Functional anatomy of the heart

The heart lies obliquely in the thorax within the mediastinum—the space between the lungs which also contains the major blood vessels, oesophagus and trachea. It typically weighs about 325 ± 75 g in men and 275 ± 75 g in women. The heart can be described as having three surfaces and an apex. About two thirds of the heart is to the left of the mid-line. The anterior surface of the heart is formed mainly by the right ventricle and is in contact with the ribs and sternum (Fig. 3.1). The inferior surface of the heart is formed mainly by the left ventricle and is in contact with the diaphragm. The posterior surface of the heart is formed mainly by the left atrium. This surface is also known as the base of the heart. The apex which is anterior to the rest of the heart consists only of the left ventricle and forms an important clinical landmark when assessing the size of the heart. The aorta and the pulmonary trunk arise from the left and right ventricles respectively at the superior pole of the heart (Fig. 3.2). The superior and inferior vena cavae open into the upper and lower parts of the right atrium. There are four pulmonary veins which open into the back of the left atrium. The junction between the atria and the ventricles is marked by the atrioventricular groove and the junction between the ventricles both posteriorly and anteriorly is marked by the interventricular grooves.

The fibrous skeleton of the heart is a framework of dense collagen around the atrioventricular junction and the arterial outflow. It provides structural support for the heart, particularly by stabilizing the heart valves, and it also prevents the heart from being overstretched (Fig. 3.3). The electrical insulation provided by this fibrous skeleton forms the barrier between the atria and the ventricles and ensures that electrical conduction normally only occurs through the bundle of His (see Chapters 2 and 7).

The heart is surrounded by the pericardium. The fibrous pericardium is a tough sac enclosing the heart and providing attachments to adjacent structures. The serous pericardium has a visceral layer which is attached to the surface of the heart and a parietal layer lining the fibrous pericardium. The smooth layers of the serous pericardium slide easily over each other and allow the heart to move within the thorax as it beats. The very narrow space between the layers of serous pericardium is filled with pericardial fluid.

The right atrium has only a thin (about 2 mm) wall. There is a rough portion belonging to the auricular appendage and a smooth portion which receives the superior and inferior vena cava and the coronary sinus, the major part of the venous drainage for the heart itself. Blood flows from the right atrium into the right ventricle through an atrio-ventricular valve which has three cusps—the tricuspid valve (Fig. 3.3). The right ventricle is more muscular (about 4–5 mm thick) than the right atrium. The tricuspid valve is attached to the papillary muscles of the ventricular wall by the chordae tendinae (heart strings) which are made of fibrous tissue and help to stabilize the valve and prevent blood from flowing back into the right atrium when the ventricle contracts.

The left atrium is thin walled (about 3 mm) and, like the right atrium, has a smooth and a roughened portion. The smooth-walled part is formed by the entrance of the four pulmonary veins. The atrioventricular opening in the left side of the heart is guarded by a valve shaped like a bishop’s mitre and formed from two cusps—the mitral valve. (In Shaw’s play, St Joan declares at her trial that ‘Bishops are never in the right’—this may help you to remember that the mitral valve is in the left side of the heart!). The left ventricle has the greatest wall thickness (8–15 mm) of all the cardiac chambers with internal muscular ridges throughout except for a small area at the aortic valve opening. At the tip of the apex the wall of the left ventricle is only about 2 mm thick. The mitral valve is anchored to the left ventricle by papillary muscles and chordae tendinae. The aortic valve has three semilunar cusps. There is a dilatation, the sinus of Valsalva, in the aortic wall just above each cusp. These dilatations allow blood to flow behind the cusps at the very beginning of ventricular relaxation in diastole and thus ensure that the aortic valve shuts and that the cusps are not flattened against the side of the aorta. They also allow blood to freely enter the right and left coronary arteries (see Chapter 5).

