Atrial Fibrillation with Transient Left Bundle Branch Block
Name and Definition
This phenomenon is called "aberrant ventricular conduction" or "Ashman phenomenon" when atrial fibrillation produces transient left bundle branch block (LBBB) patterns during episodes of rapid ventricular response. 1
The transient LBBB during AF represents rate-dependent aberrancy where the bundle branch fails to conduct normally at certain heart rates but recovers when the rate slows, producing alternating narrow and wide QRS complexes on the same ECG tracing. 1, 2
Etiology and Pathophysiologic Mechanisms
Primary Mechanism
- The transient nature occurs because the left bundle branch has a longer refractory period than normal conduction tissue, causing it to intermittently fail to conduct during the irregular rapid ventricular rates characteristic of AF. 1, 2
- When AF produces varying R-R intervals, a short cycle following a long cycle can catch the left bundle branch still refractory, producing the classic Ashman beat with LBBB morphology. 1
Underlying Cardiovascular Disease in Older Adults
The combination of AF with transient LBBB in older adults with cardiovascular disease typically reflects multiple concurrent pathologies:
Coronary Artery Disease:
- CAD is explicitly listed as a specific cardiovascular condition associated with AF and represents one of the most common underlying etiologies in older adults. 3, 4
- Intermittent LBBB is often a reflection of underlying CAD, with coronary ischemia causing transient conduction disturbances. 2
- Atrial ischemia from CAD directly promotes structural and electrical abnormalities creating an arrhythmogenic substrate for AF. 4
Hypertensive Heart Disease:
- Hypertension, particularly with left ventricular hypertrophy, is strongly associated with both AF and conduction abnormalities. 3, 5
- Hypertension causes left atrial enlargement and reduced left atrial appendage flow velocity, predisposing to AF development. 3
Structural Cardiac Remodeling:
- Atrial fibrosis develops through activation of the renin-angiotensin-aldosterone system in the setting of CAD and hypertension. 4
- Left ventricular dysfunction and heart failure are important predictive factors for both AF and conduction disturbances. 5
Degenerative Conduction System Disease:
- Primary degenerative lesions of the specialized conducting tissue occur as an isolated phenomenon, particularly in older individuals, causing baseline conduction vulnerability. 6
Critical Clinical Pitfall
The transient LBBB pattern during AF should never be dismissed as benign aberrancy in older adults with cardiovascular disease—it mandates evaluation for acute coronary ischemia, as intermittent LBBB can unmask significant coronary lesions requiring intervention. 2 The combination of new-onset AF with conduction abnormalities in the setting of acute MI portends particularly poor prognosis. 3, 4
Evaluation Algorithm
- Obtain serial ECGs to document the transient nature of LBBB (alternating narrow and wide QRS complexes). 1, 2
- Perform transthoracic echocardiography to assess for structural heart disease, left ventricular dysfunction, and valvular abnormalities. 6
- Consider coronary angiography or stress testing to exclude obstructive CAD, particularly if chest pain or ischemic symptoms are present. 7, 2
- Assess for acute reversible causes including MI, myocarditis, pulmonary embolism, hyperthyroidism, and electrolyte disturbances. 3