ECG Basics · Lesson 14 · Putting it together
Narrow-complex tachycardia
A fast narrow QRS describes ventricular activation, not the whole rhythm. Start with regularity, then look for atrial activity.
Three observations before a name
In adults, the AHA algorithm uses QRS ≥ 120 ms as the wide-complex boundary. For a narrower fast rhythm, describe the RR spacing, the atrial activity and their relationship. The patient's clinical condition matters; a rate on its own does not establish the cause.
Regular examples: sinus tachycardia and PSVT
Compare these model examples. Sinus tachycardia retains a sinus P before each conducted QRS. In the AV-dependent re-entry example, atrial activity sits close to the QRS and is harder to separate. A single strip does not identify every SVT mechanism.
AF and flutter: look between the complexes
AF has no repeating discrete P waves and usually has irregular RR intervals. AV block or pacing can make its ventricular rhythm regular. Flutter has repetitive atrial activity; fixed conduction can give regular QRS spacing. In this example, two atrial cycles produce one ventricular beat. A ventricular rate near 150/min is a clue to 2:1 flutter, not a diagnosis.
Try a comparison
Open each rhythm below. Keep speed and gain the same. Compare RR spacing and atrial activity; then add RBBB to see why a supraventricular rhythm can also have a wide QRS.
References
- American Heart Association. Adult tachyarrhythmia with a pulse algorithm, 2025. QRS-width boundary and clinical assessment.
- Page RL, et al. 2015 ACC/AHA/HRS guideline for adult supraventricular tachycardia. Rhythm definitions and atrial flutter patterns.
- ACC/AHA/ESC atrial fibrillation guideline, 2006, §II. ECG definition and the effect of AV conduction on ventricular regularity; used here for pattern description, not treatment recommendations.
SIMULATED · NOT FOR DIAGNOSIS · TracingsMD ECG Basics