What is the typical QRS complex duration in supraventricular tachycardia?

QRS Duration in Supraventricular Tachycardia

In supraventricular tachycardia (SVT), the QRS complex duration is typically narrow, defined as less than 120 milliseconds. 1

Narrow QRS Complex: The Hallmark of SVT

• When the QRS duration is less than 120 ms, the tachycardia is almost always supraventricular in origin. 1

• This narrow QRS complex reflects normal ventricular activation through the His-Purkinje system, distinguishing SVT from ventricular tachycardia in most cases. 2

• The narrow complex definition applies to all major SVT subtypes including AVNRT (atrioventricular nodal reentrant tachycardia), AVRT (atrioventricular reciprocating tachycardia), and atrial tachycardia. 1

Important Exceptions: Wide QRS Complex in SVT

SVT can present with a wide QRS complex (≥120 ms) in specific circumstances, which creates diagnostic challenges:

Three Mechanisms for Wide QRS in SVT:

1. Pre-existing or rate-related bundle branch block (BBB) 1

   • Bundle branch block may exist prior to tachycardia or develop during SVT when one bundle branch becomes refractory due to the rapid rate
   • Most rate-related BBB occurs due to long-short sequence initiation
   • QRS width >140 ms with RBBB pattern or >160 ms with LBBB pattern favors ventricular tachycardia over SVT with aberrancy 1

2. Anterograde conduction over an accessory pathway (pre-excitation) 1

   • Occurs during atrial tachycardia, atrial flutter, atrial fibrillation, AVNRT, or antidromic AVRT
   • In pre-excited tachycardias, the QRS is generally wider (more pre-excited) compared to sinus rhythm 1

3. Antiarrhythmic drug effects 1

   • Class Ic or Class Ia antiarrhythmic drugs can widen the QRS complex during SVT
   • QRS width criteria become less helpful for differentiating SVT from VT in patients taking these medications 1

Clinical Implications

• The critical diagnostic threshold is 120 ms: narrow QRS (<120 ms) strongly suggests SVT, while wide QRS (≥120 ms) requires careful differentiation from ventricular tachycardia. 1

• When encountering wide QRS tachycardia and the diagnosis cannot be definitively established, the patient should be treated as having ventricular tachycardia due to the potentially life-threatening consequences of misdiagnosis. 1

• Intravenous medications for SVT (particularly verapamil or diltiazem) may precipitate hemodynamic collapse if administered to a patient with ventricular tachycardia. 1