Normal Cardiac Anatomy and Clinical Evaluation

Abstract

The heart is a midline structure which is located in the middle, superior, and posterior regions of the mediastinum. Although it is a midline structure of the apex of the heart, normally it is positioned to the left of the midline (Fig. 2.1).

1 Introduction Normal Heart Anatomy

The heart is a midline structure which is located in the middle, superior, and posterior regions of the mediastinum. Although it is a midline structure the apex of the heart, normally it is positioned to the left of the midline (Fig. 2.1).

Fig. 2.1

Right anterior oblique external view of the human heart (Photographs the courtesy of Dr William Edwards)

1.1 Venous Return to the Heart

The heart receives venous return both from the body and lungs, and from the heart itself. Systemic venous return from the body is through the superior vena cava and inferior vena cavae. Venous return from the lungs is via pulmonary veins, and venous return from the heart is via the coronary sinus as well as sinusoidal veins in the right ventricle.

Normally, the superior vena cava is a right-sided structure as is the inferior vena cava. Usually, there are four pulmonary veins, two from the left and two from the right; however, it is not abnormal to have a total of three or five pulmonary veins. The superior and inferior vena cavae empty into the right atrium. The pulmonary veins empty into the left atrium. The venous return from the heart muscle itself enters the right atrium through a coronary sinus, the os of which is in the right atrium and to the right ventricle via the coronary veins and right ventricular sinusoids.

1.2 Atria

There is a right atrium and a left atrium. These structures are not named just for their relative position in the chest but rather for their morphological features. A morphological right atrium normally is on the right side of the body, and a morphological left atrium normally is on the left side of the body. However, there are a number of cardiac anomalies in which the morphologically right atrium may be on the left side of the body and the morphologically left atrium may be to the right side of the body. A right atrium is characterized by the limbus of the fossil ovalis, a large pyramidal-shaped atrial appendage, the presence of the crista terminalis and pectate muscles, and the fact that it receives the vena cava and the coronary sinus. Alternatively, a left atrium is characterized by the presence of a small fingerlike atrial appendage, the absence of a crista terminalis, and the absence of pectinate muscles.

1.3 Atrioventricular Valves

There are two atrioventricular valves. The tricuspid valve allows blood to travel from the right atrium to the right ventricle, and the mitral valve allows blood to move from the left atrium to the left ventricle. The tricuspid valve, as its name implies, is a three-leaflet structure. The mitral valve, as its name implies, is a bileaflet structure. The annuli of both of these valves attach to the ventricular septum, but the mitral valve attaches in a more cephalad position than the tricuspid valve (Figs. 2.2 and 2.3). Hence, there is a septum which on one side is in the left ventricle and on the other side is in the right atrium. This is termed the “atrioventricular septum.” Another feature of a tricuspid valve is that it has a papillary muscle attached to the ventricular septum, but the mitral valve does not. Mitral valves do not have papillary muscles on the ventricular septum.

Fig. 2.2

Four-chamber view of a human heart. Note that the septal leaflet of the mitral valve attaches more cephalad than the septal leaflet of the tricuspid valve (Photographs the courtesy of Dr William Edwards)

Fig. 2.3

Pathologic specimen demonstrating the relationships of the four valves of the heart to each other and the location of the proximal coronary arteries (Photographs the courtesy of Dr William Edwards)

1.4 Ventricles

There is a right ventricle and a left ventricle. These ventricles are not named simply because the right ventricle is on the right and the left ventricle is on the left. In fact the right ventricle is anterior and to the right of the left ventricle. They are morphologically dissimilar. A right ventricle is a thinner-walled structure than the left ventricle. A right ventricle has an inflow portion, body, and an outflow portion. The distribution of muscle in the right ventricle is arranged such that the right ventricle is a more compliant chamber than the left ventricle. It is designed to accommodate changes in volume. The left ventricle is a thicker-walled left compliant chamber than the right ventricle. The orientation of its muscle fibers is such that it is designed to pump higher pressure than the right ventricle is designed to pump.

1.5 Semilunar Valves

There are two semilunar valves: the pulmonary valve and the aortic valve (Fig. 2.3). Both of these valves have three cusps. The pulmonary valve has muscle beneath it (a “conus”) such that the annulus of the pulmonary valve and the annulus of the tricuspid valve are not contiguous (Fig. 2.3). On the other hand, the aortic valve annulus is contiguous with the annulus of the mitral valve. This is an important distinction when dealing with a number of congenital anomalies of the heart. The aortic valve is oriented to the right and posterior to the pulmonary valve. The coronary arteries arise from the aorta in the region of the cusps of the aortic valve. There is dilation in the area of the cusps of the aortic valve, and these areas of dilation refer to sinuses of Valsalva. Similarly, there are dilations by the cusps of the pulmonary valve. These are called the sinuses of the pulmonary valve and should not be confused with the term sinuses of Valsalva.

