Nerve Supply Of Interventricular Septum

Interventricular Septum Blood Supply

Fig. 25. Arterial supply to the interventricular septum. The right coronary artery supplies the posterior one-third of the interventricular septum, and the left coronary supplies the anterior two-thirds. The artery to the atrioventricular node commonly branches off the posterior interven-tricular artery. AV, atrioventricular; LAD, left anterior descending. Adapted from T.N. James and G.E. Burch (1958) Blood supply of the human interventricular septum. Circulation 17(3), pp. 391-396.

Fig. 25. Arterial supply to the interventricular septum. The right coronary artery supplies the posterior one-third of the interventricular septum, and the left coronary supplies the anterior two-thirds. The artery to the atrioventricular node commonly branches off the posterior interven-tricular artery. AV, atrioventricular; LAD, left anterior descending. Adapted from T.N. James and G.E. Burch (1958) Blood supply of the human interventricular septum. Circulation 17(3), pp. 391-396.

The right coronary artery also serves as an important collateral supply to the anterior side of the heart, left ventricle, and anterior two-thirds of the interventricular septum via the conus artery and communicating arteries in the interventricular septum (Fig. 25). Kugel's artery, which originates from either the right or left coronary artery, runs from anterior to posterior through the atrial septum. This artery serves as an important collateral connection from anterior arteries to the atrioventricu-lar node and posterior arteries.

8.2. Left Coronary Artery

The left coronary artery (left main coronary artery) emerges from the aorta through the ostia of the left aortic cusp within the sinus of Valsalva (Fig. 23). The plane of the semilunar valve is tilted so that the ostium of the left coronary artery is superior and posterior to the right coronary ostium. The left coronary artery travels from the aorta, and passes between the pulmonary trunk and the left atrial appendage. Under the appendage, the artery divides (and is thus a very short vessel) into the anterior interventricular (left anterior descending) artery and the left circumflex artery. The left coronary artery may be completely absent; that is, the anterior interventricular and circumflex arteries arise independently from the left aortic sinus.

The anterior interventricular artery appears to be a direct continuation of the left coronary artery that descends into the anterior interventricular groove. Branches of this artery, anterior septal perforating arteries, enter the septal myocardium to supply the anterior two-thirds of the interventricular septum (in about 90% of hearts). The first branch, the first septal perforator, supplies a major portion of the atrioventricular conduction system. In about 80% of human hearts, the second or third per forator is the longest and strongest of the septal arteries and is often called the main septal artery. This artery supplies the middle portion of the interventricular septum. Oddly, this artery also sends a branch to the moderator band and the anterior papillary muscle of the tricuspid valve (right ventricle). This artery is often called the moderator artery.

Other branches of the anterior interventricular artery extend laterally through the epicardium to supply adjacent right and left ventricular free walls. The anterior interventricular artery also sends a branch to meet the conus artery from the right coronary to form an important collateral anastomosis called the circle of Vieussens (Raymond Vieussens, French anatomist, 1641-1715), as well as branches to the anterior free wall of the left ventricle called diagonal arteries. These are numbered according to their sequence of origin as first, second, and so on diagonal arteries. The most distal continuation of the anterior interventricular artery curves around the apex and travels superiorly in the posterior interventricular sulcus to anastomose with the posterior descending from the right coronary artery.

In summary, the anterior interventricular artery and its branches supply most of the interventricular septum: the anterior, lateral, and apical walls of the left ventricle; most of the right and left bundle branches; and the anterior papillary muscle of the bicuspid valve (left ventricle). It also provides collateral circulation to the anterior right ventricle, the posterior part of the interventricular septum, and the posterior descending artery.

The circumflex artery branches off the left coronary artery and supplies most of the left atrium, the posterior and lateral free walls of the left ventricle, and (with the anterior interven-tricular artery) the anterior papillary muscle of the bicuspid valve. The circumflex artery may give off a variable number of left marginal branches to supply the left ventricle. The terminal branch is usually the largest of these branches. More likely, the circumflex artery may continue through the atrioventricular sulcus to supply the posterior wall of the left ventricle and (with the right coronary artery) the posterior papillary muscle of the bicuspid valve. In 40-50% of human hearts, the circumflex artery supplies the artery to the sinoatrial node.

In 30-60% of hearts, the left coronary artery may give off one or more intermediate branches that originate between the anterior interventricular and circumflex arteries. These extend diagonally over the left ventricle toward the apex of the heart and are thus named diagonal or intermediate arteries.

