A 2-year-old child had a murmur noted on a routine physical examination. Echocardiographic examination showed complex structural heart disease. An abdominal radiograph showed a midline stomach and an atypical liver that crossed the midline. This confirmed the diagnosis of polysplenia (bilateral left-sidedness).
Although not immediately apparent on surface anatomy, the human body has well-defined asymmetry. Perhaps nowhere is this more apparent than in the cardiovascular system. This is a consequence of the global laterality involved in development,1 but also of the specific laterality of the folding of the early embryonic heart tube. Folding of the heart tube depends on the direction of ciliary beating. Several genes responsible for correct directional beating of cilia have been identified. As examples, in the normal heart the inferior vena cava (IVC) ascends to the right of the spine and the abdominal aorta descends to the left, and there are well-defined morphologic differences in the right versus left atrium and the right versus left ventricle. Thus, one encounters reference to the morphologic (or the generally synonymous “anatomic”) right atrium, right ventricle, etc. This echo didactic will cover the morphologic features that define and differentiate the right and left cardiac structures.2,3 These are shown in Figure 1 and are described below.
The term situs refers to the entire body. Possibilities for situs include situs solitus (normal) and situs inversus (in which case all structures are mirror image). In addition there is the possibility of right-sided isomerism (asplenia syndrome) or left-sided isomerism (polysplenia syndrome). In these syndromes, rather than having distinct right and left sides, there are in fact 2 right sides or 2 left sides. This becomes apparent for organs that have asymmetric laterality. For example, there are 2 morphologic right atria and 2 morphologic right lungs (asplenia syndrome), or 2 morphologic left atria and 2 morphologic left lungs (polysplenia syndrome),4 with absence of unilateral, contralateral structures (a gallbladder, for example) (Video 1, loop 1, see Supplemental Digital Content 1, http://links.lww.com/AA/A326). These isomerisms are referred to as heterotaxy. Abdominal situs in these cases is so-called “situs ambiguous” because it is neither situs solitus nor situs inversus and is associated with malrotation of the intestinal organs.
Neither the position of the cardiac apex (the base-apex axis that can be levocardia, the normal position with the apex to the left, dextrocardia to the right, or mesocardia to the middle) nor the position of the heart within the chest (levoposition, the normal position on the left, dextroposition on the right, or mesoposition in the middle) determine situs. Cardiac situs is generally determined by the position of the morphologic right atrium, the normal condition being the morphologic right atrium to the right.
Pulmonary situs is determined by the relationship of the pulmonary artery to the adjacent bronchus, not the number of lobes, which can be variable. In the normal heart the right mainstem bronchus is generally above the right pulmonary artery (“eparterial bronchus”) and the left mainstem bronchus below the left pulmonary artery (“hyparterial bronchus”). This is the primary reason that the proximal right pulmonary is easily visualized by transesophageal echocardiography and the proximal left pulmonary artery is not. The relationship of the great vessels to the ventricles (ventriculoarterial concordance) is not a defining characteristic of situs.
The right atrium is generally marked by the entry of the vena cavae and the coronary sinus. In cases of heterotaxy, entry of the IVC may be variable. The crista terminalis separates the right atrium into anterior and posterior parts, and from it arise the numerous pectinate muscles, located throughout the right atrial wall and also located in the right atrial appendage. The ridge-like pectinate muscles are often apparent on echocardiography (Video 1, loop 2, http://links.lww.com/AA/A326). The right atrium also contains the remnant of the fetal Eustachian valve originating at the entry of the IVC, which on occasion is very prominent (Video 1, loop 3, http://links.lww.com/AA/A326). The right atrial appendage is quite distinctive from the left. It is pyramidal and fairly large (Video 1, loop 4, http://links.lww.com/AA/A326). Also defining the right atrium is the limbus of the fossa ovalis, a muscular ring on the superior and lateral sides of the fossa ovalis.
The left atrium is marked by the narrower, thin, finger-like left atrial appendage, in comparison with the right atrial appendage (Video 1, loops 5 and 6, http://links.lww.com/AA/A326). The left atrium usually receives the pulmonary veins, but may not in cases of total anomalous pulmonary venous return; thus the entry site of the pulmonary veins does not define an atrium. The left atrium, unlike the right, has no pectinate muscles outside the appendage.
The atrioventricular valves are always associated with their appropriate ventricle, viz. the tricuspid valve always empties into the morphologic right ventricle and the mitral to the left. In comparison with the mitral valve, the attachment of the tricuspid valve is more apically placed (Fig. 2 and Video 2, loop 1, see Supplemental Digital Content 2, http://links.lww.com/AA/A327), noted by comparing the attachment points of the septal tricuspid leaflet and the anterior mitral leaflet to the interventricular septum. The tricuspid and mitral components in common atrioventricular canal (endocardial cushion defect) will both be at the same level. Notably, the tricuspid valve has septal chordal attachments and 3 papillary muscles, unlike the mitral, which attaches to 2 large papillary muscles and has no septal attachments. The orifice of the tricuspid valve is more triangular than that of the mitral, which is more elliptical or fish-mouthed and has 2 leaflets and commissures.
