How to differentiate and manage left bundle branch block (LBBB) versus sinus tachycardia?

Differentiating VT from Sinus Tachycardia with LBBB

When confronted with a wide QRS-complex tachycardia showing LBBB morphology, you must assume ventricular tachycardia (VT) until proven otherwise and treat accordingly—this approach prioritizes patient survival over diagnostic precision. 1

Critical First Step: Hemodynamic Assessment

• If the patient is hemodynamically unstable, proceed immediately to DC cardioversion—this is the most effective and rapid means of terminating any wide QRS-complex tachycardia regardless of etiology 1
• Hemodynamic instability includes hypotension, altered mental status, chest pain, or acute heart failure 1

Diagnostic Approach for Stable Patients

Key Historical Features That Favor VT

• History of previous myocardial infarction is the single most important clinical predictor—a positive answer to whether the patient has had a prior MI and whether this wide complex tachycardia occurred after that MI strongly indicates VT 1
• Presence of severe structural heart disease or heart failure increases VT likelihood 1
• Syncope or presyncope with the arrhythmia suggests VT 1

ECG Features to Examine

For sinus tachycardia with LBBB, look for:

• P waves that are positive in leads I, II, and aVF, and negative in aVR 1
• P wave axis between 0 and 90 degrees in the frontal plane 1
• Gradual onset and offset (nonparoxysmal)—this is the key distinguishing feature of sinus tachycardia versus reentrant mechanisms 1
• Normal P wave contour, though amplitude may be increased 1

For VT, look for:

• AV dissociation (atrial activity independent of ventricular activity) 1
• Fusion complexes are diagnostic of VT 1
• Concordance in precordial leads (all positive or all negative deflections) 1
• QRS width >120 ms with specific morphologic criteria 1

Critical Pitfall: When ECG Criteria Become Unreliable

• QRS morphology analysis loses value when a baseline ECG showing conduction delay during sinus rhythm is available for comparison 1
• Antiarrhythmic drugs, hyperkalemia, and severe heart failure reduce the specificity of width and morphological criteria 1

Management Algorithm for Stable Wide Complex Tachycardia

When Diagnosis Remains Uncertain

If you cannot definitively distinguish VT from sinus tachycardia with LBBB despite careful evaluation, treat as VT—this is the guideline-mandated approach 1

Diagnostic Maneuver: Adenosine (Use with Extreme Caution)

• Adenosine should be used with caution when the diagnosis is unclear because it may produce ventricular fibrillation in patients with coronary artery disease 1
• Adenosine can also cause rapid ventricular rates in pre-excited tachycardias with AF 1
• Record a 12-lead ECG during adenosine administration—the response aids diagnosis even if the arrhythmia doesn't terminate 1
• If the rhythm slows or terminates with adenosine, this favors a supraventricular mechanism (sinus tachycardia with LBBB) 1
• Continuation of tachycardia with AV block virtually excludes VT and suggests atrial tachycardia 1

For Confirmed Sinus Tachycardia with LBBB

The primary management is identifying and treating the underlying cause 1:

• Evaluate for fever, hypovolemia, anemia, infection 1
• Review medications: stimulants (caffeine, nicotine), prescribed drugs (salbutamol, aminophylline, atropine, catecholamines), recreational drugs (amphetamines, cocaine, ecstasy, cannabis) 1
• Consider thyrotoxicosis, heart failure, anxiety disorders 1

Rate control when appropriate:

• Beta-blockers are first-line for symptomatic sinus tachycardia from emotional stress, post-MI patients, heart failure, and thyrotoxicosis 1
• Nondihydropyridine calcium-channel blockers (diltiazem or verapamil) are alternatives if beta-blockers are contraindicated 1

Common Pitfalls to Avoid

• Never assume a wide complex tachycardia is supraventricular based solely on hemodynamic stability—VT can present with stable vital signs 1
• Do not use IV calcium-channel blockers or beta-blockers together due to potentiation of hypotensive and bradycardic effects 1
• Avoid adenosine in patients with severe asthma 1
• Patients on theophylline may require higher adenosine doses; those on dipyridamole need lower doses 1