Normal ventricular activation and QRS-complex duration
The QRS-complex represents the electrocardiographic representation of the progress of ventricular depolarization. The free wall of the left ventricle is thicker than that of the right ventricle in adults; the amplitude of the electrical potentials generated by the activation of a muscle wall is proportional to its thickness: the potentials originating from the left ventricle are thus predominant.
Ventricular activation begins at the mid portion of the left side of the interventricular septum. The right branch which descends along the right side of the septum is long and ends near the apex of the right ventricle; the left branch on the opposite side is shorter and ends in the mid portion of the septum which explains this zone being activated first. Activation then reaches the entire depth of the septum from left to right and back to front. This primary activation is represented by a vector directed forward and to the right. The initial septal activity yields a first positive deflection (septal R wave of activation in V1) relative to the right ventricle and an initial negative deflection (q wave of septal activation in V6) relative to the left ventricle.
Endocardial activation of the ventricular free walls is rapid owing to the Purkinje network and is achieved at an average velocity of approximately 4 meters/second such that it ends before the completion of septal activation. Activation of the ventricular free walls occurs from the endocardium to the epicardium. A large amplitude R wave in the left precordial leads reflects the activation of the left ventricular free wall. Activation of the right ventricular free wall is usually not visible due to the electrical and anatomical predominance of the left ventricle.
Activation of the free walls is followed by basal activation and activation of the more posterior regions of both ventricles and of the interventricular septum; termination of this activation gives rise to a low-amplitude wave at the end of the QRS-complex: for the left leads, which record the 3 terminal vectors, the wave is always negative (final S wave indicating basal activation); for the right leads, activation of the pulmonary infundibulum is reflected by a positive wave of low amplitude.
The duration of the QRS-complex represents the total duration of ventricular depolarization and is measured from the start of the Q wave (or the R wave) to the end of the S wave at the transition with the isoelectric line. The first and last ventricular activation vectors are more or less perpendicular to the frontal plane which explains why the QRS duration appears wider in precordial leads (particularly in V1-V3) than in limb leads. The normal QRS duration is between 60 and 100 ms with no influence of heart rate and displays very little variation with age from adolescence onwards.