Segmental Anatomy

The Cardiac Segments

The cardiac segments are the anatomic and developmental “building blocks” out of which all hearts—normal and abnormal—are made.

The three main cardiac segments are the atria, the ventricles, and the great arteries.

The two connecting cardiac segments are the atrioventricular canal or junction and the infundibulum or conus.

Hence, there are five diagnostically and surgically important cardiac segments: (1) the atria, (2) the atrioventricular canal, (3) the ventricular sinuses, (4) the infundibulum or conus, and (5) the great arteries.

Atrial Situs

Where is the morphologically right atrium (RA)? Where is the morphologically left atrium (LA)? To answer these diagnostic questions, an understanding of the two main types of visceroatrial situs is helpful ( Fig. 4.1 ):

• 1.

  In visceroatrial situs solitus , the liver is predominantly right sided, the stomach and spleen are left sided, the right lung is trilobed, the left lung is bilobed, the right bronchus is eparterial, the left bronchus is hyparterial, the RA is right sided, and the LA is left sided.

• 2.

  In visceroatrial situs inversus , the liver is predominantly left sided, the stomach and spleen are right sided, the left-sided lung is trilobed, the right-sided lung is bilobed, the left-sided bronchus is eparterial, the right-sided bronchus is hyparterial, the RA is left sided, and the LA is right sided.

• 3.

  In “situs ambiguus” or visceral heterotaxy —often but not always associated with the asplenia or the polysplenia syndrome—the liver can be bilaterally symmetrical; the stomach may be left sided, right sided, or midline because of a common gastrointestinal mesentery (in which the stomach is not “tacked down”); the lungs may be bilaterally trilobed with bilaterally eparterial bronchi—often with the asplenia syndrome; or the lungs may be bilaterally bilobed with bilaterally hyparterial bronchi—often with the polysplenia syndrome; and the superior venae cavae may be bilateral.

In Fig. 4.1 , note the visceroatrial situs concordances , the situs (pattern of anatomic organization) of the viscera (abdominal and thoracic) and of the atria usually being the same: both solitus (usual, ordinary, customary—therefore normal), both inversus (a mirror image of situs solitus), or both ambiguus—as can occur with the heterotaxy syndromes (with asplenia, polysplenia, or occasionally with a normally formed spleen).

“Situs ambiguus” or visceral heterotaxy is not a third type of visceroatrial situs.

There are only two basic types of visceroatrial situs (see Fig. 4.1 ): situs solitus and situs inversus. “Situs ambiguus” (visceral heterotaxy) is the poorly lateralized or abnormally symmetrical pattern of visceral organization that can occur with the asplenia and polysplenia syndromes. However, on careful study, the heterotaxy syndromes appear to have either basically situs solitus of the viscera and atria or basically situs inversus of the viscera and atria. “Situs ambiguus” is now much less ambiguous than it used to be.

Visceral heterotaxy and “situs ambiguus,” when not otherwise qualified, indicate that the basic types of visceral situs and atrial situs have not been diagnosed. These are cases awaiting diagnosis of their anatomic types of visceroatrial situs.

The concepts of atrial isomerism and of atrial appendage isomerism , as mentioned in Chapter 3 , are wrong. To summarize, bilaterally right atria or bilaterally left atria have never been documented. In the heterotaxy syndromes of asplenia and polysplenia, in terms of size, shape, and position, the atrial appendages often are not mirror image; that is, atrial appendage isomerism (mirror imagery) often is not present. Also, it should be understood that partial isomerism (e.g., of the atrial appendages only) is not isomerism. For example, D-glucose and L-glucose would not be isomers if only some of their atoms were mirror images but others were not. Isomerism is like pregnancy; it is either present or absent—but not partial. Partial isomerism (e.g., of the atrial appendages only) is a contradiction in terms—an error in logic. The realization that the old concept of atrial-level isomerism is erroneous is important, because this understanding then “opens the door” to diagnosing the atrial situs. Atrial-level “isomerism” masks the fact that the atrial situs is undiagnosed.

However, it should also be emphasized that whenever one encounters two atrial appendages that both look “rightish” (broad, triangular, pyramidal), one should immediately think of the asplenia syndrome as a strong diagnostic possibility. Similarly, whenever one sees two atrial appendages that both look “leftish” (long, thin, finger-like), one should immediately think of the polysplenia syndrome as a diagnosis to consider. This bilateral right sidedness or bilateral left sidedness appearance of the atrial appendages is thus diagnostically very helpful.

But one should also understand that the patient really does not have two right atrial appendages (one on each side) nor two left atrial appendages (one on each side). Instead, the foregoing are diagnostically helpful anatomic appearances , not real anatomic facts . For example, the polysplenia syndrome can occur in visceroatrial situs inversus. The morphologically RA is left sided, but it may well have a finger-like tip to its appendage. Nonetheless, this is a left-sided RA, not a left-sided atrium that is mostly RA but with an LA tip of its appendage. If the latter situation were in fact the case, then this left-sided atrium would be a chimera: mostly RA, but with an LA tip of its appendage. Instead, the LA (including its appendage) is right sided, and the RA is left-sided, just as one would expect in visceroatrial situs inversus, despite the “leftish” appearance of both atrial appendages.

Thus, Dr. Jesse Edwards’ “bilateral right sidedness” and “bilateral left sidedness” concepts are very helpful when correctly understood as teaching mnemonics to help one to remember the various features of the asplenia and polysplenia syndromes, respectively. However, these ideas should not be “oversold” at the atrial level as anatomic facts, because they are not. Each human being has only one RA and one RA appendage, just as each human being has only one LA and one LA appendage.

