Heart - Heart Wall

Question 1

What are the three layers of the heart?

Answer 1

1) Endocardium
2) Myocardium
3) Epicardium

Question 2

Endocardium?

Answer 2

The innermost layer. It lines the cavities and valves of the heart.

It is composed of loose connective tissue and simple squamous epithelial tissue - similar to the composition of the lining of the blood vessels.

In addition to lining the inside of the heart, the endocardium also regulated contractions and aids cardiac embyological development.

Question 3

Clinical relevance - endocarditis?`

Answer 3

This refers to inflammation of the endocardium. It most commonly occurs on the valves of the heart, which the endocardium lines.

The main form of endocarditis is infective endocarditis - caused by a pathogen. Bacteria colonise the heart valve, and cause clumps of material called vegetations to develop. The resulting inflammation can cause permanent damage to the valve, creating a murmur which is heard when the patient is examined. Furthermore, the damaged valve is more likely to be colonised in the future, resulting in re-infection.

Question 4

Subendocardial layer?

Answer 4

The subendocardial layer lies between, and joins, the endocardium and the myocardium. It consists of loose fibrous tissue, containing the vessels and nerves of the conducting system of the heart. The purkinje fibres are located in this layer.

As the subendocardial layer houses the conducting system of the heart, damage to this layer can result in various arrhythmias.

Question 6

Myocardium?

Answer 6

It is composed of cardiac muscle and is an involuntary striated muscle. It is responsible for contractions of the heart.

Question 7

Clinical relevance - disorders of the myocardium?

Answer 7

Myocarditis

Myocarditis refers to an inflammation of the heart muscle, often due to viruses such as the adenovirus and coxsackie B. Symptoms depend on the severity of the inflammation, but often include chest pain, shortness of breath, and tachycardia.

The common sequelae of myocarditis is damage to the cardiac muscle of the myocardium. This can result in cardiac arrhythmias and heart failure.

MI

A myocardial infarction is caused by a blockage of a coronary artery. The myocardium loses it oxygen supply, and undergoes ischaemic change.

There are two main types of MI:
1) NSTEMI (non S-T elevated myocardial infarction) - the coronary artery is only partially blocked. Ischaemic damage to partial thickness of the myocardium.

2) STEMI - where the coronary artery is completely blocked. Ischaemic damage to the full thickness of the myocardium.

The most common cause of an MI is an atheroma (lipid collection in the artery walls. Risk factors for MI include obesity, high BP, smoking and diabetes).

Angina

Refers to a chest pain which arises as a result of temporary myocardial ischaemia. In this condition, the coronary arteries narrow by are not completely blocked, in the overwhelming majority as a result of atherosclerosis. The reduced blood flow causes intermittent ischaemia when oxygen demand exceeds supply.

There are two types of angina:

1) Stable angina - can be predicted, with symptoms of pain developing after exercise or under stress.
2) Unstable angina - does not require exertion to set off symptoms.

Both these conditions can be treated with GTN spray. Unstable angina is the more serious of the two, and is more likely to progress to an MI.