The body surface markings of a normal heart extend from the second left and third right costal cartilages down to the fifth left intercostal space and the sixth right costal cartilage. It should be appreciated that these markings will be affected by posture, by breathing (contraction of the diaphragm pulls the heart downwards) and by hypertrophy or dilatation of the ventricles when the apex beat may move from the fifth intercostal space in the mid-clavicular line to the sixth intercostal space in the mid-axillary line. The location of the apex beat is therefore often an important clinical sign.

The arterial blood supply of the heart is via the right and left coronary arteries which originate from the aorta near the aortic valve (see Chapter 5). The left coronary artery branches into the left circumflex and left anterior descending arteries. The right coronary artery branches into the marginal and right posterior descending arteries. The venous drainage of the heart is via a number of veins which drain into the coronary sinus on the inferior surface of the heart in the atrioventricular sulcus. The coronary sinus drains into the right atrium.

The cardiac cycle

The events which comprise the cardiac cycle on the left side of the heart are shown in Figure 3.4. The equivalent values for the right side of the heart are qualitatively similar but the pressures are lower since the pulmonary circulation is a low-resistance circu lation. The onset of ventricular systole is indicated by the first heart sound which is produced by closure of the atrioventricular valves. The end of ventricular systole is indicated by the second heart sound which is produced by closure of the aortic and pulmonary valves.

The sounds produced by valve closure and any murmurs associated with valves are best listened to in four specific areas of the chest:

On the ECG the P wave corresponds to atrial depolarization (see Chapter 7) and thus marks the onset of atrial systole. The QRS complex is associated with ventricular depolarization and thus the onset of ventricular systole. The functioning of the heart as an effective pump depends upon the valves between the atria and the ventricles (the tricuspid and mitral valves) and those between the ventricles and the main distributing arteries (the aortic and pulmonary valves) functioning correctly.

In Figure 3.4 the left ventricular pressure wave begins with a small surge of pressure when the left atrium contracts and ‘top ups’ the ventricle. In some individuals with a hypertrophied left atrium pumping blood into a stiffened left ventricle, as may occur in ischaemic heart disease or hypertension, a sound is generated which is referred to as the fourth heart sound. The increase in ventricular volume associated with atrial systole is very small as most ventricular filling is passive and occurs in the early stage of diastole when blood flows through the relaxed atrium into the relaxed ventricle. With the onset of ventricular contraction ventricular pressure rises and the mitral valve closes. This, along with the normally concurrent closure of the tricuspid valve, causes the first heart sound. If venous return is increased acutely then the closure of the tricuspid valve in the right side of the heart may be slightly delayed and the first heart sound will be ‘split’, that is closure of the two valves will be heard separately. If either valve fails to close fully then the murmur of mitral or tricuspid regurgitation, an early systolic murmur, will be heard.

In the early phase of ventricular systole all four valves are closed. Ventricular pressure rises and this is called an isovolumic (also called isometric or even isovolumetric) phase of ventricular contraction. When ventricular pressure exceeds aortic pressure the aortic valve will open. If the aortic or pulmonary valves fail to open normally then the early systolic murmur of aortic or pulmonary stenosis will be heard. An aortic stenosis murmur will often radiate up the carotid artery in the neck.

The ventricle empties the stroke volume of blood into the aorta (see Chapter 4) and ventricular volume typically falls by about 70 mL (Fig. 3.4). Ventricular and aortic pressures follow a similar path as the contraction of the ventricles opposes the elastic recoil of the aorta. Blood flow in the aorta rises substantially at this time. As ventricular relaxation proceeds, ventricular pressure falls and when it is below the pressure in the aorta the aortic valve closes. This is helped by a backflow of blood created by the sinus of Valsalva as described above. Closure of the valve creates a short-lived dip in aortic pressure called the dicrotic notch. The closure of the aortic and pulmonary valves generates the second heart sound. If either the aortic or pulmonary valve fails to close fully then the murmur of aortic or pulmonary regurgitation may be heard. This is an early diastolic murmur. Ventricular pressure continues to fall and eventually the pressure in the ventricle is less than that in the atrium. At this point the mitral valve opens and blood flows into the ventricle (Fig. 3.4). If the mitral or tricuspid valve fails to open properly then the early diastolic murmur of mitral/tricuspid stenosis may be heard. Opening of these valves between the atria and ventricles heralds the period of rapid ventricular filling and may occasionally, especially in individuals with low heart rates and high stroke volumes, produce a third heart sound which is said to be analogous to a ‘sail flapping in the wind’. After closure of the aortic valve pressure in the aorta and thus blood flow is maintained by the elastic recoil of the artery wall. The oscillations in aortic pressure are therefore not as large as those seen in the ventricle and aortic flow, although it is pulsatile, is much more uniform than it would otherwise be. This is important to ensure continued perfusion of all the tissues of the body during diastole (see Chapter 10).

