The electrical activity detected by the electrocardiogram machine is measured in millivolts. Throughout this article the duration of waveforms will be expressed as0.04 s = 1 mm = 1 small square Each large square is five small squares in width, and each small square is 1 mm wide and equivalent to 0.04 s.
The paper is divided into large squares, each measuring 5 mm wide and equivalent to 0.2 s. The electrocardiogram is recorded on to standard paper travelling at a rate of 25 mm/s. Thus, after delay in the atrioventricular mode, atrial contraction is followed by rapid and coordinated contraction of the ventricles. The impulse is propagated to the ventricles by the atrioventricular node and spreads in a coordinated fashion throughout the ventricles via the specialised conducting tissue of the His-Purkinje system. The sinoatrial node acts as a natural pacemaker and initiates atrial depolarisation. These electrical changes are recorded via electrodes placed on the limbs and chest wall and are transcribed on to graph paper to produce an electrocardiogram (commonly known as an ECG). The contraction and relaxation of cardiac muscle results from the depolarisation and repolarisation of myocardial cells. Familiarity with the wide range of patterns seen in the electrocardiograms of normal subjects and an understanding of the effects of non-cardiac disorders on the trace are prerequisites to accurate interpretation. It is an essential tool for investigating cardiac arrhythmias and is also useful in diagnosing cardiac disorders such as myocardial infarction.
Electrocardiography is a fundamental part of cardiovascular assessment.