Illustrated Textbook of Cardiovascular Pathology P Chopra
INDEX
×
Chapter Notes

Save Clear


The Normal Heart1

A thorough knowledge of the anatomy of normal heart is an essential pre-requisite before examination of the cardiac system. During postmortem examination, the heart is taken out by midline thoracoabdominal incision which extends from the neck to the symphysis pubis. The heart is usually removed along with the lungs en bloc which is then separated out from the rest for systematic analysis. Before severing all the connections it is essential to examine and follow the systemic venous drainage and pulmonary veins by blunt dissection. The relationship of the great vessels should be noted carefully. Normally the pulmonary artery lies anterior and to the right of the aorta near the cardiac roof. With the heart in situ one should also note the position of atrial appendages. Following this a small nick should be made into the anterior surface of the pulmonary trunk to look for the presence of pulmonary embolus. After excluding any obvious external abnormality one can now safely remove the heart for further detailed examination.
The heart can be opened in one of the following two methods:
Sequential segmental analysis In this method the heart is opened following the flow of blood sequentially exposing the right atrium, right ventricle, pulmonary artery and then the left atrium, left ventricle and aorta by inflow-outflow transvalvular incisions. This dissection is an excellent approach for the demonstration of normal cardiac anatomy, valvular heart diseases and congenital heart diseases. The following sequential incisions should be made to expose the various chambers:
  1. An apical incision should be made 1–2 cm from the apex, parallel to the posterior atrioventricular groove. The size and shape of the ventricular cavities and relative thickness of both the ventricular walls are assessed.
  2. Opening of the right atrium is done by joining the superior and inferior vena caval orifices by scissors. This incision exposes the right atrial cavity. An alternative method of exposure includes an incision made 0.5 cm above the inferior vena cava upto the tip of the right atrial cavity.
  3. Using a knife, the right ventricle is cut posteriorly through the tricuspid valve 1–1.5 cm away and parallel to interventricular septum, until the first transverse cut through the ventricles is met.
  4. The anterior papillary muscle of the right ventricle is identified and the anterior ventricular wall is cut to the right of this structure through the right ventricular outflow tract, pulmonary valve and pulmonary trunk. This cut along with the previous incision exposes the entire right ventricular cavity.
  5. The left atrium is then opened by an incision through the left atrial appandage near its base in the midway between the right and left pulmonary veins. The cut is then extended downwards upto the atrioventricular junction.
  6. An incision is made by a knife through the atrioventricular valve over the posterior left ventricular wall 1–1.5 cm away and parallel to the interventricular groove. This should be done cautiously so that the chordae tendinea are not damaged.
  7. If the anterior mitral valve cusp is normal then a probe should be passed behind this leaflet through the left ventricular outflow tract into the aortic valve orifice. The anterior left ventricular musculature then should be incised carefully 2along the probe keeping the anterior mitral leaflet intact. The incision can be extended upwards into the ascending aorta. Prior to this incision, a blunt dissection between the pulmonary trunk and ascending aorta is needed for a neat exposure of the structures.
Alternatively, the heart can be cut by the “bread-loaf technique” where it is sliced by making several incisions parallel to the posterior atrioventricular groove. This technique is an excellent approach to know the extent of myocardial infarction. However, it is not recommended in cases of valvular heart disease.
The right atrium (Fig. 1.1) On opening, the right atrium is seen to have a smooth posterior part and a rough anterior trabeculated appendage, the junction between which is marked by a well-formed muscle bundle, the crista terminalis from which the pectinate muscles arise. Externally there is a groove corresponding to the crista terminalis. This is known as sulcus terminalis which is an important landmark to identify the sinoatrial node. In complex cardiac anomalies, it is the atrial appendageal structures which remain reasonably constant for identification of right and left atrial chambers. The right atrial appendage is triangular, blunt and joins the atrial cavity with a broad base while the left atrial appendage is tubular, crenalated and has a narrow base connecting with the cavity. The right atrial septal surface is characterized by fossa ovalis which is a depression at the site of foramen ovale – the interatrial communication in the fetal life. This hole shunts oxygenated blood from the right to the left atrium in fetal life and has a prominent rim or limbus. The floor of the fossa ovalis is a flap valve of thin partition which is sufficiently large to close the fossa ovalis. However, in approximately 25% of normal hearts, it may not be adherent at its superior margin and a probe can be passed through the right to left atrium producing a condition termed as probe patency of foramen ovale. The inferior vena cava opens into the junction of the posterior wall and the floor which is often guarded by a rudimentary valve (Eustachian valve). The size of the opening of the coronary sinus is variable and it may have a valve (Thebesian valve). In about 2% of adult individuals, lace like fibrous tags extend from the margin of the valves guarding the inferior vena cava and coronary sinus. This is known as Chiari's network which represents incomplete resorption of the right sinus venosus valve during development. Similar remnants may be seen across the fossa ovalis which represent remnants of the left sinus venosus valve.