The pressure wave in the left atrium is shown in Figure 3.4. The right atrial pulse is qualitatively similar and, since there are no valves at the entrances to the atria, a similar pulse may be seen in the jugular vein if the patient’s head and thorax are tilted back at about 45 degrees. The ‘a’ wave of atrial pressure (7 mm Hg in the left atrium and 5 mm Hg in the right atrium) corresponds to atrial systole, the ‘c’ wave corresponds to the closure of the atrioventricular valves which bulge back into the atria as they close. The ‘v’ wave of the jugular or atrial pulse results from atrial filling and immediately precedes the opening of the tricuspid and mitral valves.

The case history of a young girl with a suspected cardiac valve problem is introduced in Case 3.1:1.

Valve pathology

History taking for cardiac disease

The classical symptoms of cardiac disease are:

None of these symptoms is specific to cardiac disease. Chest pain for example may arise from many causes—severe asthma, pneumonia, gastro-oesophageal reflux or musculoskeletal problems. As with any medical assessment the function of history taking is to seek evidence which will support or refute the hypothesis that symptom x is caused by problem y. The wise clinician should always keep an open mind however, as it is easy to be deceived by an atypical presentation of common illness or by classical symptoms for one pathology actually arising from another cause. The clinician should consider the history as a series of important clues. The clues may be reinforced by findings on examination and the results of appropriate tests.

Cardiac pain can broadly be divided into two groups, firstly the pain associated with acute insufficiency of coronary blood flow as a result of coronary vessel obstruction causing either myocardial infarction or unstable angina and secondly the pain related to blood flow insufficiency becoming apparent on exertion which is classical angina pectoris. Cardiac chest pain is typically central, crushing and radiates to the left arm (usually) or both arms or the neck but it is not well localized. Onset of pain during exercise and relief after rest is typical of exertional insufficiency. The resting blood flow may be adequate but the myocardium becomes ischaemic under increased demand. The character of the pain of infarction is similar but more severe. It frequently occurs at rest and is of longer duration. Men frequently describe the pain of infarction as the most severe they have ever known, though this may not be the case for women who have been through childbirth!

In arriving at a diagnosis the associated symptoms and timing may be important. A sharp, stabbing chest pain which is specifically localized may indicate pleuritic pain. A sudden onset of pain accompanied by dyspnoea may suggest a pneumothorax or pulmonary embolism. If the pain is associated with fever and related symptoms then pneumonia may be the underlying cause. A sharp, burning, retrosternal pain linked with eating patterns may point to gastro-oesophageal reflux as the cause.

A careful note of risk factors for cardiac disease should be made:

The above factors will all be useful in identifying heart failure (see Chapter 6) which in the majority of cases will be preceded by some form of chronic illness. Exercise intolerance and peripheral oedema will point to heart failure as will orthopnoea. This is an inability to breath comfortably when lying flat which causes the individual to prop themselves up with several pillows in order to sleep. Orthopnoea is predominantly a symptom of left heart failure. Sitting up reduces the preload (venous return) on the right side of the heart and hence reduces the output into the pulmonary circuit. Pulmonary engorgement (increased lung stiffness) which generates the sensation of dyspnoea is therefore reduced.

