ECG Case 038

15-year old patient presenting with rapid palpitations and dizziness. Symptoms recur in ED. Describe the ECG.


Describe and interpret this ECG


Main Abnormalities

  • Very rapid, regular broad-complex tachycardia (~ 200 bpm).
  • LBBB morphology (dominant S wave in V1).
  • No clear atrial activity — no flutter waves or fibrillatory waves.
  • No obvious diagnostic features for VT

Differential Diagnosis

In a patient presenting with a regular broad-complex tachycardia and no evidence of atrial activity, the main diagnostic considerations are:

Although diagnostic criteria exist to aid in differentiation of these rhythms, none of them have 100% sensitivity or specificity — leading many authors to recommend treating as VT if uncertain.

However, clinical context is everything…

This patient has two strong indicators of SVT with aberrancy:

  • Young age — the vast majority of BCTs in children are SVT with aberrancy.
  • Evidence of WPW on previous ECGs.


This is an example of antidromic AV reentry tachycardia — a reentrant SVT seen in WPW where the impulse travels from atria to ventricles via the accessory pathway, recycling backwards through the AV node (hence “antidromic”).

Activation of the ventricles via the accessory pathway produces a broad complex that may be indistinguishable from VT. This is in comparison to orthodromic SVT, where the impulse travels forwards through the AV node producing a normal-looking, narrow QRS.

This patient reverted back to sinus rhythm with vagal maneouvres. The WPW pattern was once again visible on his sinus rhythm ECG.

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Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |


  1. Hi

    To be more precise, it’s better to describe the differences in differentials:
    – SVT with aberrancy due to BBB.
    – SVT with accessory pathway to do e.g. WPW.

  2. ECG features:
    1. During sinus rhythm, QRS is normal or displays different degrees of manifest pre-excitation with LBBB morphology.
    2. During tachycardia, antidromic AVRT over a Mahaim fiber has a LBBB morphology, various QRS patterns and axis.

    Mahaim syndrome is also possible.

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