.
With chest radiography studies and the ECG, one can deduce the location of the atria and the ventricles in dextrocardia (as well as in mesocardia). One can gain a more conclusive diagnosis of the segmental relationship through two-dimensional echocardiography or MRI (or invasively with angiocardiography).
1. Classic mirror-image dextrocardia (I, L, I) (see Fig. 17-5, A) shows:
a. The liver shadow on the left and the stomach bubble on the right on radiography and the P axis between +90 and +180 degrees on the ECG (situs inversus)
b. Q waves in V5R and V6R (V5R and V6R are right-sided precordial leads, the mirror-image positions of V5 and V6, respectively)
2. Normal heart shifted toward the right side of the chest with the normal right-to-left relationship maintained (dextroversion) (S, D, S) (see Fig. 17-5, B) shows:
a. The liver shadow on the right and the stomach bubble on the left on radiography and the P axis between 0 and +90 degrees on the ECG (situs solitus)
b. Q waves in V5 and V6
FIGURE 17-5 Examples of common conditions in which the apex of the heart is in the right chest. A, Classic mirror-image dextrocardia. B, Normally formed heart shifted toward the right side of the chest. C, Congenitally corrected transposition of the great arteries (L-TGA) with situs solitus. D, Situs ambiguus seen with splenic syndromes. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle. (From Park MK, Guntheroth WG: How to Read Pediatric ECGs, 4th ed. Philadelphia, Mosby, 2006.)
3. L-TGA with situs solitus (S, L, L) (see Fig. 17-5, C) shows:
a. Situs solitus of abdominal viscera on radiography and the P axis in the normal quadrant (0 to + 90 degrees) on the ECG
b. Q waves in V5R and V6R
4. Undifferentiated cardiac chambers (see Fig. 17-5, D) are often associated with complicated cardiac defects and may show:
a. Midline liver on radiography and shifting P axis or superiorly oriented P axis on the ECG
b. Abnormal Q waves in the precordial leads (similar to those described for single ventricle; see Chapter 14)
Acquired Heart Disease
18. Primary Myocardial Disease
19. Cardiovascular Infections
20. Acute Rheumatic Fever
21. Valvular Heart Disease
22. Cardiac Tumors
23. Cardiovascular Involvement in Systemic Diseases
Acquired Heart Disease
Among acquired heart diseases, emphasis is placed on the more common pediatric diseases, such as cardiomyopathies; cardiovascular infections, including myocarditis and infective endocarditis; acute rheumatic fever; and valvular heart disease. Although the cause of Kawasaki’s disease is not entirely clear, it is discussed in the chapter on cardiovascular infection. Mitral valve prolapse is discussed in the chapter on valvular heart disease. A chapter on cardiac involvement in some systemic diseases also is presented.