electrocardiogram

electrocardiogram This means of studying the activity of the heart from electrical signals detectable from the body surface stemmed directly, early in the twentieth century, from the invention of the string galvanometer by the Dutch physiologist, Einthoven. Electrocardiography was demonstrated to the Royal Society in London in 1909.

The ‘ECG’ (or sometimes still ‘EKG’ in the US, from the German spelling) has become an icon representing the heart's activity. The waveform is the most familiar ‘high tech’ sign of the electrical behaviour of the heart. In various versions, its characteristic shape (see figure) reporting a healthy rhythm, or the flat line suggesting the patient's demise, is familiar to any viewer of television medical soap operas. A clever variation on the theme forms the distinctive logo for the British Heart Foundation, the largest UK charity dedicated to funding cardiovascular research.

The electrocardiogram (as a paper trace or a TV monitor display) shows the changes in the voltage, detectable during the time course of the heart beat, between pairs of electrodes placed at certain points on the skin. The basis of the ECG is that the heart, like other muscles, is triggered to contract by electrical activity. The heart is a relatively large piece of tissue, so the flow of electrical current associated with (and immediately preceding) contraction produces detectable voltages (typically a few millivolts) on the surface of the body. Electrode pairs can be placed at various positions on the body to yield information about the status of the heart. The classic ‘limb leads’ are attached to one leg and two arms; other pairings are placed at defined positions on the chest itself. Even more detail can be obtained with leads inserted in the oesophagus (the gullet) or even from within the heart itself (with the electrode introduced via a vein). Abnormal enlargement (hypertrophy) of the heart's various chambers produces characteristic distortions of the ‘ideal’ ECG form which are readily interpreted by experienced users.

The spread of the electrical wave across the heart varies in speed (see heart). Simple physics dictates that where a change in potential of a large fraction of the heart occurs in a relatively brief time, the resulting ECG wave is large too. When most of the heart is at a similar potential, no voltage difference will appear at the surface. Thus, prominent waves in the ECG indicate the synchronized start (or finish) of activity in significant fractions of the heart.

These potentials amount to one or two milli-volts. The impulse of electrical activity causing the heart to contract in a co-ordinated manner progresses through the heart in a complex three-dimensional pattern. The appearance of the electrocardiogram, therefore, varies from person to person as heart shape and position can be significantly different even in entirely normal individuals. Any person's pattern further alters with the location of the recording electrodes. Nevertheless, there are significant, consistently observed deflections and intervals in a typical electrocardiogram; the main ‘peaks’ are labelled as P, QRS, and T (see figure).

The most basic feature of the ECG is that the time from any one such ‘peak’ to the same one in the next cycle indicates precisely how long the heart cycle is taking. At slow rates, the timing of the waves can be easily correlated to the heart sounds heard with the stethoscope. But far more precise information can be gleaned once the relationship of the waves to the phases of the cycle is understood:

The P-wave

indicates the electrical activity associated with contraction of the cardiac atria, the heart's upper chambers.

The P–R interval

is the delay between the beginning of activity in the atria and the ventricles (atrio–ventricular conduction time). In adults, normal P–R intervals range between 120 and 200 milliseconds, occasionally being shorter in children and slightly longer in the aged. The P–R interval shortens at high heart rates (e.g. due to exercise or to fever) and increases at lower heart rates (e.g. during sleep).

The QRS complex

indicates the onset of contraction of the ventricles. The shape of the QRS complex may be modified by a number of physiological factors (e.g. body position and breathing pattern). In normal adults, the duration of the QRS complex varies between 60 and 100 milliseconds; in children it tends to be shorter.

The Q–T interval

is measured from the beginning of the QRS complex to the end of the T-wave and represents the time between activation of electrical activity in the ventricles and their return to the resting state. Like the P–R interval, the Q–T interval shortens at high heart rates and increases at lower rates.

The T-wave

indicates when the electrical activity associated with the cells in the cardiac ventricle returns to the resting state after electrical activation. Thus, it signals the start of relaxation of the ventricle walls. It tends to be longer lasting than QRS because the onset of relaxation across the ventricle is less tightly synchronized than that of contraction.

Some stark deviations from this classical sequence can occur, including the chaotic waves associated with ventricular fibrillation. This is the uncoordinated, apparently random electrical activity (and thus contraction) of the ventricles that can readily prove fatal without defibrillation. Heart block is a condition readily identified by ECG analysis.

There are characteristic changes in the wave pattern of the ECG in myocardial ischaemia (inadequate blood supply to the heart), which may be evident at first during exercise in sufferers from angina, and which may confirm or exclude an ischaemic episode or myocardial infarction in instances of unexplained chest pain.

David J. Miller, and Niall G. MacFarlane

See also heart; heart attack.