In the asplenia syndrome, for example, when we are unable to diagnose the morphologic anatomic identity of the atria and hence are unable to diagnose the anatomic type of atrial situs, we make the diagnosis of situs ambiguus of the atria , which means that we do not know what the atrial situs is. The latter is an honest statement, which we think is vastly preferable to the diagnosis of “right atrial isomerism” or “right atrial appendage isomerism,” because the latter diagnoses are erroneous in terms of literal anatomic accuracy.

Anatomic accuracy is the “gold standard” that we have endeavored to employ throughout this study. Accuracy we regard as the basic principle of science. Congenital heart disease is so complex that the only way to avoid confusion is by anatomic accuracy. Accurate pathologic anatomy is the basis of diagnosis in congenital heart disease.

An accurate physiologic diagnosis is also of great importance. But in congenital heart disease, the pathologic anatomy usually determines the pathophysiology . Consequently, accurate pathologic anatomy remains the basis of diagnosis in congenital heart disease. For example, cyanosis with decreased pulmonary blood flow and venoarterial shunting can have many different anatomic causes, which must be diagnosed accurately and treated effectively. Hence, the anatomic details are of paramount practical importance.

Occasionally in the heterotaxy syndromes, the situs of the abdominal viscera and the situs of the atria can be different, that is, visceroatrial situs discordance can occur. ^,

Although visceroatrial situs concordance is the rule in situs solitus and in situs inversus (see Fig. 4.1 ), visceroatrial situs discordance can occur in the heterotaxy syndromes.

Inversion is defined in anatomy as mirror imagery. In situs inversus , there is right–left reversal but without antero-posterior or superoinferior change (see Fig. 4.1 ), as in a mirror image. The mirror is the sagittal plane between the diagrams of situs solitus and situs inversus in Fig. 4.1 .

The first of the main cardiac segments is thus the viscera and the atria—not just the atria. This is why the diagnosis of the anatomic type of visceroatrial situs (see Fig. 4.1 ) usually is helpful for atrial localization. If the plain posteroanterior chest x-ray shows situs solitus (the usual arrangement) of the viscera—with liver shadow to the right and stomach bubble to the left ( Fig. 4.2 ), then very probably the atria are also in situs solitus—with RA to the right and LA to the left.

Conversely, if the liver shadow is to the left and the stomach bubble is to the right ( Fig. 4.3 ), situs inversus of the viscera strongly suggests that the atria also will be in situs inversus.

But if the liver shadow is bilaterally symmetrical ( Fig. 4.4 ), this strongly suggests “situs ambiguus,” or the heterotaxy syndrome of the viscera and of the atria.

Although the diagnosis of the anatomic type of visceroatrial situs (solitus, inversus, or “ambiguus,” see Fig. 4.1 ) is a useful first approximation concerning atrial identification, in complex cases one must go much further:

• 1.

  Where is the inferior vena cava (IVC)? Is it right sided or left sided? Does it switch sides at the level of the liver? The atrium to which the IVC connects directly, in our experience, always has been the RA.

• 2.

  Where is the ostium of the coronary sinus? The atrium into which the ostium of the coronary sinus opens always has proved to be the RA. (A coronary sinus septal defect—or an unroofed coronary sinus—must not be mistaken for the right atrial ostium of the coronary sinus.)

• 3.

  What is the shape and size of the atrial appendage? Is the appendage large, triangular, and anterior? If so, this is typical of the RA. Is the appendage small, finger-like, and posterior? If so, this is typical of the LA.

• 4.

  What is the morphology of the atrial septal surface? Is it characterized by septum secundum’s superior and inferior limbic bands? If so, this is typical of the RA. Is septum primum well seen on the atrial septal surface? If so, this is typical of the LA.

(Please see Chapter 3 for more details concerning the morphologic anatomy of the RA and the LA.)

For convenience and brevity, situs solitus of the viscera and atria may be abbreviated as S . Situs inversus of the viscera and atria may be symbolized as I . “Situs ambiguus” of the viscera and atria may be abbreviated as A .

The three main cardiac segments—the atria, the ventricles, and the great arteries—may be regarded as the elements of a set. The standard mathematical symbol meaning “the set of” is braces: {}.

The segmental situs set is written in sequential, blood-flow order: {atria, ventricles, great arteries}. The three elements of the set are separated by commas, this being conventional set notation.

The segmental situs set thus may begin as {S,-,-} or as {I,-,-} or as {A,-,- —meaning situs solitus of the viscera and atria (S), or situs inversus of the viscera and atria (I), or “situs ambiguus” of the viscera and atria (A), respectively.

The diagnostic questions concerning atrial localization (where is the morphologically RA, and where is the morphologically LA?) are answered by determining the anatomic type(s) of visceral situs and atrial situs that is (are) present (see Fig. 4.1 ). In situs solitus and in situs inversus, there virtually always is visceroatrial situs concordance: the situs of the viscera and the situs of the atria are both the same, both solitus, or both inversus. But, as noted heretofore, in the heterotaxy syndromes (asplenia, polysplenia, and heterotaxy with a normally formed spleen), there can be visceroatrial situs discordance. The situs of the viscera and the situs of the atria can be different —a fact that is relevant to diagnostic atrial localization. To the best of our present knowledge, the atrium with which the IVC connects always is the morphologically RA. (Please see Chapter 3 for the other anatomic features of the morphologically right and left atria.)