The Morphologically Right Atrium

Angiocardiographically, the broad triangular shape of the right atrial appendage, both in the posteroanterior view ( Fig. 3.1 ) and in the left lateral view ( Fig. 3.2 ), is highly characteristic.

Anatomically, the external appearance of the right atrium is characterized by a broad triangular right atrial appendage ( Fig. 3.3 ). The right atrial appendage resembles Snoopy ∗

∗ Dog, one of the characters in Peanuts , created by American cartoonist Charles M. Schulz (born 1922).

Note that the inferior vena cava can drain into the left atrium, for example, when a prominent right venous valve (Eustachian valve) is associated with a deficient or absent septum primum. But in this situation, the inferior vena cava connects normally with the right atrium. It is important to distinguish between connections and drainage .

Externally, the sulcus terminalis indicates the termination of the medial, venous, sinus venosus component of the right atrium and the beginning of the more lateral, muscular, contractile portion of the right atrium. The sulcus terminalis, or sinoatrial sulcus , lies just lateral to the ostia of the superior vena cava and the inferior vena cava. This shallow furrow runs superoinferiorly on the posterior external aspect of the right atrium, between the venous component of the right atrium medially that joins the superior and inferior vena caval ostia and the musculi pectinati (pectinate muscle) portion of the right atrial appendage that lies more laterally. The sulcus terminalis (terminal sulcus), also known as the sinoatrial sulcus, corresponds to the crista terminalis (terminal crest) that is seen on inspection of the interior of the right atrium.

The importance of the sulcus terminalis is that this is where the sinoatrial node , the pacemaker of the heart, resides. Although not visible upon external inspection, the head of the sinoatrial node is located to the right of, or lateral to the superior vena cava (in visceroatrial situs solitus), just beneath the epicardium, in the sulcus terminalis. The tail of the sinoatrial node extends like the tail of a comet—almost down to the level of the inferior vena cava. Hence, the sinoatrial node is so called because it lies at the junction of the sinus venosus medially and the atrium (appendage) laterally; thus, the pacemaker really is sinoatrial in location.

Because the sinoatrial node is invisible upon careful external inspection, one must know where it is—particularly if one is a surgeon about to do a right atriotomy.

What is the blood supply of the sinoatrial node? The answer is: about 55% from the right coronary artery and about 45% from the left coronary artery, typically from the left circumflex branch.

The sinoatrial nodal artery runs through the sinoatrial node like a shish kabob skewer. The sinoatrial nodal tissue is located around the sinoatrial nodal artery. Surgically, if at all possible, one should avoid producing thrombosis of the sinoatrial nodal artery in order to avoid sick sinus syndrome. Dr. “Billy” Kreutzer, the famous congenital heart surgeon from Buenos Aires and coinventor of the Fontan–Kreutzer procedure, told me how he avoids the sinoatrial node. Just after he has put cardioplegia solution into the aortic root, he watches very carefully to see if the sinoatrial nodal artery is coming from the right coronary artery and into the “front door” of the head of the sinoatrial node or if the sinoatrial nodal artery is coming from the left circumflex coronary artery, in which case it can come around behind the superior vena cava to enter the sinoatrial node via “the back door.” Cardioplegia solution usually is water clear; so a surgeon who watches carefully at this point in the procedure often can identify exactly where the sinoatrial nodal artery is. It is not enough just to avoid surgical transection of the sulcus terminalis. The surgeon must also, if possible, avoid transection of the sinoatrial nodal artery before it reaches the sinoatrial node in the sulcus terminalis.

The external appearance of the right atrium (see Fig. 3.3 ) is so characteristic that an experienced eye usually needs only a quick glance to make this morphologic anatomic identification.

The internal appearance of the morphologically right atrium is equally distinctive ( Fig. 3.4 ). The most highly reliable diagnostic features of the right atrium are: (1) the ostium of the inferior vena cava, (2) the ostium of the coronary sinus, (3) the superior limbic band of septum secundum, and (4) the large broad triangular right atrial appendage and its musculi pectinati (pectinate muscles).

Normally, the superior vena cava enters the right atrium (see Fig. 3.4 ). However, if a persistent left superior vena cava is present in visceroatrial situs solitus and if the coronary sinus is unroofed due to the presence of a coronary sinus septal defect, then the persistent left superior vena cava will drain into the morphologically left atrium. Hence, the superior vena cava is not as highly reliable a diagnostic marker of the right atrium as is the inferior vena cava.