The degree of exercise intolerance as evidenced by the onset of angina may be a useful guide to the level of coronary insufficiency. Chest pain on vigorous exertion such as walking up stairs implies a lower level of insufficiency than chest pain walking on the level. Recording these parameters will allow the progress of disease to be monitored and give a yardstick for benefit from intervention (see Chapters 5 and 13).

Palpitations, simply the awareness of one’s own heartbeat, is a common presentation but a rarer symptom of actual cardiac disease. Awareness of an abnormal heart rhythm may precede a collapse, which is usually strong evidence of an arrhythmia compromising cardiac output. However collapse may be the first ‘symptom’ of a serious arrhythmia. The description by witnesses to a collapse may be crucial, with the major differential diagnosis being neurological events such as seizures. If the individual is aware of the palpitations the key points to elicit in history taking are onset/offset and regularity. The heart cannot be partly in an abnormal rhythm so paroxysmal arrhythmias are typified by sudden onset and offset. Inappropriate sinus tachycardia, the most common cause of palpitations, generally starts and stops gradually. Regularity of palpitations is another important characteristic. Ectopic beats tend to give an occasional irregularity with most beats coming at a normal interval. Bigeminy and trigeminy (couplets and triplets) tend to be irregular in a consistent manner. The commonest arrhythmia in clinical practice is atrial fibrillation (see Chapter 7). The chaotic nature of the heart rate means that it is very distinctly ‘irregularly irregular’.

Children and young adults may demonstrate sinus arrhythmia, a perfectly normal acceleration and deceleration of the heart rate related to breathing. This usually disappears with advancing age. The author’s unquantified guesstimate based on conducting ECG practicals with medical students is that about a third of 18–20 year olds will still show sinus arrhythmia.

Ankle swelling and other signs of peripheral oedema represent evidence of increased right atrial pressure and a resultant increase in hydrostatic pressure in the capillary bed (see Chapter 11). It is particularly a result of right ventricular failure as opposed to the pulmonary oedema of left heart failure (see Chapter 6). Frequently both ventricles will fail together, especially as the commonest cause of right heart failure is left heart failure.

Peripheral oedema will occur in the dependent parts of the body, that is the lowest part with respect to gravity. This is usually the feet and ankles but could also be the sacrum and buttocks in those who are bed bound for most of the time. With mild right heart failure the feet and ankles may swell during the day when an individual is standing for much of the time but go down when they ‘put their feet up’. The severity of failure is to some degree evidenced by how far up the legs the oedema can be detected, oedema reaching up to the knee being generally worse than simple swelling of the ankles.

Clinical examination of the cardiovascular system

Detailed discussion of the normal and abnormal features of cardiac disease are beyond the scope of this book and you should refer to a textbook of clinical examination. To summarize the examiner is looking for signs attributable directly to cardiac dysfunction such as an irregular pulse or murmurs and signs which are secondary to cardiovascular dysfunction such as liver distension and oedema. Before performing the examination it is wise to consider the possibilities suggested by the history. Chest pain of itself is unlikely to relate to other physical signs. However ischaemia, either acute or chronic, may leave evidence of heart failure such as a raised jugular venous pressure, peripheral oedema or displacement of the apex beat due to cardiac enlargement.

A thorough examination of the cardiovascular system will encompass at least the following points but may require much more detail;

The history taken or aspects of the examination may point the way to other specific signs such as checking the retina for Roth’s spots in suspected endocarditis or flame haemorrhages in hypertension.

Investigations will lead on from history taking and examination. An ECG (see Chapter 7) is extremely useful and as it is cheap and non-invasive is nearly always appropriate; even a baseline ECG is worth having for future reference. Chest X-ray, blood count and electrolyte assessment are all commonly performed at least once. Only the timing of the test is of significant debate since out of hours tests are generally more expensive than those performed during the normal working day. Blood cultures should always be taken if sepsis is a possibility; traditionally at least three sets of samples from different sites in the case of endocarditis and antibiotic treatment should be withheld if at all possible until this has been done so that a causative organism can be identified.

