A 68-year-old woman with a history of myocardial infarction 6 months ago presents with recurrent episodes of sustained monomorphic ventricular tachycardia (VT). She is haemodynamically stable during the arrhythmia. Baseline ECG shows evidence of prior anterior wall MI with reduced ejection fraction (EF 35%). Before initiating long-term antiarrhythmic therapy, which investigation is most appropriate to assess the substrate and guide treatment decisions?

Explanation

## Investigation of Choice for Post-MI Ventricular Tachycardia Substrate Assessment ### Clinical Context Sustained monomorphic VT in the setting of prior MI reflects a re-entrant circuit within scar tissue. Understanding the extent, location, and characteristics of the scar is essential for: 1. Risk stratification and prognosis 2. Determining candidacy for catheter ablation vs. antiarrhythmic drugs 3. Predicting response to therapy 4. Identifying patients who may benefit from ICD implantation ### Why Cardiac MRI with Late Gadolinium Enhancement (LGE) is Optimal **Key Point:** Cardiac MRI with late gadolinium enhancement is the gold standard for visualizing myocardial scar and fibrosis because: - **Spatial resolution:** Detects scar as small as 1–2 mm with high precision - **Scar characterization:** Distinguishes dense scar (core) from border zone (penumbra), which defines re-entrant circuit anatomy - **Substrate mapping:** Identifies regions of slow conduction and heterogeneous tissue — the substrate for VT - **Ablation planning:** Provides 3D scar geometry for catheter ablation guidance - **Prognostic value:** Scar burden correlates with arrhythmia recurrence and mortality **High-Yield:** LGE-MRI findings in post-MI VT: - Transmural or subendocardial scar (bright signal = fibrosis) - Scar in distribution of infarct-related artery (e.g., anterior wall = LAD territory) - Heterogeneous scar with border zone — site of re-entrant circuit - Scar volume >10 cm³ associated with higher VT recurrence **Clinical Pearl:** In patients with EF ≤35% and prior MI, LGE-MRI helps determine if VT is due to scar-related re-entry (amenable to ablation) vs. other mechanisms, and guides ICD implantation decisions. ### Comparison of Investigations for VT Substrate Assessment | Investigation | Scar Detection | Spatial Resolution | Guides Ablation | Prognostic Value | |---|---|---|---|---| | **Cardiac MRI with LGE** | Excellent | High (1–2 mm) | Yes, 3D mapping | Yes, scar burden | | **Holter monitor** | No | N/A | No | Limited (arrhythmia burden) | | **Transthoracic echo** | No | Poor | No | Wall motion abnormality only | | **Coronary angiography** | No | N/A | No | Assesses coronary patency | ### Antiarrhythmic Strategy Based on Substrate **Mnemonic: SCAR** — **S**car burden, **C**ore vs. border zone, **A**blation candidacy, **R**ecurrence risk. **If extensive scar with poor EF:** - First-line: ICD + amiodarone (Class III agent) - Consider catheter ablation if recurrent ICD shocks **If limited scar with preserved EF:** - Antiarrhythmic monotherapy (sotalol, flecainide, amiodarone) - Catheter ablation may be curative ### Why Other Options Are Incorrect **Holter monitor** — detects arrhythmia frequency and burden but provides no information about substrate anatomy, scar location, or ablation feasibility. It is useful for monitoring treatment response, not for initial substrate assessment. **Transthoracic echocardiography** — assesses global and regional wall motion abnormalities and EF but cannot visualize scar tissue or fibrosis in detail. It does not guide ablation or predict VT mechanism. **Coronary angiography** — identifies coronary stenosis and infarct-related artery but does not characterize myocardial scar or define the VT substrate. It is indicated for revascularization assessment, not arrhythmia mechanism.