Lateral to the entry of the superior vena cava—to the right in visceroatrial situs solitus (see Fig. 3.4 ) and to the left in visceroatrial situs inversus—lies the crista terminalis (terminal crest) . The crista terminalis internally (see Fig. 3.4 ) corresponds to the sulcus terminalis externally (see Fig. 3.3 ), which is where the sinoatrial node is located. If deep sutures are put into the crista terminalis, for example, while doing the Mustard procedure or the Fontan–Kreutzer procedure, then the sinoatrial nodal artery may be traumatized. Thrombosis of the sinoatrial nodal artery results in ischemic necrosis of the sinoatrial node, which in turn leads to the sick sinus syndrome .

The tinea sagittalis intersects with the crista terminalis just anterior to the entry of the superior vena cava (see Fig. 3.4 ). Externally, the tinea sagittalis corresponds to the bridge of Snoopy’s nose. From the practical standpoint, right behind the tinea sagittalis is a favorite place for the tips of cardiac catheters to get caught. A little injudicious push of the cardiac catheter at this point can lead to perforation of the right atrial appendage.

In the posteroanterior image intensifier, the catheter tip may seem to be quite far to the left. But looking in the lateral image intensifier reveals that the catheter tip is very anterior—rather than more posterior, as the left atrium is. If in doubt concerning the position of the catheter tip, the catheteer should also look at the color of the blood—dark red meaning right atrium, bright red indicating left atrium. Hence, checking the location of the catheter tip in both the posteroanterior and lateral projections and checking the oxygen saturation of the blood will help to avoid a perforation of the right atrial appendage.

But let us say you have just perforated the right atrial appendage with a cardiac catheter. What do you do now? The answer is: nothing. Don’t quickly retract the catheter, pulling it out of the hole in the right atrial appendage. The wall of the right atrial appendage, particularly between the pectinate muscles, is very thin—less than 1 mm in thickness. Instead, leave the catheter where it is. Call your surgical colleagues. Prepare to do a pericardiocentesis. The anatomic point I seek to make is that the right atrial appendage free wall is very thin and may bleed into the pericardial sac following removal of the perforating cardiac catheter. Unfortunately, the right atrial free wall is not a self-sealing “tire,” as the much thicker ventricular walls often are. So, the risk of cardiac tamponade following removal of a perforating cardiac catheter is greater following atrial free-wall perforation than following ventricular free-wall perforation. This, then, is the practical importance of the tinea sagittalis. Perhaps it should be added that right atrial perforations can also occur at many other sites; they do not occur only behind the tinea sagittalis.

The atrioventricular node and the proximal unbranched portion of the atrioventricular (or His) bundle unfortunately are invisible. So you have to know where they are, particularly if you are a catheteer, an electrophysiologist, or a cardiac surgeon. The atrioventricular node and the atrioventricular bundle lie between the ostium of the coronary sinus posteriorly and the membranous septum anteriorly ( Figs. 3.4 and 3.5 ).

Where is the membranous septum? From the right atrial standpoint, the membranous septum lies between the anterior and septal leaflets of the tricuspid valve (see Figs. 3.4 and 5 ). The atrioventricular portion of the membranous septum extends above the commissure between the anterior and septal leaflets of the tricuspid valve and separates the left ventricle below from the right atrium above. A defect in the atrioventricular portion of the membranous septum results in a left ventricular-to-right atrial shunt , often referred to as a Gerbode defect. The interventricular portion of the membranous septum lies just below the anterior leaflet/septal leaflet commissure of the tricuspid valve. A defect of the interventricular portion of the membranous septum results in a membranous ventricular septal defect .

So, the atrioventricular node and the proximal unbranched portion of the His bundle extend in a straight line from the ostium of the coronary septum posteroinferiorly to the membranous septum anterosuperiorly. The tail of the atrioventricular node may extend a short way into the ostium of the coronary sinus. Hence, a coronary sinus rhythm may represent a high nodal rhythm. If you are a cardiac surgeon and if you are showing a younger colleague where the atrioventricular conduction system is, don’t point with a sucker. As the late Dr. Bill Mustard (of Mustard procedure fame) once told me, the tip of a sucker can easily become stuck and adherent to the right atrial endocardium. Then, when you have to pull the sucker away from the right atrial endocardium, complete and permanent heart block can result, because the atrioventricular node and His bundle are immediately subendocardial. So, point out the location of the atrioventricular node and His bundle with a probe or some other instrument that will not inadvertently injure the conduction system.

