ECG findings in re-entrant tachycardias

The rhythm is regular and the rate is between 130 and 250 beats per minute.

The QRS complex is narrow unless:

• there is rate-related bundle branch block
• there is pre-existing bundle branch block
• anterograde conduction occurs down an accessory pathway which is distinct from the AV node

P waves:

• are inverted
• in AV-nodal re-entry tachycardia may be masked by the QRS complex
• in AV re-entry tachycardia occur half-way between the QRS complexes
• there is one P wave for each QRS complex

Reference

1. Kotadia ID, Williams SE, O'Neill M; Supraventricular tachycardia: An overview of diagnosis and management. Clin Med (Lond). 2020 Jan;20(1):43-47.