Every muscle can perform only one movement at a time, namely, the shortening of its fibres by contraction. This also applies to the heart muscle. Each action of a muscle is associated with electric currents which change in the course of the contraction and which, after passing through the various tissues, reach the surface of the body. The record thus obtained on a chart is called an electrocardiogram. In its original form the instrument was based on the principle of the string galvanometer, which was founded in 1903 by Einthoven, a Dutch physiologist (Fig.1).

Alexander Muirhead attached wires to a feverish patient's wrist to obtain a record of the patient's heartbeat while studying for his Doctor of Science (in electricity) in 1872 at St Bartholomew's Hospital. This activity was directly recorded and visualized using a Lippmann capillary electrometer by the British physiologist John Burdon Sanderson. His electrocardiograph machine consisted of a Lippmann capillary electrometer fixed to a projector. This allowed a heartbeat to be recorded in real time. In 1911 he still saw little clinical application for his work.

The electric currents produced by the contraction of the heart muscle and picked up by the electrodes applied to the patient’s body are passed through this filament, which undergoes a deflection whose direction and magnitude will depend on the direction and strength of the current. This movement of the filament is projected, by means of an optical system, as a spot of light on to a moving paper strip chart which is coated with a light-sensitive compound.

When no current is flowing thorough the filament, a straight line is traced on the chart. Currents associated with the muscular action of the heart cause the spot of light to oscillate and thus trace a typical curve on the light-sensitive chart. Any irregularities in the functioning of the heart appear as corresponding irregularities in the curve which enable the heart specialist to diagnose the disease or other cause of these deviations from the normal pattern.

The string galvanometer instrument has now been superseded by the electronic cardiograph which operated with amplifier tubes (Fig.2). The currents from the body electrodes enter the amplifier at E1 and E2. The greatly amplified currents are fed to a mirror galvanometer. The movements of the mirror cause a reflected spot of light to oscillate on the light-sensitive chart(film) and thus trace the required curve.

The electrodes are affixed to the human body at certain definite points (Fig.3): left arm and right arm (a), the left leg and right arm (b), left leg and left arm. These were the points originally selected by Einthoven. Nowdays other points of attachment to the limbs and also to the wall of the chest are used.

Modern cardiograph records do not record these various curves separately, in successive operations, but record them simultaneously. The normal electrocardiogram presents the appearance shown in Fig.4. The portion of the curve between P and Q corresponds to the contraction of the articles, while the portion between Q and T corresponds to the contraction of the ventricles of the heart.