When the His bundle reaches the region of the membranous septum, the His bundle dives or penetrates just proximal to the membranous septum. The penetrating portion of the atrioventricular bundle normally passes behind and below the membranous septum.

When the atrioventricular bundle reaches the ventricular level, it branches. The branching portion of the His bundle first gives off the left posterior radiation, then the left middle radiation, then the left anterior radiation, and lastly the right bundle branch.

Another approach to localizing the grossly invisible atrioventricular node and the proximal unbranched portion of the atrioventricular bundle is the triangle of Koch (see Fig. 3.5 ). The sides of this triangle are as follows: (1) the tendon of Todaro, which is the anterior extension of the eustachian valve of the inferior vena cava; (2) the thebesian valve of the coronary sinus; and (3) the attachment of the septal leaflet of the tricuspid valve.

The difficulties associated with the triangle of Koch are as follows: (1) The tendon of Todaro is invisible grossly, although it is well seen histologically. However, if one grasps the eustachian valve of the inferior vena cava and pulls down gently in the direction of the inferior vena cava, then the tendon of Todaro (pronounced tod′-a-ro) stands out clearly in the floor of the right atrium. (2) The eustachian valve of the inferior vena cava can be small or absent. But even in such cases, one can usually see where the eustachian valve “should” have been. (3) The thebesian valve of the coronary sinus can be small or absent. Nonetheless, knowing where the ostium of the coronary sinus is, one can usually see where the thebesian valve of the coronary sinus “should” have been. (4) The origin of the septal leaflet of the tricuspid valve can be downwardly displaced, distinctly below the right atrioventricular junction.

Consequently, the triangle of Koch can have poorly demarcated sides, and it is quite large. The largeness of the triangle of Koch can be reduced by the understanding that the atrioventricular node and the proximal unbranched portion of the His bundle are located toward the apex of the triangle of Koch—remote from the thebesian valve that forms the base of this triangle.

Because the tendon of Todaro is invisible, and because the eustachian, thebesian, and septal tricuspid leaflets are quite variable, we have found that the coronary sinus–membranous septum line is the easiest and most accurate method of localizing the atrioventricular node and the proximal unbranched portion of the His bundle.

The conduction system is considered in greater detail in Chapter 28 .

If my friend and mentor, the late Dr. Maurice Lev, were writing this chapter, he would say that the morphologically right atrium is the atrium that on its septal surface displays the superior limbic band of septum secundum (see Figs. 3.4 and 3.5 ). By contrast, as will be seen, the morphologically left atrium is the atrium that on its septal surface displays septum primum.

When we speak of septum secundum , we mean the superior limbic band of septum secundum (see Figs. 3.4 and 3.5 ). Limbus means border (Latin). The superior limbic band forms the superior border of the foramen ovale or fossa ovalis. The superior limbic band is a muscular structure. It is the anterior interatrial plica (fold) of muscle, between the right and left atrial appendages. Normally, the ascending aorta lies just in front of the superior limbic band of septum secundum. If one excises septum secundum completely, one ends up outside the heart, in the pericardial cavity, or one can cut into the ascending aorta. Thus, in doing a surgical atrial septectomy, one should excise the membranous septum primum, but one should avoid generous excision of septum secundum’s superior limbic band in the anterosuperior direction. Usually septum secundum can be left untouched.

Septum secundum is said also to have an inferior limbic band that forms the inferior border of the foramen ovale or fossa ovalis (see Figs. 3.4 and 3.5 ). The inferior limbic band begins as a venous structure. The inferior limbic band is part of the origin of septum primum—also a venous structure. Later in development, the inferior limbic band undergoes muscularization. Hence, the septum secundum, as ordinarily defined, is a composite structure: The superior limbic band is muscular, whereas the inferior limbic band is primarily venous.

In utero, the atrial septum is a valve , normally, a unidirectional flap valve that permits only right-to-left blood flow. The atrial septum may be likened to a door that normally opens only from the right atrium into the left atrium. Septum primum is the door. Septum secundum is the door jamb.

There are seven valves in the heart. In addition to the four obvious ones, the other three valves are: (1) the eustachian valve of the inferior vena cava, (2) the thebesian valve of the coronary sinus, and (3) the atrial septum in utero.

