## Mechanisms of Atrial Fibrillation **Key Point:** Atrial fibrillation arises from two principal mechanisms: focal automaticity (especially from pulmonary veins in paroxysmal AF) and reentry circuits (dominant in structural heart disease). ### Mechanism by AF Type | AF Type | Primary Mechanism | Substrate | Clinical Setting | |---------|-------------------|-----------|------------------| | Paroxysmal (especially lone AF) | Pulmonary vein automaticity | Minimal atrial remodeling | Younger, structurally normal hearts | | Persistent/Permanent | Reentry circuits | Atrial scarring, fibrosis, conduction block | Structural heart disease, hypertension, heart failure | | Structural heart disease | Reentry (dominant) | Atrial dilation, fibrosis, heterogeneous conduction | Post-MI, cardiomyopathy, valvular disease | **High-Yield:** In **structural heart disease** (dilated atria, myocardial scarring, post-infarction), reentry is the dominant mechanism because: 1. Atrial dilation creates longer conduction pathways 2. Fibrosis and scarring create areas of slow conduction and unidirectional block 3. These substrate changes favor reentrant circuits over simple automaticity **Clinical Pearl:** Pulmonary vein isolation (ablation) is highly effective in paroxysmal AF because it targets the automatic foci; however, in structural AF with reentry, ablation must address the substrate (scar-based circuits), making it less effective and requiring more extensive ablation patterns. **Warning:** Do not confuse the mechanism with the trigger — pulmonary veins may trigger paroxysmal AF, but the underlying mechanism in structural disease is reentry, not ectopy.
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