Tachycardias are broadly categorized based upon the width of the QRS complex on the electrocardiogram (ECG) .
●A narrow QRS complex (<120 milliseconds) reflects rapid activation of the ventricles via the normal His-Purkinje system, which in turn suggests that the arrhythmia originates above or within the His bundle (ie, a supraventricular tachycardia). The site of origin may be in the sinus node, the atria, the atrioventricular (AV) node, the His bundle, or some combination of these sites.
●A widened QRS (≥120 milliseconds) occurs when ventricular activation is abnormally slow. The most common reason that a QRS is widened is because the arrhythmia originates below the His bundle in the bundle branches, Purkinje fibers, or ventricular myocardium (eg, ventricular tachycardia). Alternatively, a supraventricular arrhythmia can produce a widened QRS if there are either pre-existing or rate-related abnormalities within the His-Purkinje system (eg, supraventricular tachycardia with aberrancy), or if conduction occurs over an accessory pathway. Thus, wide QRS complex tachycardias may be either supraventricular or ventricular in origin. (See "Approach to the diagnosis and treatment of wide QRS complex tachycardias".)
This topic will provide a broad overview of the different causes of narrow QRS complex tachycardia and an approach to their evaluation and diagnosis. Detailed discussions of specific narrow complex tachycardias (eg, AVNRT, AVRT, and AT) and a broad discussion of wide complex tachycardias are presented separately. (See "Approach to the diagnosis and treatment of wide QRS complex tachycardias".)
Reentry is the most common cause of narrow QRS complex tachycardia. Increased automaticity and triggered activity occur less frequently .