The septum spurium (spurious septum) is the superior commissure of the sinoatrial valve. The right venous valve (i.e., the right leaflet of the sinoatrial valve) runs along the inferior rim of the crista terminalis. The left venous valve runs over the superior limbic band of septum secundum to the left of the entry of the superior vena cava. The right and left venous valves come together, forming the superior commissure of the sinoatrial valve, where the crista terminalis and the superior limbic band of septum secundum unite. When unusually prominent, this superior commissure of the sinoatrial valve has been called the septum spurium.

The tendon of Todaro is part of the inferior commissure of the sinoatrial valve.

The superior limbic band of septum secundum is also called the crista dividens (the dividing crest) of the perinatal physiologists. When the oxygenated blood from the placenta comes up the inferior vena cava and enters the right atrium, the placental blood stream divides on the superior limbic band of septum secundum, which functions as a dividing crest. This crest divides the oxygenated placental blood stream into the via sinistra (the left road) and the via dextra (the right road). The via sinistra goes to the left atrium, left ventricle, ascending aorta, and the brain, whereas the via dextra sends blood to the right ventricle, pulmonary artery, and via the patent ductus arteriosus to the abdominal viscera.

The inferior rim of the superior limbic band of septum secundum is the limbic ledge in the catheterization laboratory. If one wishes to cross the atrial septum with a catheter, one can advance the catheter from the inferior vena cava up into the superior vena cava, then turn the catheter tip toward the left atrium and slowly withdraw the catheter. As the tip passes under the superior limbic band, the tip then moves leftward, against septum primum. Immediately beneath this limbic ledge is where one may wish to do a Brockenbrough procedure to cross an intact atrial septum into the left atrium.

If there is a crista dividens (dividing crest), there should be a crista reuniens (reuniting crest). But where is it? Where do the via sinistra and the via dextra reunite? The answer is: at the aortic isthmus/patent ductus arteriosus junction. This is the aortic arch 4–aortic arch 6 junction. The crista reuniens is the spur between the aortic isthmus and the patent ductus arteriosus.

Developmentally, what is the morphologically right atrium? The right atrium consists of three main embryonic components: (1) the sinus venosus , that is, the ostium of the inferior vena cava; the ostium of the superior vena cava, the smooth or venous-like tissue between these two ostia medially (the right sinus horn); and the ostium of the coronary sinus (the left sinus horn); (2) the primitive atrium , which forms the right atrial appendage; and (3) the common atrioventricular canal , which contributes to the septum of the atrioventricular canal (the atrioventricular septum) and the tricuspid valve leaflets.

So, what is the morphologically right atrium? It consists of the dominant horn of the sinus venosus (normally the right sinus horn), a large triangular well-incorporated atrial appendage, and an atrioventricular canal component. This is why the ostium of the inferior vena cava, the ostium of the coronary sinus, and a well incorporated (large, triangular) atrial appendage are its cardinal diagnostic features.

Etymologies and History. Some of the terms that have been used in Figs. 3.3, 3.4, and 3.5 are likely to be strange and unfamiliar to you. But as soon as you know the root meanings of these terms, they become much easier to understand and to remember. Each of the words we use has a story. (Parenthetically, this section is intended for those readers who are interested in the deep understanding provided by a knowledge of the relevant history and root meanings of our terminology. However, those readers not enthralled by history and language should feel free to skip this section.)

Atrium is derived from ancient Roman architecture. The earliest Roman house was a circular one-roomed hut with wattled walls and a conical thatched roof. Later, the ancient Roman house became rectangular, built of cubical bricks faced with stucco. The roof was tiled, usually sloped inward, and beneath a central opening in the roof was a tank for collecting rainwater. This house contained only one chamber—the living-room or atrium , in which all the activities of daily life went on. It got its name from ater (black, Latin), the color that was imparted to the interior by the circling smoke of the hearth fire. Chimneys were unknown until very late, and windows were few and small. Hence, atrium literally means black room.

In later Republican times, enormous mansions were built, but they followed the earlier plan. The entrance door opened into a hall that led to the atrium, along the side of which tiny windowless bedrooms were partitioned off. At the rear of the atrium was a private office where the father of the family kept his papers and money. Beside the private office there were passages leading to an open courtyard—with lawn, flower bed, and central fountain. The dining room opened off the courtyard. At the back of the house were the kitchens, bathrooms, and service quarters. Often there was a second and even a third story, where the household slaves lived. These mansions were solidly built of concrete faced with slabs of colored marble. The walls were carefully smoothed and covered with mural paintings, and the floors were made of concrete finished with mosaics.

