Can left anterior fascicular block (LAFB) be intermittent?

Intermittent Left Anterior Fascicular Block

Yes, left anterior fascicular block (LAFB) can be intermittent in nature, particularly in the context of acute or chronic ischemic coronary artery disease.

Pathophysiology and Evidence for Intermittent LAFB

Left anterior fascicular block is one of the conduction disorders affecting the fascicles of the left bundle branch. The most recent evidence strongly supports that LAFB can manifest as an intermittent phenomenon in several clinical scenarios:

• The most common cause of intermittent LAFB is critical proximal stenosis of the left anterior descending coronary artery before its first septal perforator branch 1
• It can be observed during acute coronary syndromes including vasospastic angina 1
• It may appear during exercise-induced myocardial ischemia 1
• It can be phase 4 bradycardia rate-dependent or induced by early atrial extrastimulus 1

Clinical Significance of Intermittent LAFB

Intermittent LAFB has important clinical implications:

• It may have similar clinical significance as "Wellens syndrome" and the "de Winter pattern" in acute coronary syndrome settings 1
• The transient/intermittent nature of LAFB is considered an important characteristic that has been incorporated into diagnostic electrocardiographic criteria 1
• When LAFB appears intermittently, it's typically a second-degree or transient block rather than a complete block 2

Diagnostic Evaluation for Suspected Intermittent LAFB

For patients with suspected intermittent LAFB, the following diagnostic approach is recommended:

1. 12-lead ECG: Essential to document the LAFB when present

   • Look for left axis deviation (typically -30° to -90°)
   • Delayed intrinsicoid deflection in lead aVL 3

2. Ambulatory ECG monitoring: Recommended to capture the intermittent nature

   • 24-48 hour Holter monitoring or longer if needed 4
   • To document transitions between normal conduction and LAFB

3. Echocardiography: Recommended to assess for structural heart disease 4

   • Particularly important to evaluate for ischemic heart disease

4. Stress testing: Consider in patients with suspected ischemia-induced LAFB 4

   • May help document exercise-induced LAFB

Management Considerations

Management should focus on:

1. Addressing the underlying cause, particularly if ischemic heart disease is suspected

   • Coronary evaluation may be warranted in patients with new or intermittent LAFB

2. Risk stratification:

   • Isolated intermittent LAFB without structural heart disease generally has a benign prognosis 4
   • When associated with other conduction abnormalities (bifascicular block), closer monitoring is warranted 4

3. Follow-up:

   • Regular clinical evaluation with ECG is recommended 4
   • More frequent monitoring if associated with symptoms or other conduction abnormalities

Important Caveats

• Permanent pacing is not indicated for isolated LAFB, even if intermittent, in the absence of symptoms or higher-degree AV block 5
• The diagnosis of LAFB should not be made solely on the presence of left axis deviation, as other criteria should be considered 3
• When LAFB is associated with other conduction abnormalities (such as RBBB forming bifascicular block), the risk of progression to complete heart block increases 4

The intermittent nature of LAFB is well-documented and should prompt evaluation for underlying cardiac conditions, particularly coronary artery disease, which is the most common cause of this phenomenon.