The “QRS complex” is the combination of the Q wave, R wave and S wave and represents ventricular depolarization. This term can be confusing since 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, but only a R wave and S wave, yet 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 0.80 and 0.10 seconds (80 and 100 ms). 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 causing 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 in the ECG criteria review sections.