Atherosclerosis (also called atheroma) is an inflammatory, degenerative disease of large and medium-sized arterial vessels. It is characterised by the development of fibrolipid plaques within the intima of the vessel wall. In smaller arteries, including the coronary vessels that supply the myocardium, these plaques can cause severe narrowing (stenosis) of the lumen, with significant impairment of blood flow. The clinical consequences depend on the speed of development of the stenosis, and on whether the affected tissue has any additional source of blood supply (known as collateral supply). In large arteries, such as the abdominal aorta, the inflammatory atherosclerotic process also damages the muscular wall causing weakness and dilatation. This is known as aneurysm formation (see Box 17). As the aneurysm enlarges there is an increasing risk of rupture, with resultant catastrophic haemorrhage.

Four major risk factors are recognised for atherosclerosis:

Minor risk factors include increasing age, male sex, obesity, family history and stress.

Atheromatous plaques are probably initiated by injury to the endothelial cells of the arterial intima. An inflammatory response is evoked to the damage, which results in an infiltrate of macrophages, and proliferation of smooth muscle cells from the media of the vessel wall. These smooth muscle cells migrate into the intima and begin to produce collagen. Both the macrophages and the smooth muscle cells may accumulate lipid within their cytoplasm, giving a vacuolated appearance on light microscopy (‘foam cells’). Free cholesterol and necrotic inflammatory debris also become incorporated within the plaque lesion.

Figure 33 shows a normal muscular artery and a typical atheromatous plaque with a fibrous tissue cap and a lipid-rich, necrotic centre. It is important to realise that the actual composition of individual plaques varies, and can change over time. Plaques that are particularly rich in lipids may be more unstable and susceptible to rupture or haemorrhage. Older plaques can become heavily calcified. The plaque surface is prone to develop superimposed thrombosis, which can rapidly increase the severity of the obstruction to blood flow. At autopsy, it is common to find a recent thrombus complicating an atheromatous coronary artery plaque in patients who died after acute myocardial infarction.

Aortic dissection is also called dissecting aneurysm, although this is an inaccurate term as the aorta is not significantly dilated. In aortic dissection, blood tracks into the muscular wall of the blood vessel. The entry point of blood into the aortic media is through an intimal tear, usually within the proximal 10cm of the ascending aorta. Occasionally, there is a second distal luminal tear, through which blood re-enters the circulation, producing a ‘double-barrelled’ aorta. More commonly, however, the haemorrhage extends outwards with vessel rupture and catastrophic extramural haemorrhage into:

• the pericardium, causing cardiac tamponade

• the mediastinum, causing haemothorax

• the abdominal cavity (haematoperitoneum).

The dissection can also involve the great vessels of the neck, compromising cerebral blood flow. Dissection of the coronary arteries is a rare cause of acute myocardial ischaemia.

There is a strong association with systemic hypertension and with Marfan’s syndrome.

Ischaemia is due to lack of oxygen, and in the overwhelming majority of ischaemic heart disease (IHD) cases this is due to reduced blood flow through coronary arteries narrowed or occluded by atherosclerosis. However, the left ventricular myocardium is also at risk of ischaemia when there is pathological hypertrophy, for example in systemic hypertension or aortic valve stenosis. Under these conditions, cardiac perfusion may be insufficient to meet the metabolic needs of the increased muscle mass. Rarely, cardiac ischaemia can result from decreased oxygen-carrying capacity of the blood in severe anaemia (even though the coronary arteries may be normal).

Chronically raised systemic blood pressure is a major cause of morbidity and mortality. Hypertension can cause, or significantly contribute to:

‘Normal’ blood pressure varies within a population and with age, but evidence suggests that a sustained pressure of 140/90mmHg or greater is associated with increased risk of disease. Persistent diastolic blood pressure in excess of 100mmHg requires treatment. Very high blood pressure (e.g. 240/120mmHg) can result in accelerated disease – ‘malignant hypertension’.

In approximately 10% of cases, systemic hypertension is secondary to another disease (Box 18). The mechanism of primary hypertension is unclear but the following defects have been found in patients:

Family history, obesity and excessive alcohol intake are further associated factors. The aetiology of essential hypertension is complex and clearly involves a combination of genetic and environmental factors. Modest, but clinically significant, reductions in blood pressure may be achieved by weight loss, regular physical exercise, moderation of alcohol intake and possibly by reduction of salt in the diet.

Cardiomyopathy is defined as heart muscle disease not caused by ischaemic, hypertensive, valvular or congenital heart disease. Although uncommon, cardiomyopathies are an important cause of cardiac failure and sudden death in young adults. The aetiology of specific cardiomyopathies is shown in Box 19. Ninety per cent are of the dilated type.

Myocarditis is inflammation of the heart muscle. It may be asymptomatic, or cause:

Acute myocarditis may be suggested at autopsy by a pale, flabby heart, but histological examination is necessary for diagnosis. Microscopically there is multifocal myocardial chronic inflammation associated with death of the cardiac muscle cells. The aetiology of myocarditis is shown in Box 20.