1.6 Great Arteries

There are two great arteries, the pulmonary artery and the aorta. The pulmonary artery arises from the right ventricle and divides into the right and left pulmonary arteries. The aorta arises from the left ventricle. The initial branches of the aorta are the right and left coronary arteries. The next branch is the innominate artery that gives rise to the right carotid artery and right subclavian artery. The next branch of the aortic arch is the left carotid artery and the next branch is the left subclavian artery.

The ductus arteriosus connects the aorta and the pulmonary artery. This structure is patent in the fetus but normally should close within several days of birth. The ductus arteriosus remains patent (PDA) in 1 of 2500–5000 live births, but, in infants born prematurely, the incidence is 8 of 1000 live births. It represents 9–12 % of congenital heart defects. Persistent patency of the ductus results in an excess blood volume transiting the pulmonary circulation, the left atrium, and the left ventricle. This can produce signs and symptoms of congestive heart failure. If the PDA is large and is allowed to persist for several years, it can cause pulmonary vascular obstructive disease. Thus, a clinically significant patent ductus arteriosus should be closed with a transvenous device or surgery.

1.7 Coronary Arteries

There are two coronary arteries, the right and the left (Fig. 2.3; see also Chap. 43). The left coronary artery rises from the left sinus of Valsalva, and the right coronary artery arises from the right sinus of Valsalva. There are two major branches of the left coronary artery, the circumflex coronary artery and the left anterior descending coronary artery. There are numerous additional branches of the left coronary artery. The right coronary gives rise to the posterior descending coronary artery in about 80 % of the cases. The right coronary artery is also characterized by a cone branch.

1.8 Electrical System

The electrical system of the heart consists of a sinus node and atrioventricular node and a His-Purkinje system. The electrical activity of the heart begins in the sinus node (Fig. 2.4). The sinus node is located at the junction of the SVC and the right atria. The electrical activity travels through both atria. Normally, there is only one route of the electrical activity to travel from the atria to the ventricle and that is through the AV node and bundle of His. Conduction through the AV node is relatively slow and this accounts for the PR segment of the ECG. The AV node is located in the triangle of Koch which is boarded by the tricuspid valve annulus, the tendon of Todaro, and the coronary sinus. After the electrical activity travels through the AV node and His bundle, the electrical activity is distributed throughout the ventricles through the right and left bundle branches and the Purkinje system.

Fig. 2.4

Diagram of the cardiac conduction system

2 Techniques, History, and Physical Examination

2.1 The History

The historical points of interest, of course, will vary considerably depending upon the age of the patient and the presenting signs, symptoms, and complaints. For an infant, one wants to know if there is a family history of congenital heart disease or premature death and if the baby was exposed to any teratogenic agents and how well the baby is feeding.

For older patients, one needs to ascertain the presence of cardiac symptoms such as dyspnea, shortness of breath, palpitations, syncope, etc.

2.2 Physical Examination

Physical examination has four components: inspection, palpation, auscultation, and percussion.

The following can be ascertained by inspection: respiratory rate, chest retractions, pallor, clubbing, cyanosis and differential cyanosis, sweating, quality of fat stores, dysmorphic features, edema, and abnormal superficial vascular patterns

The following can be ascertained by palpation: pulse volume, precordial impulses, thrill, temperature, liver and spleen size, peripheral edema, and ascites.

Percussion is useful to determine the presence of pleural effusion, ascites, and size and position of the liver and spleen.

Cardiac auscultation involves more than just describing the presence of murmurs. It is used to describe the heart sounds, clicks, and rubs in addition to murmurs.

One needs to note if the intensity (i.e., loudness) of the first (S1) and second (S2) heart sound is normal, reduced, or increased. The intensity S2 will be increased if the chest wall is very thin, if there is pulmonary hypertension, or if the aorta is relatively anterior in the chest such as occurs with transposition of the great arteries, tetralogy of Fallot, and pulmonary atresia. S2 normally splits with inspiration and becomes single or nearly single with expiration. If S2 is widely split and never becomes single, one must suspect the presence of an ASD or right bundle branch block.

