Tachycardias are broadly categorized based upon the width of the QRS complex on the electrocardiogram (ECG).
- A narrow QRS complex (<120 msec) reflects rapid activation of the ventricles via the normal His-Purkinje system, which in turn suggests that the arrhythmia originates above or within the atrioventricular (AV) node (ie, a supraventricular tachycardia).
- A widened QRS (≥120 msec) occurs when ventricular activation is abnormally slow, most commonly because the arrhythmia originates outside of the normal conduction system (eg, ventricular tachycardia), or because of abnormalities within the His-Purkinje system (eg, supraventricular tachycardia with aberrancy). Much less common are pre-excited tachycardias; these are supraventricular tachycardias with antegrade conduction over an accessory pathway into the ventricular. This only occurs in a minority of patients with pre-excitations syndromes (Wolff-Parkinson-White Syndrome).
A wide complex tachycardia (WCT) represents a unique clinical challenge for two reasons:
- Diagnosing the arrhythmia is difficult – Although most WCTs are due to ventricular tachycardia (VT), the differential diagnosis includes a variety of supraventricular tachycardias (SVTs). Diagnostic algorithms are complex and imperfect.
- Urgent therapy is often required – Patients may be unstable at the onset of the arrhythmia or deteriorate rapidly at any time. Therapeutic decisions are further complicated by the risks associated with giving therapy for an SVT to a patient who actually has VT [1-4]. (See 'Pharmacologic interventions' below and 'Uncertain diagnosis' below.)
Diagnostic uncertainty — There is no single criterion or combination of criteria that provides complete diagnostic accuracy in evaluating a WCT. Algorithms that perform well in initial reports, with selected populations and experienced ECG analysts, are likely to be less accurate in general practice. (See 'Algorithm performance' below.)