The anterior interventricular artery is the most commonly occluded of the coronary arteries. It is the major blood supply to the interventricular septum and the bundle branches of the conducting system. It is easy to see why coronary artery disease can lead to impairment or death (infarction) of the conducting system. The result is a "block" of impulse conduction between the atria and the ventricles; this block is known as right/left bundle branch block. Furthermore, branches of the right coronary artery supply both the sinoatrial and atrioventricular node in at least 50% of hearts. An occlusion in this artery could result in necrosis of the sinoatrial or atrioventricular nodes, thus preventing or interrupting the conduction of electrical activity across the heart. (For more information on coronary artery stenting, see Chapter 6.)

8.3. Cardiac Veins

The coronary arteries supply the heart with nutrients and oxygen. At the same time, waste products and carbon dioxide must be removed. An extensive network of intercommunicating veins provides venous drainage from the heart. The venous drainage of deoxygenated blood from all tissues is collected in the right atrium; this includes the venous drainage of the heart. Venous drainage of the heart is accomplished through three separate systems: (1) the cardiac venous tributaries, which converge to form the coronary sinus; (2) the anterior cardiac (anterior right ventricular) veins; and (3) the smallest cardiac (Thebesian) venous system (Fig. 26).

Most of the myocardium is drained by the cardiac veins that course parallel to the coronary arteries. These three large veins (the great, middle, and small cardiac veins) converge to form the coronary sinus.

On the anterior side of the heart, the great cardiac (anterior interventricular) vein lies within the anterior interventricular sulcus and runs from inferior to superior beside the anterior interventricular artery (Figs. 26 and 27). At the base of the heart, near the bifurcation of the left coronary artery, it turns and runs within the atrioventricular groove around the left side of the heart to the posterior. In the atrioventricular groove, on the posterior side of the heart, the great cardiac vein becomes the coronary sinus, which then empties into the right atrium. From the inside of the right atrium, it can be seen that the coronary sinus opens into the right atrium, forming an opening or os located anteriorly and inferiorly to the orifice of the inferior vena cava. There is a valve (Thebesian valve) that covers to varying degrees the opening of the coronary sinus to prevent backflow. The great cardiac vein is formed by the confluence of small venous tributaries from the left and right ventricles and anterior portion of the interventricular septum. As it ascends toward the coronary sinus, it receives small venous tributaries from the left atrium and left ventricle; it also receives a large left marginal vein, which runs parallel to the left marginal artery.

There are two structures that serve as the boundary between the termination of the great cardiac vein and the beginning of the coronary sinus. The first is the valve of Vieussens, which has the appearance of a typical venous valve and functions to prevent the backflow of blood from the coronary sinus into the great cardiac vein. The second is the space between the entry points of the oblique vein of the left atrium (of Marshall; John Marshall, English anatomist, 1818-1891) and the posterior vein of the left ventricle. The oblique vein of Marshall runs superior to inferior along the posterior side of the left atrium, providing venous drainage of the area. The posterior vein ascends to the coronary sinus from the inferior portion of the left ventricle and provides drainage of the area.

In addition to the great cardiac vein, the coronary sinus receives the middle cardiac vein (Figs. 26 and 28). Located on the posterior surface of the heart, it arises near the posterior aspect of the apex of the heart and runs from inferior to superior through the posterior interventricular sulcus. It then joins the coronary sinus within millimeters of the sinus entering into the right atrium. The middle cardiac vein is formed from venous confluence of tributaries that drain the posterior left and right ventricles and the interventricular septum.

The coronary sinus also receives the highly variable small cardiac vein. The small cardiac vein arises from the anterior/ lateral/inferior portion of the right ventricle. It ascends and runs inferior to, and roughly parallel with, the marginal branch of the right coronary artery until it reaches the right atrioventricular sulcus. At this point, it turns and runs horizontally around to the posterior side of the heart and enters the coronary sinus with the middle cardiac vein. The small cardiac vein is extremely small or absent in 60% of humans. In about 50% of hearts, the small cardiac vein enters the right atrium directly.

Typically, about 85% of the heart's venous drainage occurs through the great, middle, and small cardiac veins through the coronary sinus to the right atrium. This elaborate system of veins drains the left ventricle, some of the right ventricle, both atria, and the anterior portion of the interventricular septum.

The second system of venous drainage of the heart is the anterior cardiac veins (Figs. 26 and 29). This system is distinguished from the other cardiac venous system because the anterior cardiac veins do not drain into the coronary sinus. Two to four anterior cardiac veins originate and drain the anterior right ventricular wall, travel superiorly to cross the right atrioventricular sulcus, and enter the right atrium directly. The sulcus is usually packed with adipose tissue. Through this adipose tissue run the anterior cardiac veins, the right coronary artery, and a branch of the coronary artery, the right atrial or nodal artery. The anterior cardiac veins pass over the right coronary artery in close proximity and in a perpendicular angle. A right marginal vein (when present) runs parallel with the right marginal artery before entering the right atrium directly

Anterior Cardiac Vein
Fig. 26. Venous drainage of the heart. Three separate venous systems carry blood to the right atrium: the coronary sinus and its tributaries, the anterior cardiac veins, and the smallest (Thebesian) cardiac veins.

and is usually considered part of the anterior cardiac venous system.