Investigations of heart disease: imaging the heart

The heart is the most dynamic organ in the body. It beats between 50 and 100 times a minute for your whole life. This movement has until relatively recently proved a barrier to accurate imaging.

Sudden cardiac death

Sudden cardiac death is defined as an event that is non-traumatic, non-violent, unexpected and results from sudden cardiac arrest within 6 hours of previously witnessed normal health. The sudden and unexpected death of an individual is a particularly sad event coming as it does ‘out of the blue’. The vast majority (>90%) of sudden cardiac deaths occur due to underlying coronary artery disease. The actual terminal event may come unexpectedly even in an individual who has known coronary artery disease and is probably the result of a severe arrhythmia such as ventricular fibrillation. Alternatively the sudden and complete occlusion of a major part of the coronary arterial tree may lead to sudden loss of function in such a large portion of the cardiac muscle that the heart is no longer capable of sustaining a sufficient output to maintain brain function. Infrequently this is the first presentation of ischaemic heart disease.

It is estimated that between 100 and 500 young people die of cardiac disease in the UK each year. Frequently the death of an affected family member is the first sign of an inherited disorder. The commonest cause of sudden cardiac death in young people is hypertrophic cardiomyopathy. This is an autosomal dominant condition in which abnormal structural proteins such as the β-myosin heavy chain cause physiological dysfunction which induces compensatory hypertrophy of the myocardium. This may be recognized at post mortem or detected using echocardiography. It is now standard practice for professional footballers to undergo routine screening to exclude this condition. Unfortunately the fact that the heart of a highly trained sportsman tends to undergo physiological hypertrophy makes screening a more difficult task. Other defects of the heart muscle such as dilated cardiomyopathy may also cause sudden cardiac death.

Post-mortem examination of the heart may be unremarkable if the underlying event leading to death is an arrhythmia as the cause is not structural. An example of this is the long QT syndrome in which defective ion channels in the myocardial cell membrane cause abnormal prolongation of the repolarization interval (beginning of QRS complex to end of T wave on the ECG). Individuals are at considerable risk of sudden cardiac death due to a particular form of ventricular fibrillation known as ‘torsades de pointes’. If recorded on ECG monitoring the axis of ventricular activity is shown to rotate about the heart. The condition was first described in 1957, and some of the genes responsible for long QT variants have since been identified and include genes encoding for the proteins which form sodium and potassium channels. Typical histories include loss of consciousness with exertion, particularly swimming, possibly due to the effect of diving or exercising in cold water or cardiac arrest when startled or woken suddenly from sleep.

Medical management of these conditions involves screening families for asymptomatic but affected individuals using ECG, echocardiography and exercise testing. Significant reductions in risk may be achieved using β-blockade pointing to the adrenergically mediated initiation of the arrhythmia in these conditions. Standard advice is to refrain from vigorous exercise. For refractory or high-risk individuals, implantable cardioversion defibrillators can be used. These devices are developments of the pacemaker technology designed to detect severe arrhythmias and deliver a cardioverting DC electric shock to the myocardium. They have recently entered the National Institute for Health and Clinical Excellence Guidelines in the UK. Unfortunately though effectively lifesaving the pain and discomfort of automatic defibrillation can be extremely debilitating and may lead to morbidity of its own, particularly if the patient does not lose consciousness prior to the delivery of the life-saving shock.

Genetic testing remains a challenge in these conditions. The variability in functional molecular phenotypes means that selecting dysfunctional genotypes from the spectrum of normal variants is difficult, particularly where dysfunctional genotypes may be extremely rare. Where a number of family members are known to be affected, genetic linkage analysis may be helpful. Unfortunately the majority of cases involve a single individual within one family who has died suddenly. Screening children is particularly difficult since lack of evidence of the condition may not exclude its development in the longer term, for example with hypertrophic cardiomyopathy.

Further reading