A 62-year-old woman with dilated cardiomyopathy and an ejection fraction of 25% is found to have frequent premature ventricular contractions on 24-hour Holter monitoring. Which is the most common cause of ventricular arrhythmias in patients with dilated cardiomyopathy?

## Mechanism of Arrhythmias in Dilated Cardiomyopathy **Key Point:** Reentry in areas of myocardial fibrosis and scar is the most common mechanism of ventricular arrhythmias in dilated cardiomyopathy (DCM), regardless of the underlying etiology (viral, genetic, idiopathic, or ischemic). ### Pathophysiology of DCM-Related Arrhythmias ```mermaid flowchart TD A[Dilated Cardiomyopathy]:::outcome --> B[Myocardial dysfunction] B --> C[Ventricular remodeling] C --> D[Fibrosis and scar formation] D --> E[Slow conduction zones]:::action E --> F[Unidirectional block]:::action F --> G[Reentrant circuits]:::outcome G --> H[Sustained VT/VF]:::urgent ``` ### Why Reentry Dominates in DCM 1. **Extensive fibrosis** — Progressive replacement of myocardium with collagen in DCM 2. **Abnormal conduction** — Fibrosis creates areas of slow, discontinuous propagation 3. **Anatomical substrate** — Scar and fibrotic tissue provide the anatomical framework for reentry 4. **Electrogram fragmentation** — Mapping shows late potentials and split electrograms in fibrotic zones 5. **Reproducibility** — Reentrant circuits are stable and reproducible, explaining sustained VT **High-Yield:** In DCM, the arrhythmia substrate is structural (fibrosis), not functional (ischemia). This is why DCM patients have sustained VT even without coronary artery disease. ### Comparison of Mechanisms in Different Cardiomyopathies | Mechanism | DCM | HCM | ARVC | Ischemic CM | |-----------|-----|-----|------|-------------| | Reentry (fibrosis/scar) | **Most common** | Common | Most common | Most common | | Early afterdepolarization | Rare | Rare | Rare | Uncommon | | Abnormal automaticity | Uncommon | Uncommon | Rare | Uncommon | | Triggered activity | Rare | Possible | Rare | Rare | **Clinical Pearl:** Electrophysiology studies in DCM patients reveal that VT circuits are located in areas of scar tissue identified by late gadolinium enhancement on cardiac MRI. This supports reentry as the primary mechanism. **Mnemonic:** **FIBROSIS REENTRY** — Fibrosis In Both ventricles Reentry Occurs Slow conduction In Scar = reentry is the dominant mechanism in DCM.