A combination of the Q wave, R wave and S wave, the “QRS complex” represents ventricular depolarization. This term can be confusing, as not all ECG leads contain all three of these waves; yet a “QRS complex” is said to be present regardless.
For example, the normal QRS complex in lead V1 does not contain a Q wave — only a R wave and S wave — but the combination of the R wave and S wave is still referred to as the QRS complex for this lead.
The normal duration (interval) of the QRS complex is between 0.08 and 0.10 seconds — that is, 80 and 100 milliseconds. When the duration is between 0.10 and 0.12 seconds, it is intermediate or slightly prolonged. A QRS duration of greater than 0.12 seconds is considered abnormal.
The QRS duration will lengthen when electrical activity takes a long time to travel throughout the ventricular myocardium. The normal conduction system in the ventricles is called the His-Purkinje system and consists of cells that can conduct electricity quite rapidly. Thus, normal conduction of an electrical impulse through the AV node, then to the ventricles via the His-Purkinje system, is fast and results in a normal QRS duration. When electrical activity does not conduct through the His-Purkinje system, but instead travels from myocyte to myocyte, a longer time is necessary, and the QRS duration is widened.
A widened QRS duration occurs in the setting of a right bundle branch block, left bundle branch block, non-specific intraventricular conduction delay and during ventricular arrhythmias such as ventricular tachycardia — all of which are discussed in detail inside their respective sections in ECG Reviews and Criteria.