zoom view
Fig. 1.1: Important landmarks: opened up right atrium
  1. Superior vena cava
  2. Right pulmonary artery
  3. Right superior and inferior pulmonary veins
  4. Interatrial septum
  5. Limbus fossa ovalis
  6. Fossa ovalis
  7. Valve of inferior vena cava
  8. Pericardial reflection
  9. Ascending aorta
  10. Pectinate muscles of the right atrial appendage
  11. Crista terminalis
  12. Membranous septum
  13. Septal leaflet of tricuspid valve
  14. Opening of coronary sinus
  15. Valve of coronary sinus
3
zoom view
Fig. 1.2: Opened up left atrium and left ventricle with mitral valve removed: left lateral view
  1. Pulmonary trunk
  2. Left atrial appendage
  3. Great cardiac vein
4 to 6. Semilunar cusps of aortic valve
The left atrium (Fig. 1.2) This is the posterior most structure of the heart. The left atrium is entirely smooth walled and receives the four pulmonary veins, two on each side. It is the left atrial appendage which is used for identification of morphological left atrium in complex congenital heart diseases. The left atrial appendage is much smaller than that of the right atrium and is not marked by any specific crista internally or sulcus externally. The trabeculae of the left atrial appendage are less pronounced. The pectinate muscles in the left atrium are confined predominantly within the appendage. The septal surface consists of the left atrial surface of the fossa ovalis also known as fossa lunata. There is no rim or limbus to the fossa on the left atrial side. However, anteriorly the fibromuscular flap valve is usually plastered with the anterior left atrial wall and the junction is marked by several rough ridges.4
zoom view
Fig. 1.3: Opened up right ventricle: right lateral view
  1. Left common carotid artery
  2. Left subclavian artery
  3. Left brachiocephalic vein
  4. Arch of aorta
  5. Ligamentum arteriosum
  6. Left pulmonary artery
  7. Transverse pericardial sinus
  8. Pulmonary trunk
  1. Conus arteriosus
  2. Septal band
  3. Septal (medial) papillary muscle
  4. Posterior papillary muscle
  5. Chordae tendinea
  6. Moderator (septomarginal) band
  7. Anterior papillary muscle
  8. Inferior vena cava
  1. Right atrium
  2. Supraventricular crest
  3. Right coronary artery
  4. Right atrial appendage
  5. Branches of right pulmonary artery
  6. Transverse pericardial sinus
  7. Superior vena cava
  8. Right brachiocephalic vein
  9. Brachiocephalic trunk
The right ventricle (Fig. 1.3) The right and left ventricles have discrete morphological features. In the normal heart, the right ventricle lies anterior and to the right of the left ventricle. However, in congenital cardiac malformation, the position of the ventricles may be altered. For these reasons, the ventricles are better distinguished in terms of their morphology rather than their position as the morphologically right and morphologically left ventricles.
The morphologically right ventricle has three portions (1) the inlet component; (2) the apical trabecular component, and (3) the outlet component. The limit of the inlet zone is the distal attachment of the chordae tendinea of the tricuspid valve. The apical trabecular part extends inferiorly beyond the attachments of the papillary muscles towards the ventricular apex. The trabeculations are coarser in the right ventricle and on its septal surface there is a prominent muscular band known as septomarginal trabeculation. The apical trabecular part is the most constant and characteristic component of the morphologically right ventricle. The right ventricle also has several septoparietal trabeculations which extend from the septomarginal trabeculation to the right ventricular parietal wall. Another muscular sling, known as the moderator band is prominent and crosses from the septomarginal trabeculation to the anterior papillary muscle and then to the parietal wall. The outlet component of the right ventricle or infundibulum is a muscular tube which supports the pulmonary valve. The inlet and outlet components are separated by a prominent muscular bar known as crista supraventricularis.5
The tricuspid valve The normal tricuspid valve has septal, antero-superior and inferior leaflets. The valve usually lacks a distinct fibrous annulus as seen in the mitral ring. Instead, the valve fibrosa blends with the adipose tissue of the atrioventricular sulcus. Each commissure is tethered by a fan-shaped chordae arising from the apex of the papillary muscles. The commissures are not always supported by the corresponding papillary muscles. The major anterior papillary muscle is the largest and usually springs directly from the body of the septomarginal trabeculation. At the point of insertions of the papillary muscles and chordae, the right ventricular inlet blends with the coarsely trabeculated component of the right ventricle. The most important distinguishing feature of the tricuspid valve is the direct attachment of chordae from the septal leaflet into the septum. In the morphologically left ventricle, chordal attachments to the interventricular septum are never seen.
