How can a Belhassen (verapamil‑sensitive idiopathic left‑ventricular fascicular) tachycardia be distinguished from supraventricular tachycardia?

Distinguishing Belhassen Tachycardia from SVT

Belhassen tachycardia (verapamil-sensitive idiopathic left ventricular fascicular tachycardia) is distinguished from SVT by its characteristic ECG pattern of right bundle branch block morphology with superior axis deviation, relatively narrow QRS (100-140ms), and specific response to verapamil rather than adenosine. 1

Key ECG Features to Differentiate

Belhassen Tachycardia Characteristics:

• QRS morphology: Right bundle branch block (RBBB) pattern with superior axis (most common form) 1
• QRS width: Relatively narrow (100-140ms), which can mislead clinicians into thinking it's SVT 2, 3
• Rate: Typically sustained tachycardia, often 160-220 bpm 4, 5
• Axis: Superior axis is classic; less commonly inferior axis with alternate reentry paths 1

SVT Characteristics:

• QRS morphology: Narrow QRS (<120ms) unless aberrant conduction present
• No consistent RBBB pattern with superior axis
• Responds to adenosine (unlike Belhassen)

Critical Diagnostic Algorithm

Step 1: Evaluate QRS morphology and axis

• If RBBB + superior axis → suspect Belhassen tachycardia
• If narrow QRS without RBBB pattern → more likely SVT

Step 2: Trial of adenosine

• Adenosine fails in Belhassen but works in most SVT 4, 6, 3
• Failure of adenosine in a young patient with RBBB/superior axis pattern should immediately raise suspicion for fascicular VT 4

Step 3: Response to verapamil

• Verapamil terminates Belhassen tachycardia (acts on verapamil-sensitive slow conduction zone in the reentry circuit) 1, 7
• Verapamil may also work in some SVT but is less specific

Mechanism Distinguishes the Two

Belhassen tachycardia involves reentry through the left ventricular Purkinje system (typically left posterior fascicle as retrograde limb, with verapamil-sensitive tissue as anterograde limb), not the AV node like typical SVT 1. This explains why:

• Adenosine (AV nodal blocker) fails 4, 6
• Verapamil succeeds (blocks the slow conduction zone in the fascicular circuit) 1, 7

Common Pitfalls to Avoid

1. Misdiagnosing as SVT with aberrancy: The relatively narrow QRS (compared to typical VT) frequently causes this error 6, 2, 3. The RBBB + superior axis pattern is the key distinguishing feature.

2. Assuming all wide-complex tachycardias respond to standard VT protocols: Belhassen is unique among VTs in being verapamil-sensitive rather than requiring amiodarone or cardioversion 1, 6

3. Using verapamil incorrectly: In infants, verapamil must be given as slow infusion over 10-30 minutes, not rapid push, to avoid cardiovascular collapse 4. The FDA label contraindication for verapamil in wide-complex VT 7 does NOT apply to Belhassen tachycardia specifically, which is the exception to this rule 1, 4.

4. Missing the diagnosis in young patients: Belhassen typically affects young patients without structural heart disease 1, 8, 2. If you see RBBB + superior axis tachycardia in a young person, think Belhassen first.

Clinical Context Matters

• Patient age: Young adults (teens to 30s) favor Belhassen 8, 6, 2
• Structural heart disease: Absent in Belhassen 1, 8
• Presentation: Often palpitations, may be exercise-related 1
• Hemodynamic stability: Usually stable, unlike other VTs 4, 5

Treatment Confirmation

For acute termination: Intravenous verapamil is recommended (Class I, Level B-NR) 1. Successful termination with verapamil essentially confirms the diagnosis retrospectively 1, 6, 5.

Note: Not all fascicular VTs are verapamil-sensitive unless the specific verapamil-sensitive reentry pathway is involved 9, but the classic posterior fascicular form (Belhassen) responds reliably.