Consequently, atrium acquired the connotation of the room that one entered first, which then led elsewhere.

In even earlier ancient Greek, atrion meant entrance hall. Much medical Latin was derived directly from Greek, often via Claudius Galen (130–201, CE), a Greek from Pergamos in Asia Minor who spent much of his professional life in Rome.

Thus, both in Greek and Roman usage, atrion or atrium was the place one entered first in a house, which then led elsewhere.

Vena cava means hollow vein [Latin].

Pectinate muscles means comblike muscles ( pecten = comb, in Latin). In the right atrial appendage, the pectinate muscles tend to be parallel and straight, like the teeth of a comb or like a cock’s comb. The crista terminalis is like the spine of a comb, whereas the pectinate muscles resemble its teeth (see Fig. 3.4 ).

Tinea sagittalis means sagittal worm. Sagitta means arrow (Latin). The sagittal plane is thus the anteroposterior plane, as though an arrow were shot straight through the body from front to back from ventral surface to dorsal surface). Hence, the tinea sagittalis is a wormlike muscle that lies in the sagittal plane (see Fig. 3.4 ).

Tricuspid means three points ( cuspis = a point, especially of a spear, in Latin). Tricuspid in Greek is triglochin . The meaning of glochin was any projecting point, such as the end of a yoke strap or the barb of an arrow. Erasistratos (Erasistratus in Latin), the brilliant young Alexandrian contemporary of Herophilos (c 300 BCE), discovered, described, and named the tricuspid valve and the mitral (or bicuspid) valve. It was also Erasistratos who discovered that the heart is a pump.

Coronary means encircling like a crown ( corona , Latin). The Greek precursor of corona was stephanos , meaning wreath—as in the laurel or wild olive wreath of ancient Greek Olympic champions. The coronary sinus and the coronary arteries encircle the head of the heart (its base) like an Olympic wreath. Sinus means a bending, curve, or fold (in Latin). Thus, the coronary sinus literally means a fold that encircles the head of the heart like a garland.

The triangle of Koch honors the German surgeon Walter Koch, born in 1880. The atrioventricular node has been referred to as Koch’s node . The atrioventricular node has also been called Aschoff’s node and the node of Aschoff and Tawara .

The tendon of Todaro immortalizes Francesco Todaro , an Italian anatomist who lived from 1839 to 1918.

The sinoatrial node is also known as Keith’s node , or as the node of Keith and Flack .

Although Aristotle (384–322 BCE) discovered the cardiovascular system (please see Chapter 1 ), he thought that the human heart normally has three ventricles, by which he meant three chambers. Aristotle did not include what we call the right atrium as part of the heart. He thought that the right atrium is a dilatation of the great vein, that is, the inferior vena cava plus the superior vena cava. Aristotle thought that the heart begins at what we call the right ventricle and the left atrium.

Herophilos (c 300 BCE) (Herophilus in Latin), the Greek physician who founded the medical school at Alexandria in Egypt, was one of the earliest proponents of the idea that the atria really are part of the heart, not just a dilatation of the great veins.

Some 300 years later, during the time of Jesus Christ, Rufus of Ephesus called what we term the base of the heart the head of the heart. Rufus, who was also a Greek, observed that of the two ventricles, the left is thicker and artery-like, while the right is thinner and vein-like but has the larger volume. It is noteworthy that Rufus recognized only two ventricles (not three, as Aristotle had).

Rufus of Ephesus stated: “On each side of the head of the heart are things like wings. They are hollow and soft and pulsate with the rest of the heart and are called its ears.” This was because these structures were on either side of the head of the heart. “Ears” are “auricles” (auricula in Latin). So it was that the ears or auricles were definitely identified as parts of the heart and distinguished from the ventricles. Venter means “belly” (Latin). Ventriclus means “little belly” or “womb” (Latin).

However, Galen opposed this new understanding of Rufus of Ephesus. Galen continued to regard the tricuspid valve as “the insertion of the vena cava into the heart.”