The presence of third heart sound (S3) is normal in children. It must be distinguished, however, from a mitral murmur or tricuspid diastolic flow murmur which are abnormal.

Heart murmurs result from turbulent blood flow within the heart. They are described by intensity (loudness), frequency or pitch, timing within the cardiac cycle, location on the chest wall, and effects of position of the patient on the quality of the murmur. For systolic murmurs, standard convention is to describe the intensity as from 1–6/6. A grade 4/6 murmur is accompanied by a palpable thrill which is a vibratory sensation felt by the examiner with the examiner’s hand applied to the patient’s chest. A 5/6 murmur can be heard with the stethoscope removed several centimeters from the chest, and a grade 6/6 murmur can be heard without the use of a stethoscope. For diastolic murmurs, the intensity is described as a range of 1–4/4.

The description of the change of the intensity of the murmur throughout the duration of the murmur is important. A “crescendo-decrescendo” murmur is typical of the murmur produced by aortic or pulmonary stenosis. A “decrescendo” murmur is typical of aortic or pulmonary valve insufficiency. The term “holosystolic” means that the murmur occurs throughout systole.

The frequency or pitch refers to the quality of the sound. Frequency and pitch are described as low, mid, or high. For example, the murmur of a VSD is a low-frequency murmur and that of mitral insufficiency is a high-frequency murmur. The murmurs of aortic and pulmonary stenosis are mid frequency.

2.3 The Electrocardiogram

The electrocardiogram (ECG) is the measurement of the electrical activity of the heart. It allows the examiner to determine if the rhythm of the heart is normal. The voltage of the electrical signals is related to the thickness of the heart muscle so it allows the clinician to determine if the heart muscle is hypertrophied. To perform an ECG, leads are placed on all four extremities and six leads are placed on the chest (Fig. 2.5).

Fig. 2.5

Diagram of the placement of the precordial chest leads for recording an electrocardiogram

2.4 Chest X-Ray

The chest x-ray is useful to determine the presence of cardiac enlargement and whether or not there is increased or decreased pulmonary blood flow.

2.5 Echocardiography

Echocardiography (cardiac ultrasound) allows one to visualize the heart and to determine the anatomy and accurately define the presence of congenital cardiac anomalies (Figs. 2.6 and 2.7). In addition, using Doppler technology, one can map the flow of blood through the heard and calculate pressure gradients within the heart.

Fig. 2.6

Four-chamber echocardiographic view of the normal heart

Fig. 2.7

Long-axis echocardiographic view of the normal heart

2.6 Cardiac Catheterization

Prior to the introduction of echocardiography, cardiac catheterization was necessary to determine the presence or absence and the severity of cardiac defects. It involves advancing a catheter from the femoral vein into the various chambers of the heart and measuring the blood oxygen saturations and pressures in the various parts of the heart and blood vessels (Figs. 2.8 and 2.9). In addition, contrast (dye) can be injected into the heart to produce angiographic pictures of the heart and blood vessels. With the advent of echocardiography, cardiac catheterization is used much less frequently to outline the extent of cardiac anomalies.

Fig. 2.8

Heart diagram demonstrating blood oxygen saturations in a normal heart. Note that the blood oxygen saturation is 65 % as it returns to the heart and remains at that level until it returns to the heart from the lung. Since, in the lungs, it became oxygenated, the blood returning to the left atrium from the lungs is 96 % and remains at that level as it passes through the left ventricle and is pumped into the aorta and subsequently delivered to the body. Abbreviations: RA right atrium, RV right ventricle, LA left atrium, LV left ventricle, pulm v. pulmonary valve, ao v. aortic valve, tric v tricuspid valve, mitral v mitral valve

Fig. 2.9

Heart diagram with intracardiac pressures in a normal heart. Abbreviations: RA right atrium, RV right ventricle, LA left atrium, LV left ventricle, pulm v. pulmonary valve, ao v. aortic valve, tric v. tricuspid valve, mitral v. mitral valve

However, over the past 25 years, a number of “interventional” cardiac catheterization has evolved, and a number of congenital heart defects can be treated in the cardiac catheterization laboratory sparing the patient on open chest operation. Stenotic valves can be dilated; areas of vascular narrowing such as coarctation of the aorta can be dilated or held open with insertion of a stent. Certain atrial septal defects can be closed. Fistulas can be closed. In certain cases, tissue valves can be inserted.