HEALTH / Common Procedures: Electrocardiography

'HAS HE had a heart attack?' The answer to this anxious question from relatives of someone pale and sweating and gripped by chest pain is almost certainly 'Yes'. But the final verdict usually comes from the paper trace of an electrocardiogram or ECG.

The test was invented and introduced by the Dutch scientist Willem Einthoven, who won the Nobel Prize for medicine in 1924. He discovered that the electrical changes that drive the muscular contractions of the human heart can be detected by attaching metal plates to the arms, legs and chest, and connecting these to a sensitive electric meter. The instrument records our own natural electricity; it does not pass any sort of electric current into the person being examined.

The changes in voltage with each heartbeat produce a complex wave with peaks and troughs called the P wave, the QRS wave, and the T wave. These may be recorded on a paper strip or displayed on a miniature television-type screen.

Interpretation of an ECG tracing is often straightforward but sometimes specialist skills may be required. The trace shows the rate and the rhythm of the heartbeat, including sometimes quite subtle changes in the regularity of the beat; it also gives information about the relative size of the different chambers, and the health of the muscle in each of them.

The most common heart problem in Britain is coronary heart disease, in which the coronary arteries which supply the heart with blood become narrowed or blocked by a combination of cholesterol and blood clots. When the heart muscle is suddenly starved of blood, pain develops in the chest - a heart attack - and more gradual starvation may lead to chest pain on exertion (angina). There are many other causes of chest pain, and an ECG is a very reliable test for distinguishing pain originating in heart muscle from other causes, such as inflammation of the gullet. Sometimes the diagnostic abnormalities may appear on the ECG only if it is recorded during exercise - usually walking on a moving track in a hospital laboratory.

A second valuable type of information given by an ECG is the regularity of the heartbeat. Someone who has been having dizzy turns or blackouts will have an ECG done early in the investigation of their symptoms. Sometimes he or she may be asked to wear a tape recorder attached to a miniature ECG machine to record the waves for a continuous period of 24 hours. This Holter monitoring should be able to show whether there is any intermittent fault in heart rhythm.

Very few medical tests are infallible, and the ECG may sometimes be misleading. In particular, someone who has had a heart attack due to recent blocking of a coronary artery may have an ECG which looks reasonably normal - though abnormalities will usually appear within 24 hours or so. Similarly, there cannot be 100 per cent certainty when someone is reassured that a normal ECG reading means that their pain in the chest is not angina.

But the ECG gives the right answer virtually every time; it is cheap, absolutely safe, and though many new and ingenious ways have been introduced for assessing the health of the heart, this is still the best understood by most doctors.