Pulmonary valve The pulmonary valve has three cusps of approximately equal size which are semilunar in shape. Two of the cusps face the aortic coronary cusps and are known as right-facing and left-facing pulmonary cusps. The third cusp is called the non-facing cusp. The free edge of each cusp is thickened at its center to form nodule Arantii. Sometimes the valve cusps can have small fenestrations which are of no pathological significance.
The left ventricle (Figs 1.2 and 1.4) The left ventricle also has an inlet, apical trabecular and outlet components. Unlike the right ventricle, the inlet and outlet portions are not demarcated by the muscular cuff of the crista supraventricularis. The inlet extends from the atrioventricular junction to the attachment of the papillary muscles. The most characteristic feature to decide the morphological leftness is the fine trabecular nature of the apical component. The septal surface of the left ventricle is smooth as it does not have a septomarginal trabeculation or a moderator band. Another characteristic feature of the left ventricle is that the mitral valve never possesses chordal attachments to the septum. The membranous part of the interventricular septum is best identified from this chamber, which lies between the right coronary and non-coronary cusps. The outlet component of the left ventricle is deficient posteriorly so that the aortic and mitral valves are in fibrous continuity.
zoom view
Fig. 1.4: Opened up left ventricle: left lateral view
  1. Ligamentum arteriosum
  2. Pulmonary trunk
  3. Transverse pericardial sinus
  4. Left atrial appendage
  5. Left anterior descending artery
  6. Posterior (mural) leaflet of mitral valve
  7. Anterior (aortic) leaflet of mitral valve
  8. Anterior papillary muscle
  9. Chordae tendinea
  10. Apical trabeculations of left ventricle
  11. Posterior papillary muscle
  12. Apex of heart
  13. Right pulmonary artery
  14. Left pulmonary artery
  15. Left pulmonary veins
  16. Left atrium
  17. Oblique vein of left atrium
  18. Coronary sinus
  19. Inferior vena cava
(Figs 1.1 to 1.4 Courtsey: ADAM Student Altas of Anatomy, Williams & Wilkins, 1996, Editor Todd R Olson pp. 82–84, 88–90)
6
The interventricular septum is predominantly muscular with a small component of apical membranous septum. The membranous septum is part of the fibrous skeleton of the heart which supports the atrioventricular musculature. Because of the more apical placement of the tricuspid valve in comparison to that of the mitral valve, the septal leaflet of tricuspid valve divides the membranous septum into atrioventricular and interventricular components. The muscular septum has inlet, trabecular and outlet portions. Again, on account of more apical attachment of tricuspid valve than that of the mitral valve, this septum is divided into atrioventricular and interventricular components. This atrioventricular muscular septum should not be confused with the atrioventricular membranous septum as discussed earlier. Attachment of the septal leaflet of the tricuspid valve separates the inlet of the right ventricle from the outlet of the left ventricle. The trabecular part of the interventricular septum has coarser trabeculations on the right ventricular aspect. The outlet component of the interventricular septum is considerably small. It should be mentioned here that the posterior wall of infundibulum beneath the pulmonary valve is not a part of the septum. It separates the infundibulum from the outside of the heart. The outlet septum separating the ventricular outlets lies below the distal part of the infundibulum. With increasing age, the septum becomes sigmoid and the interventricular component of the membranous septum increases.
The mitral valve The mitral valve has two leaflets, the anterior or aortic and posterior or mural leaflet which are separated by the posteromedial and anterolateral commissures. The anterior leaflet is triangular which has fibrous continuity with the aortic valve leaflets. The posterior leaflet is quadrangular, has three scallops and takes up two-thirds of the annular circumference. The posterior leaflet throughout its length is attached to the mitral annulus while the anterior leaflet is in fibrous continuity with the aortic valve, the two valves sharing a common annulus. The anterior leaflet has a rough and clear zone and the posterior leaflet, similar to the tricuspid valve has a basal zone. The leaflets are supported by two groups of papillary muscles situated in the posteromedial and anterolateral positions.