The right ventricle is associated with the tricuspid valve and contains its papillary muscles. Other cardinal features are that it is heavily trabeculated, has a series of well-defined muscle bundles, the septal and parietal bands, and a large apical trabeculation, the moderator band (Video 2, loop 1, http://links.lww.com/AA/A327). Unlike the left ventricle, there is a muscular outflow tract, the infundibulum or conus, which separates the tricuspid valve from the semilunar valve (Video 2, loop 2, http://links.lww.com/AA/A327). In the lesion double outlet right ventricle, for example, in which both great vessels originate from the right ventricle, 2 infundibula can be seen, identifying this ventricle as a morphologic right ventricle (Video 2, loop 3, http://links.lww.com/AA/A327). In cross-section (in the midesophageal short-axis view), the right ventricle is crescentic.
The left ventricle, unlike the right, lacks trabeculations and receives the mitral valve. It lacks the muscle bundles of the right ventricle, but on occasion can have a so-called “false tendon” across its mid portion. Critically, it lacks a muscular outflow tract, allowing for mitral–aortic fibrous continuity (Video 2, loop 4, http://links.lww.com/AA/A327). The left ventricle is circular in cross-section.
Name: Victor C. Baum, MD.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Victor C. Baum approved the final manuscript.
Name: P. Nicole Duncan, BS, RDCS, RCCS.
Contribution: This author helped conduct the study.
Attestation: P. Nicole Duncan approved the final manuscript.
APPENDIX: VIDEO CAPTIONS
Video 1. Vena cavae and atria. Loop 1, The transesophageal echocardiography (TEE) probe has been advanced caudad to the heart showing direct entry of hepatic veins to the right atrium with an absent inferior vena cava. This absence of a right-sided structure is consistent with a diagnosis of polysplenia (bilateral left-sidedness). RA = right atrium. Loop 2, Midesophageal 4-chamber view rotated to center the right atrium, showing the ridged pectinate muscles (arrows). The presence of these muscles outside the appendage identifies this chamber as a morphologic right atrium. Loop 3, Transgastric image demonstrating the liver, IVC, and a prominent Eustachian valve. The Eustachian valve is a normal fetal structure that serves to direct oxygenated umbilical venous blood across the foramen ovale to the left side of the heart. The entry of the IVC and the presence of the Eustachian valve serve to identify this as a morphologic right atrium. In this image the probe has been advanced caudally to image the entry of the IVC to the right atrium. IVC = inferior vena cava. Loop 4, Midesophageal 4-chamber view demonstrating the right atrial appendage. This patient also has a fatty tricuspid annulus, a normal variant. RAA = right atrial appendage. Loop 5, Midesophageal 2-chamber view demonstrating the left atrial appendage, which is narrower than the rounded right atrial appendage. LA = left atrium, LAA = left atrial appendage (arrow), LV = left ventricle. Loops 6a and b, The atrial appendages (arrows) in this child with heterotaxy are broad based and appear morphologically identical, indicating that there is isomerism. Loop a shows the appendage of the right-sided atrium and loop b the left-sided atrial appendage.
Video 2. Ventricles. Loop 1, Midesophageal 4-chamber view: this patient with L-transposition of the great arteries has ventricular inversion, the morphologic left ventricle is to the patient's right, and the morphologic right ventricle is to the patient's left. There is a prominent moderator band in the left-sided ventricle, identifying this as a morphologic right ventricle. In addition, the relative apical displacement of the atrioventricular valve of the left-sided ventricle identifies this as a tricuspid valve, confirming the identity of this ventricle. Loop 2, Midesophageal right ventricle inflow–outflow view. The muscular infundibulum of the right ventricular outflow track is interposed between the tricuspid valve and the semilunar valve, here, the pulmonary valve. Loop 3, This is a transthoracic echo from the subcostal window of a patient who has double outlet right ventricle (DORV) as 1 component of her complex congenital heart disease. The transducer has been angled somewhat anteriorly to image the right ventricular outflow tract. There are 2 outflow tracts from this ventricle, both with a muscular infundibulum below its semilunar valve, identifying this as a right ventricle. Loop 4, Midesophageal long-axis view demonstrating the mitral valve, left ventricular outflow track, and aortic valve. Unlike the right ventricle where a muscular infundibulum is interposed between the tricuspid and semilunar valve, in this morphologic left ventricle there is fibrous continuity without intervening muscle of the anterior leaflet of the mitral valve and the aorta. Compare with the situation in the right ventricle (Video 2, loop 2).
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