The third system of venous drainage of the heart is the smallest cardiac venous system. This system is composed of a multitude of small intramural ("within the walls")/intramyocardial veins also called Thebesian veins. These are minute vessels that begin in the capillary beds of the myocardium and open directly into the chambers of the heart. Although called veins, they are valveless communications between myocardial capillaries and a chamber of the heart. These veins drain primarily into the right atrium, and to a lesser extent the right ventricle, near the septa. The openings of these veins can be seen macro-scopically (Thebesian foramina) in the endocardium of the right atrium.

Thebesian Veins

Fig. 27. The great cardiac vein. On the anterior side of the heart, the great cardiac vein lies within the anterior interventricular sulcus and runs from inferior to superior beside the anterior interventricular artery. At the base of the heart, it runs within the atrioventricular groove around the left side of the heart to the posterior. In the atrioventricular groove, on the posterior side of the heart, the great cardiac vein becomes the coronary sinus and empties into the right atrium.

Fig. 27. The great cardiac vein. On the anterior side of the heart, the great cardiac vein lies within the anterior interventricular sulcus and runs from inferior to superior beside the anterior interventricular artery. At the base of the heart, it runs within the atrioventricular groove around the left side of the heart to the posterior. In the atrioventricular groove, on the posterior side of the heart, the great cardiac vein becomes the coronary sinus and empties into the right atrium.

Groove For Confluence Sinuses

Fig. 28. The middle cardiac vein. The middle cardiac vein, located on the posterior surface of the heart, arises near the posterior aspect of the apex of the heart and runs from inferior to superior through the posterior interventricular sulcus before entering the coronary sinus. The middle cardiac vein is formed from venous confluence of tributaries that drain the posterior left and right ventricles and the interventricular septum.

Fig. 28. The middle cardiac vein. The middle cardiac vein, located on the posterior surface of the heart, arises near the posterior aspect of the apex of the heart and runs from inferior to superior through the posterior interventricular sulcus before entering the coronary sinus. The middle cardiac vein is formed from venous confluence of tributaries that drain the posterior left and right ventricles and the interventricular septum.

Posterior Interventricular Sulcus

Fig. 29. Anterior cardiac veins. Two to four anterior cardiac veins originate and drain the anterior right ventricular wall. These veins travel superiorly to cross the right atrioventricular sulcus and enter into the right atrium. These veins are part of the smallest cardiac venous system that empties oxygen-poor blood directly into the right atrium without a communication with the coronary sinus.

Fig. 29. Anterior cardiac veins. Two to four anterior cardiac veins originate and drain the anterior right ventricular wall. These veins travel superiorly to cross the right atrioventricular sulcus and enter into the right atrium. These veins are part of the smallest cardiac venous system that empties oxygen-poor blood directly into the right atrium without a communication with the coronary sinus.

8.4. Myocardial Bridges

The coronary arteries typically course on the myocardium or under/within the epicardium of the heart. Frequently, a portion of an artery deviates from its usual subepicardial position to follow an intramyocardial (intramural) course, either by traveling a significant length within the myocardium or beneath an arrangement of muscular slips ("myocardial bridges"). Myocardial bridging is most common in the middle segment of the anterior interventricular artery (1). The myocardial fibers that cover or "bridge over" the anterior interventricular artery are direct extensions of the myocardium of the conus arteriosus of the right ventricle and cross the artery in a perpendicular direction. Myocardial bridges over the right coronary and the circumflex arteries are much less common. When present, these bridges are extensions of the respective atrial myocardium (2). The prevalence of myocardial bridges from various sources is reported to occur in 5.4-85.7% of human hearts when measured from the cadaver (3,4) and 0.5-16% when measured from angiography in catheterization labs (4-6).

Coronary arteries and their branches have a tortuous pattern as they run across the heart. Interestingly, studies employing angiography followed by detailed microdissection showed that a coronary artery with a typical tortuous shape takes on a perfectly straight pattern when it follows an intramyocardial course (7).

Angiography has also shown that myocardial bridges are associated with narrowing of the lumen of the coronary artery.

The narrowing appears during systole and disappears during diastole (1). The appearance of straight running or systolic narrowing patterns seems to be an important diagnostic technique during angiography to discover intramyocardial segments of coronary arteries (1). Myocardial bridging is usually a benign condition. Although there is contrasting evidence, atherosclerosis is uncommon within a myocardial bridge (3); bridging might be providing some protection against plaque formation (1).

Essentials of Human Physiology

Essentials of Human Physiology

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