Consequently, the discovery of Rufus of Ephesus that the atria are indeed parts of the heart, but significantly different from the ventricles, had to wait for 1500 years for confirmation and acceptance by Leonardo da Vinci (1452–1519). In his investigation of the heart, Leonardo described “lower ventricles,” “upper ventricles,” and “ears.” His upper ventricles are our atria. His “ears” are our auricular appendages.

Finally, the concept of Rufus of Ephesus was fully accepted by Andreas Vesalius (1514–1564) in the first edition of his De Humani Corporis Fabrica in 1543. For Vesalius, there were two atria and two ventricles, period.

Thus, it took about 1500 years for the right atrium to be accepted as a part of the heart.

The Morphologically Left Atrium

Angiocardiographically, the morphologically left atrium has an appendage that is long and thin—poorly incorporated into the main cavity of the left atrium ( Figs. 3.6 and 3.7 ). The left atrial appendage looks like a pointing finger or a map of Central America or a windsock at an airport. The characteristic shape of the left atrial appendage is often better seen in the lateral projection (see Fig. 3.7 ) than in the posteroanterior projection (see Fig. 3.6 ).

Anatomically, the left atrial appendage is long and thin ( Fig. 3.8 ), very different from the external appearance of the right atrial appendage (see Fig. 3.3 ). Normally, the pulmonary veins also connect with the left atrium (see Fig. 3.8 ). However, because of totally and partially anomalous pulmonary venous connections, the pulmonary veins are not a highly reliable diagnostic marker of the morphologically left atrium. For example, totally anomalous pulmonary venous connection makes it clear that it is possible for a morphologically left atrium to exist with no pulmonary venous connection being present.

The internal anatomy of the left atrium is distinctive ( Figs. 3.9 and 3.10 )—very different from that of the right atrium (see Figs. 3.4 and 3.5 ). The left atrial septal surface displays septum primum (see Figs. 3.9 and 3.10 ), the flap valve of the foramen ovale. This is the “door” of the interatrial communication, which is on the left atrial septal surface because septum primum (the first septum) opens into the left atrium out of the right atrium.

Photograph of the interior of the morphologically left atrium (LA) showing the flap of the foramen ovale—septum primum (SI) —on the septal surface, the normal connection of the pulmonary veins (PV), the left atrial appendage (LAA) that is not incorporated into the cavity of the LA, the pectinate muscles barely visible within the LAA, and the deep anterior leaflet of the mitral valve (MV).

The interior of the morphologically left atrium (LA). The LA is characterized by septum primum (Sept I) —the flap valve of the foramen ovale—on its septal surface, because septum primum opens into the LA, away from the right atrium (RA). Septum primum and septum secundum (Sept II) together form a unidirection flap valve that opens out of the RA and into the LA. The pulmonary veins (PV) normally connect with the dorsal wall of the LA. The left atrial appendage (LAA) is so poorly incorporated into the LA that few pectinate muscles are visible from within the cavity of the LA, unless the LAA is cut open. Note that the opened mitral valve (MV) and the opened left ventricle (LV) can also be seen.

From Van Praagh R, Vlad P. Dextrocardia, mesocardia, and levocardia: the segmental approach to diagnosis in congenital heart disease. In Keith JD, Rowe RD, Vlad P, eds. Heart Disease in Infancy and Childhood. 3rd ed. New York: Macmillan Publishing; 1978:638, with permission.

The left atrial appendage normally is so poorly incorporated into the main cavity of the left atrium that one can barely see the pectinate muscles of the left atrial appendage from the center of the left atrial cavity (see Fig. 3.9 ).

Because this interatrial “door” opens from the right atrium into the left atrium, the right atrial septal surface is characterized by septum secundum—the door jamb (see Fig. 3.4 ), whereas the left atrial septal surface is characterized by septum primum—the door (see Fig. 3.8 ).

What is septum primum really? We think that the answer to this question is: Septum primum is the main component of the left venous valve mechanism, that is bifid (it consists of two parts). In other words, septum primum is a big venous valve—normally the largest venous valve leaflet in the body. This is why septum primum normally is directly continuous with the left wall of the inferior vena cava. The other, normally much smaller component of the left venous valve mechanism is known as the left venous valve . It is applied to the right atrial surface of septum primum inferiorly and is often difficult to see grossly. When visible, there may be an interseptovalvular space , a space between septum primum to the left and the left venous valve to the right.

Because septum primum is really a large venous valve, this appears also to explain why septum primum often contains numerous very small fenestrations . Venous valves, like septum primum, often also contain small fenestrations.