Aortic valve The aortic valve is a semilunar valve which has three cusps two of which are named according to the origin of the two coronary arteries as right coronary and left coronary cusps. The remaining one is known as non-coronary cusp. The coronary arteries arise from the sinuses located behind the semilunar cusps. The free edge of each cusp is thickened at its center to form a nodule. The aortic root is the area occupied by the semilunar cusps. The anterior mitral leaflet is in fibrous continuity with the adjacent aortic cusps where the aortic and mitral valve rings are fused together. The cusps may be unequal in size. Sometimes the aortic valve may be bicuspid which may lead to calcific aortic stenosis. The two coronary arteries usually arise from the center of the sinus and are of approximately equal size. Occasionally the right coronary artery may have two ostia one of which is for the first conal artery.
7
The conduction system (Fig. 1.5) It includes the sinoatrial (SA) node, the atrioventricular (AV) node, the bundle of His and the bundle branches with the terminal ramifications. SA node is a cigar-shaped structure lying immediately subpericardially within the terminal groove on the lateral aspect of the junction of the superior vena cava and the right atrium. As the SA node is grossly invisible, the entire block of tissue from the suspected area should be taken and serially sectioned either parallel or perpendicular to the long axis of the vessel. The node usually has a single large nodal artery which serves as an important landmark on histological sections.
The AV node is arranged as a continuous axis which extends from the atrioventricular septum, penetrates the atrioventricular membranous septum and divides on the crest of the muscular interventricular septum. The atrial component of the AV node is contained exclusively within the Koch's triangle. This triangle is demarcated by the septal leaflet of the tricuspid valve, eustachian valve, and the tendon of Todaro. The apex of the triangle denotes the point at which the common bundle of His penetrates the membranous septum to reach the left ventricle. It then emerges in the subaortic outflow tract beneath the commissure between the non-coronary and right coronary leaflets of aortic valve. The axis branches on the crest of the muscular septum. The left bundle branch then fans out in a continuous cascade splitting into anterior, septal and posterior divisions towards the ventricular apex. The right bundle branch turns back through the interventricular septum as a cord like structure before crossing in the moderator band and ramifying into the right ventricular myocardium. Thus the tissue excised for the study of the conduction system must include the atrioventricular septum, the membranous septum and the rest of the interventricular septum.
zoom view
Fig. 1.5: The conduction system
  1. Sinoatrial node
  2. Right atrium
  3. Internodal pathways
  4. Atrioventricular node
  5. Atrioventricular (AV) bundle
  6. Right branch of AV bundle
  7. Left branch of AV bundle
  8. Purkinje fibers
Coronary artery circulation In the normal heart, only two of the three aortic sinuses give rise to coronary arteries. The two aortic sinuses that usually support the coronary arteries are always adjacent to the subpulmonary infundibulum. The third sinus is the nonadjacent/nonfacing sinus. The right 8coronary artery (RCA) emerges from the right sinus. The proximal course of the RCA is almost at right angle to the aortic sinus from which it emerges. The artery gives rise to infundibular and a series of anterior, middle and posterior atrial arteries that run upward to supply the atrial musculature. One of these branches usually the anterior one is prominent and forms the artery to the sinus node in 60% of cases. Other branches variable in number, run downward from the right coronary artery to supply the right anterior wall of the ventricle. These are generally called the right anterior ventricular branches. Of these the infundibular and acute marginal arteries have specific names. As the right coronary artery passes along the diaphragmatic surface of the heart, it reaches the crux in 90% of individuals where it gives off the artery to the AV node and the posterior descending artery. The dominance is determined by the artery that gives rise to the posterior descending and AV nodal artery.
