Cardiology · Conduction Abnormalities
The facts most likely to be tested
A new-onset LBBB in the setting of acute chest pain is considered a STEMI equivalent and requires immediate cardiac catheterization.
The ECG diagnostic criteria for LBBB include a QRS duration ≥ 120 ms and broad, monomorphic R waves in leads I, aVL, V5, and V6.
LBBB typically presents with absent Q waves in lateral leads and deep S waves in leads V1 and V2.
Discordant ST-segment changes (ST elevation in leads with negative QRS complexes and ST depression in leads with positive QRS complexes) are expected findings in LBBB.
The Sgarbossa criteria are used to diagnose myocardial infarction in the presence of a pre-existing LBBB.
LBBB is most commonly associated with underlying structural heart disease, particularly hypertensive heart disease, coronary artery disease, or dilated cardiomyopathy.
Left ventricular dyssynchrony caused by LBBB can lead to reduced left ventricular ejection fraction and may be an indication for cardiac resynchronization therapy (CRT).
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A 68-year-old male with a history of hypertension and heart failure presents to the emergency department with 2 hours of substernal chest pressure and diaphoresis. His vitals are stable, and he is currently pain-free after sublingual nitroglycerin. An ECG shows a QRS duration of 140 ms, broad monomorphic R waves in leads I and V6, and deep S waves in V1. There are discordant ST-segment elevations in leads V1 and V2. His initial troponin I is elevated.
What is the most appropriate management for this patient?
Emergent cardiac catheterization (percutaneous coronary intervention)
A new-onset LBBB in the presence of ischemic symptoms is treated as a STEMI equivalent, necessitating immediate reperfusion therapy.
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Etiology / Epidemiology
Associated with structural heart disease, specifically hypertension, aortic stenosis, and ischemic cardiomyopathy.
Clinical Manifestations
Often asymptomatic; look for wide QRS >120ms and discordance between QRS and T-waves.
Diagnosis
12-lead ECG is the gold standard; look for monomorphic R waves in leads I, aVL, V5, and V6.
Treatment
Treat underlying cause; cardiac resynchronization therapy (CRT) for symptomatic heart failure with LVEF ≤35%.
Prognosis
New-onset LBBB in the setting of chest pain is treated as an STEMI equivalent until proven otherwise.
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Epidemiology & Etiology
LBBB is rarely found in healthy individuals and strongly correlates with underlying cardiac pathology. Common etiologies include hypertensive heart disease, dilated cardiomyopathy, and calcific aortic stenosis. It is a frequent finding in patients with congestive heart failure.
Pertinent Anatomy
The left bundle branch divides into the anterior and posterior fascicles. A complete block requires damage to the main left bundle trunk, resulting in delayed activation of the left ventricle.
Pathophysiology
The right ventricle depolarizes first, followed by the left ventricle via slow cell-to-cell conduction. This creates a prolonged QRS duration and abnormal septal activation. The resulting discordance means the T-wave is typically directed opposite to the terminal QRS vector.
Clinical Manifestations
Patients are frequently asymptomatic, but may present with syncope or dyspnea related to underlying heart failure. On ECG, look for broad, notched R waves in lateral leads (I, aVL, V5, V6) and deep S waves in V1. New-onset LBBB in a patient with acute chest pain is a cardiac emergency requiring immediate evaluation for myocardial infarction.
Diagnosis
The 12-lead ECG is the diagnostic tool of choice. Criteria include a QRS duration ≥120ms, absence of Q waves in lateral leads, and monomorphic R waves in I, aVL, V5, and V6. The Sgarbossa criteria are used to identify ischemia in the presence of LBBB.
Treatment
Management focuses on the underlying etiology, such as blood pressure control or valve replacement. For patients with LVEF ≤35% and persistent symptoms despite optimal medical therapy, cardiac resynchronization therapy (CRT) is indicated. Avoid prophylactic pacing in asymptomatic patients without structural disease.
Prognosis
LBBB is an independent predictor of increased cardiovascular mortality. Patients require regular monitoring for the development of heart failure and progression to high-degree AV block.
Differential Diagnosis
Right Bundle Branch Block: rSR' pattern in V1
Ventricular Tachycardia: AV dissociation or fusion beats
Wolff-Parkinson-White: presence of a delta wave
Hypertrophic Cardiomyopathy: deep narrow Q waves in lateral leads
Ventricular Paced Rhythm: presence of pacing spikes