Have you ever thought how very strange it is that the largest venous valve in the body, the sinoatrial valve, is normally grossly incompetent (regurgitant)? We are aware of no other vascular valve in the body that normally is severely regurgitant. We think that this interpretation is not really true as far as the sinoatrial valve is concerned. Septum primum, which is the main component of the left leaflet of the sinoatrial valve, normally prevents regurgitation from the left atrium into the right atrium and the venae cavae. Consequently, the body’s largest venous valve, the sinoatrial valve, normally is not incompetent . The sinoatrial valve only becomes incompetent if septum primum is deficient, thereby creating an ostium secundum type of atrial septal defect. Aristotle and Galen thought that what we call the right atrium is part of the great vein (superior vena cava plus inferior vena cava), because the right atrium is not separated by valves from the venae cavae but instead is confluent with these great veins.

Septum primum is the flap valve of the foramen ovale that is torn during balloon atrial septostomy, which was developed by Dr. Bill Rashkind to increase mixing at the atrial level in physiologically uncorrected transposition of the great arteries.

Septum primum grows upward (cephalically) not downward, as many textbooks of embryology say.

Septum primum does not grow downward to close ostium primum, as has been stated erroneously. The septum of the atrioventricular canal is closed initially by the endocardial cushions of the atrioventricular canal. Common atrioventricular canal is an endocardial cushion defect (not a septum primum defect). It is possible to have a complete form of common atrioventricular canal with a normally formed septum primum. The atrial septum may be intact, even though a large atrioventricular septal defect coexists.

Deficiency of septum primum is the most common cause of an ostium secundum type of atrial septal defect. Other causes of a secundum type of atrial septal defect include a deficient superior limbic band of septum secundum, as with left-sided juxtaposition of the atrial appendages; deficiency of both septum primum and the superior limbic band of septum secundum; and distention of the right atrium, as with a vein of Galen shunt in the head but without deficiency of either septum primum or septum secundum. In the latter situation, the secundum atrial septal defect undergoes spontaneous closure following successful clipping of the vein of Galen shunt, with volume unloading of the right atrium and disappearance of right atrial distention.

Thus, a secundum atrial septal defect may be associated with deficiency of septum primum, deficiency of septum secundum (superior limbic band), or both—or neither (with right atrial distention).

What is a patent foramen ovale? An open (patent) oval foramen (foramen ovale) is a communication between the right and left atria through which a probe can be passed. The probe passes beneath the downwardly facing concavity of the superior limbic band of septum secundum (see Figs. 3.4 and 3.5 ) and above the upwardly facing concavity of septum primum (see Figs. 3.9 and 3.10 ). The patent foramen ovale has some right-left length, as it passes beneath septum secundum and above septum primum. A patent foramen ovale typically does not permit left-to-right shunting; otherwise the communication would be regarded as an ostium secundum type of atrial septal defect. Hence, patent foramen ovale has the connotation that it is a probe-patent interatrial communication that does not permit shunting.

An intact atrial septum is known as a fossa ovalis (oval ditch or depression). A fossa ovalis has the connotation that it cannot possibly permit any kind of shunting (left-to-right or right-to-left) no matter how distended the atria may become, because the atrial septum is anatomically sealed. Some people speak of a fossa ovalis type of atrial septal defect. We regard this designation as an oxymoron.

What is the difference between a patent foramen ovale and an ostium secundum? A patent foramen ovale is an interatrial communication with some right-left length. It passes under the superior limbic band of septum secundum (see Figs. 3.4 and 3.5 ), and it passes over the superior margin of septum primum (see Figs. 3.9 and 3.10 ). In contrast, ostium secundum is the space above the superior margin of septum primum (see Figs. 3.9 and 3.10 ). Ostium secundum does not extend beneath the superior limbic band of septum secundum. Ostium secundum has almost no right-to-left length, being only as wide as septum primum. Fenestrations or defects in septum primum are called that. (We do not call them ostia secunda. As will be explained in Chapter 9 on interatrial communications, we think that the conventional account of the formation of ostium secundum is incorrect.)

Embryologically , the left atrium consists of three main components:

• 1.
  

  The venous component is the common pulmonary vein, which normally is incorporated into the left atrium up to just beyond the primary division of each branch. There is really only one pulmonary vein, not four (two left and two right) or five (two left and three right). The appearance of four or five pulmonary veins is produced by the incorporation of the left and right branches of the common pulmonary vein up to just beyond the first division of each branch. The common pulmonary vein normally appears at 27 days in the human embryo.