The main stem of the left coronary artery (LCA) after its origin from the left hand facing aortic sinus runs a very short course behind the pulmonary trunk. Within 2.5 cm or so, the main stem branches into left anterior descending (LAD) and left circumflex (LCF) arteries. In one-third of individuals, the stem of the left main coronary artery trifurcates. The branch between the anterior descending and circumflex branches is called the intermediate branch. The first septal perforating artery is a relatively large branch which takes origin from the anterior descending branch close to the origin of the first diagonal branch. This branch can greatly enlarge in coronary atherosclerosis. LAD runs directly into the interventricular groove giving oblique branches and diagonal branches to the ventricle. The first diagonal branch is usually a small branch to the right ventricular infundibulum. In cases of occlusion of the LAD, this branch may link with the counter part of the right side to form the ring of Vieussens. The other branches are the septal perforating arteries which run perpendicularly into the substance of the muscular septum. Beyond the first diagonal artery the LAD runs intramyocardially for some distance. The usual course of LAD is then across the apex of the heart, where it turns toward the cardiac base of the posterior interventricular groove. The circumflex artery after its origin passes beneath the left atrial appendage to enter the left atrioventricular groove. In 10% of cases the circumflex artery is much larger giving the posterior descending artery and the artery to the AV node. In this pattern, known as the left dominance, the circumflex artery gives rise to the posterior descending artery and the artery at the crux to the AV node. It then continues into the right AV groove to supply the diaphragmatic wall of the RV. It is one of the superior atrial branches of the circumflex artery which in 40% of the population provides the artery to the SA node.
The posterior interventricular artery (90% from RCA and 10% from LCF) runs in the posterior interventricular groove and gives rise to a series of perforating arteries that run forward to the muscular septum often connecting with the branches of the anterior interventricular artery. The posterior descending artery then terminates at the apex connecting with the terminal branches of LAD. In a small percentage of cases, the terminal branches of the circumflex and right coronary arteries descend parallel to either side of the posterior interventricular groove which is known as the “balanced pattern”.
Histology of the heart The normal cardiac myocyte has a diameter of 10 to 15 µm. It has a single central nucleus. The cytoplasm, in addition to the organelles contain lipofuscin pigment, the amount of which increases with age. The myocardium has syncytial like arrangement of cardiac myocytes which take origin from the fibrous skeleton of the heart. The Purkinje fibers, present in the subendocardial location of all heart chambers are more prominent in the sections from the ventricles. They are larger than the normal cardiac myocytes and have a vacuolated cytoplasm and central nucleus.
The atrioventricular valve leaflets have a fibrous core (lamina fibrosa) which is continuous with the valve ring on one side and chordae tendinea on the other side. Over this layer, lies a loose connective tissue zone known as lamina spongiosa. A thin fibroelastic layer covering the spongiosa is continuous with the atrial and ventricular subendocardial layers and are known as “atrialis” and “ventricularis” respectively. The former contains 9more elastic tissue. The valve leaflets are avascular and a thin layer of endothelium covers the fibroelastic structure of the leaflets.
The semilunar valves also have a dense lamina fibrosa and a loose spongiosa. They have a thin fibroelastic layer beneath the endothelial lining. The elastic tissue strands are more prominent over the ventricular aspect of the valve than on the arterial side of the cusp. The normal semilunar valve cusps are avascular.
Electron microscopy of cardiac myocytes The myocytes have a syncytial arrangement and they are separated from each other by intercalated discs. At subcellular level the myofilaments are arranged in bundles as myofibrils with intervening mitochondria and sarcoplasmic reticulum. The sarcomere, the functional unit of cardiac contractile mechanism is formed by the organization of the myofibril. The sarcomere is delimited on each side by a Z line with thick and thin filaments in between arranged perpendicularly to the Z line. The thick filaments contain myosin which form the A band. The actin filaments or the thin filaments with the regulatory proteins troponin and tropomyosin extend from the Z line up to the A band through the I band. The force of contraction is generated from the interaction of these myofilaments. The extent of overlap between the adjoining thick and thin filaments decides the amount of force that can be generated for myocardial contraction.
 
SUGGESTED READING
  1. Anderson RH, Becker AE: Cardiac Anatomy: An Integrated Text and Colour Atlas. Churchill Livingstone,  Edinburgh,  1980.
  1. Anderson RH, Becker AE: Normal cardiac anatomy. In Robertson WB (Ed): The Cardiovascular System Part A (3rd edn) Churchill Livingstone.  10: 3–26, 1993.
10
APPENDIX
Various weight and measurements of adult heart.
Weight
250–300 g
(0.45% of body weight in males)
200–250 g
(0.4% of body weight in female)
Wall thickness
Atria
1–2 mm
Right ventricle
3–5 mm
Left ventricle
10–15 mm
The wall thickness of right ventricle is measured 2 cm below to the pulmonary valve and that of left ventricle is measured 2 cm below the mitral valve. The measurement should exclude the trabeculated subendocardial muscles.
Valve circumference
Aortic valve
7.5 cm (6-7.5 cm)
Pulmonary valve
8.5 cm (7–9 cm)
Mitral valve
10 cm (8-10.5 cm)
Tricuspid valve
12 cm (10-12.5 cm)