1. The P wave due to the activation (depolarisation) impulse passing over the atrium;
2. The QRS complex due to the activation impulse in the ventricles;
3. The T wave due to the recovery (repolarisation) impulse in the ventricle;
4. The U wave. This is an additional small deflection that follows the T wave in some cases.

This may be a single deflection, but it is often biphasic or triphasic, consisting of deflections above and below the baseline. Any deflection above the baseline is an R wave. A deflection below the baseline preceding the R wave is a Q wave. A deflection below the baseline following the R wave, is an S wave. When the entire QRS is directed downwards it is referred to as a QS wave. Occasionally there is a second R wave and this is referred to as an R' or a secondary R wave. In any QRS complex the largest deflection is designated by a capital letter and small deflections by lower case letters.

P-R interval is the distance from the beginning of the P wave to the beginning of the QRS. It represents the time the impulse takes to pass from the sino-atrial node to the ventricular myocardium. Normal duration = 0.12 - 0.20 seconds.

QRS duration is the distance from the beginning to the end of the QRS complex. It represents the time taken by the depolarisation wave to pass through the ventricular myocardium. Normal upper limit = 0.12 seconds.

S-T segment is the segment between the end of the QRS and the beginning of the T wave. The end point, that is junction of the segment, often merges smoothly into the proximal limb of the T. Thus there are no figures for the duration of the segment and this measurement is not used in diagnosis. However, the contours of the S-T segment and its level relative to its baseline may be of great importance. The point of junction of QRS complex and S-T segment is known as the J (junction) point.

R-R interval is the distance between two successive R waves and therefore represents the time between successive heart beats.

The ECG is recorded on paper divided into large and small squares of 5 mm and 1 mm respectively. Horizontal measurements represent time. The usual rate of movement of paper in the machine is 2.5 cm per second. Thus the 5 mm graduations represent 0.20 seconds and the 1 mm graduations 0.04 seconds. Vertical measurements represent voltage, and standardisation of the machine is such that 1 cm represents 1 millivolt.

Each large square on the paper represents 0.20 seconds. Thus 300 large squares represent 1 minute. Heart rate per minute can be determined by counting the number of large squares between two QRS complexes (R-R interval) and dividing this figure into 300. Greater accuracy can be obtained by using fractions of squares. When the rhythm is irregular it is necessary to obtain an average R-R interval and divide this into 300. ECG rulers graduated for measurement of heart rates are now widely available.

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