A 68-year-old woman with chronic atrial fibrillation on digoxin therapy presents with nausea, visual disturbances, and palpitations. Serum electrolytes show hypokalaemia (K⁺ 3.1 mEq/L). An ECG shows a slow ventricular rate with frequent premature ventricular contractions. Which investigation is most appropriate to confirm the suspected diagnosis of digoxin toxicity?

## Diagnosis of Digoxin Toxicity **Key Point:** Serum digoxin level is the most specific and direct investigation to confirm digoxin toxicity in a patient with clinical features and risk factors. ### Clinical Context of Digoxin Toxicity The patient presents with the **classic triad of digoxin toxicity:** 1. **Gastrointestinal:** Nausea (direct effect on chemoreceptor trigger zone) 2. **Visual:** Blurred vision, yellow-green halos (direct retinal toxicity) 3. **Cardiac:** Arrhythmias (enhanced automaticity + AV block) **Risk Factor:** Hypokalaemia significantly increases digoxin toxicity risk by reducing Na⁺-K⁺-ATPase inhibition threshold. ### Why Serum Digoxin Level is Diagnostic | Investigation | Role in Digoxin Toxicity | |---|---| | **Serum digoxin level** | Direct measurement; therapeutic range 0.5–2.0 ng/mL; toxicity often >2.0 ng/mL (though clinical correlation essential) | | **Holter ECG** | Shows arrhythmia pattern but non-specific; cannot differentiate digoxin toxicity from other causes | | **EPS** | Invasive; not diagnostic for drug toxicity; reserved for ablation planning | | **Coronary angiography** | Rules out acute coronary syndrome but not relevant to digoxin toxicity diagnosis | **High-Yield:** Serum digoxin level **must be drawn 6–8 hours post-dose** for accurate interpretation. Levels drawn earlier may be falsely elevated due to incomplete tissue distribution. ### ECG Findings in Digoxin Toxicity **Characteristic but non-specific findings:** - **Sagging ST segment** ("reverse tick" or "Salvador Dalí" appearance) - **Shortened QT interval** - **Prolonged PR interval** (AV block) - **Arrhythmias:** Frequent PVCs, bigeminy, trigeminy, atrial fibrillation with slow ventricular response, junctional tachycardia **Clinical Pearl:** The arrhythmia pattern "atrial fibrillation with a slow ventricular rate" in a patient on digoxin is pathognomonic for digoxin toxicity (due to enhanced AV nodal block). **Mnemonic — Digoxin Toxicity Features: NAUSEA-VT** - **N**ausea - **A**rrhythmias (PVCs, bigeminy, junctional tachycardia) - **U**nderstanding (confusion, disorientation) - **S**agging ST segment - **E**xtrasystoles - **A**V block - **V**isual disturbances (yellow-green halos) - **T**oxicity confirmed by serum level ### Management Implications Once digoxin toxicity is confirmed: 1. **Discontinue digoxin** 2. **Correct hypokalaemia** (K⁺ repletion to 4.0–5.0 mEq/L) 3. **Treat arrhythmias:** Phenytoin or lidocaine (avoid quinidine, which increases digoxin levels) 4. **Severe toxicity:** Digoxin-specific antibody fragments (DigiFab) if life-threatening arrhythmias [cite:KD Tripathi 8e Ch 12]