SVT with Bundle‑Branch Block (BBB)
A supraventricular tachycardia conducted with a pre‑existing or rate‑dependent bundle‑branch block produces a regular broad‑complex tachycardia. Emergency priorities are rapid assessment of stability and safe initial management while avoiding misclassification (VT versus SVT with BBB).
The working rule in undifferentiated regular broad‑complex tachycardia is: treat as ventricular tachycardia (VT) unless you are confident otherwise (Resuscitation Council UK).
Clinical clues Favors SVT with BBB:
- Younger patient.
- No structural or ischaemic heart disease.
- Known chronic BBB on baseline ECG.
- Prior documentation of the same broad complex morphology with a supraventricular rhythm.
Favors VT:
- Older patient.
- Prior myocardial infarction.
- Left ventricular dysfunction.
- Prior VT or severe cardiac disease.
ECG features - practical points
- No ECG sign is 100% specific; always interpret morphology in the clinical context.
- Features that suggest SVT with BBB:
- QRS widening that resembles the patient’s known LBBB or RBBB pattern.
- QRS usually less extremely wide or bizarre than typical VT.