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.
Chamber Localization
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 ).
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.
- 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 17-2
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 ).