  
  
• 2.
  

  The primitive atrial component is the atrial appendage.

  
  
• 3.
  

  The atrioventricular canal component consists of the mitral valve and the lower portion of the atrial septum, that is, that portion of the atrioventricular septum that lies between the mitral annulus below and the atrial septum (inferior limbic band component) above.

Why do the right and left atrial appendages have such different shapes? (See Fig. 3.3 versus Fig. 3.8 .) We think that the answer, at least in part, is hemodynamics. The blood flows from the placenta, up the inferior vena cava and into the right atrium, distending the right atrial appendage and thereby incorporating the appendage into the main cavity of the right atrium.

Some of the inferior vena caval return arches over the top of septum primum like a waterfall. On the left atrial side, the blood passes down through the mitral valve and into the left ventricle. In other words, the blood of the via sinistra passes downward behind the left atrial appendage as it passes through the mitral valve and into the left ventricle. The blood that arches over septum primum does not flow into the left atrial appendage, distending it. The right-to-left cascade of the via sinistra occurs behind the left atrial appendage. Consequently, the left atrial appendage remains an appendix to the left atrial cavity rather than being distended and incorporated into it.

In the heterotaxy syndrome with asplenia , why may both atrial appendages look quite rightish?

Conversely, in the heterotaxy syndrome with polysplenia , why may both atrial appendages look quite leftish? Again, we think the answer is hemodynamics. In the asplenia syndrome , the inferior vena cava almost always is intact (not interrupted). Consequently, the blood from the placenta flows up the inferior vena cava and into the right atrium. Because the atrial septum is often very defective and common atrioventricular canal frequently coexists, the blood returning to the heart by way of the inferior vena cava flows into both atrial appendages, distending both and making both look rightish.

By contrast, in the polysplenia syndrome , the inferior vena cava is often interrupted from the renal veins below to the hepatic veins above. Consequently, the blood from the lower body must return to the heart by way of the azygos vein(s). The azygos vein can be right sided, left sided, or even bilateral. Hence, the blood from the placenta and lower body returns to the heart via a markedly enlarged azygos vein that joins a superior vena cava. The blood then passes down the superior vena cava, into the atrium, and passes behind both atrial appendages as it goes through an atrioventricular valve to reach the ventricular level. Since the augmented superior vena caval blood flow does not flow into either atrial appendage—but instead passes behind the appendages—consequently both atrial appendages remain undistended and hence both appear leftish.

We think that the concept of atrial isomerism, or atrial appendage isomerism, is wrong. The right atrium, or the right atrial appendage, is not really bilateral. Similarly, the left atrium, or the left atrial appendage, also is not really bilateral. In some, but by no means all cases of viscera heterotaxy with asplenia or polysplenia, bilaterally similar appearing appendages can be found—we think for the aforementioned hemodynamic reasons. In many cases of visceral heterotaxy with asplenia or polysplenia, however, the atria and the atrial appendages are morphologically very different (not “isomeric”).

Hence, we think that just as each human being has only one morphologically left ventricle and one morphologically right ventricle, so too each human being has only one morphologically right atrium or right atrial appendage and only one morphologically left atrium or left atrial appendage.

Once one understands that the concept of atrial or atrial appendage isomerism is erroneous, then it is readily possible to diagnose the morphologic identities of the atria in virtually all cases of the polysplenia syndrome and in the majority of cases of the asplenia syndrome. Diagnoses such as “atrial situs ambiguus,” “right atrial isomerism,” “right atrial appendage isomerism,” “left atrial isomerism,” or “left atrial appendage isomerism” mean that the morphologic anatomic identities of the atria are un diagnosed. All such cases await accurate morphologic anatomic diagnosis (see Figs. 3.3, 3.4, 3.7, and 3.8 ). For detailed consideration of the heterotaxy syndromes with asplenia, polysplenia, and occasionally with a normally formed spleen, please see Chapter 29 .

The morphologic anatomic features of the right atrium and left atrium are summarized in Table 3.1 .

The Morphologically Right Ventricle

Angiocardiographically, the morphologically right ventricle is the coarsely trabeculated one ( Fig. 3.11 ). The infundibulum (or conus) normally appears relatively smooth and well expanded in the posteroanterior projection.

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