The voltage of the QRS-complexes is expressed in mV, while the amplitude is measured in mm using the horizontal lines. The amplitude of the QRS-complexes is extremely variable depending on the leads and according to numerous factors. Indeed, many parameters disrupt the relationship between the thickness of the myocardium and the amplitude of the QRS-complexes: individual factors such as age, gender or body morphology but also position of the heart in the chest as well as variable conduction properties of pericardial structures depending on patients.
If there is a q wave, one measures its depth which should not physiologically exceed 2 mm in lead I, 3 mm in leads II, III, V5, V6; it should remain below 25% of the ensuing R wave.
The amplitude of the R wave is usually maximal in leads II, V5 or V6. A value of 14 mm in lead I, 13 mm in aVL, 20 mm in aVF, 5 mm in V1 and 25 mm in V6 can be used as the upper limit of normal. The amplitude of the S wave should not exceed 20 mm in V1 and 25 mm in V2. The R/S ratio must remain below 1 in V1 and above 1 in V5.
Measuring the amplitude of the QRS-complexes as well as various indices (Lewis, Sokolow) is an integral component of the systematic analysis of an ECG since it allows the diagnosis of ventricular hypertrophy. Both ventricles are of unequal size, weight and volume; indeed, the left ventricle has a greater wall thickness, a larger epicardial surface and a higher weight. The predominance of the left ventricle has 2 important electrophysiological consequences:
- it is common to observe a moderate widening of QRS duration in the case of left ventricular hypertrophy; but a right ventricular hypertrophy rarely leads to a widening of the QRS since a considerable increase in right ventricular mass is needed for its activation time to exceed that of the left ventricle;
- left ventricular activation vectors outweigh those from the right ventricle. Left ventricular hypertrophy merely accentuates this predominance; a right ventricular hypertrophy must conversely be extensive in order to be reflected on the ECG;
Various electrocardiographic abnormalities can be observed during ventricular hypertrophy:
- the prolongation of ventricular depolarization is highlighted by the delayed recording of the intrinsicoid deflection in leads near the hypertrophied ventricle (V1, V2 for right ventricular hypertrophy, V5, V6 for left ventricular hypertrophy);
- the redirection of the principal depolarization vectors is significant in right ventricular hypertrophy which attracts vectors to the right and front; since the forces are normally essentially leftward, left ventricular hypertrophy has very little effect on their direction;
- the increase in amplitude of QRS-complex voltages is a function of the thickening of the hypertrophied ventricular mass with a significant voltage of the R wave in the precordial leads opposite the ventricle of interest (V1, V2 for right ventricular hypertrophy, V5, V6 for left ventricular hypertrophy);
- the changes in repolarization consist in a displacement of the T-wave vector opposite that of the QRS. This opposition is due to a reversed direction of repolarization within the hypertrophied ventricle.