A 52-year-old man with a history of hypertension presents with palpitations. His 12-lead ECG shows a prolonged QT interval. Which of the following is the most common cause of acquired QT prolongation in clinical practice?

Explanation

## Most Common Cause of Acquired QT Prolongation **Key Point:** Hypokalaemia is the most frequent electrolyte disturbance causing QT prolongation in everyday clinical practice, particularly in patients on diuretics, vomiting, or diarrhoea. ### Mechanism of QT Prolongation in Hypokalaemia Low serum potassium (K⁺ < 3.5 mEq/L) delays ventricular repolarization by: 1. Reducing the outward K⁺ current (I_K) 2. Prolonging the action potential duration in ventricular myocytes 3. Increasing susceptibility to early afterdepolarizations (EADs) 4. Creating a substrate for torsades de pointes arrhythmias ### Comparison of QT-Prolonging Conditions | Cause | Mechanism | Frequency | Clinical Context | |-------|-----------|-----------|------------------| | **Hypokalaemia** | ↓ K⁺ current (I_K) | Most common | Diuretics, GI losses, RTA | | Hypothyroidism | ↓ Metabolic rate, ↓ repolarization | Uncommon | Myxoedema, autoimmune | | Acute MI | Ischaemic injury, inflammation | Moderate | Acute coronary event | | Subarachnoid haemorrhage | Catecholamine surge, autonomic storm | Rare | Thunderclap headache | **High-Yield:** Hypokalaemia is the **single most preventable** cause of QT prolongation. Checking serum K⁺ and correcting it is standard practice in any patient with QT prolongation. ### Clinical Pearl The combination of **hypokalaemia + QT prolongation + syncope = high risk for torsades de pointes**. This is a medical emergency requiring urgent IV potassium repletion and cardiac monitoring. **Mnemonic — Causes of QT Prolongation (ABCDEFG):** - **A**ntiarrhythmics (Class IA, III) - **B**radycardia, **B**ath (cold water immersion) - **C**alcium channel blockers (some), **C**ongenital (Romano-Ward, Jervell-Lange-Nielsen) - **D**iuretics (hypokalaemia), **D**rugs (macrolides, fluoroquinolones, antipsychotics) - **E**lectrolyte abnormalities (hypokalaemia, hypomagnesaemia, hypocalcaemia) - **F**emale sex (longer baseline QTc), **F**ever - **G**astrointestinal losses (K⁺ depletion) **Warning:** Do NOT confuse QT prolongation with QRS prolongation. QT reflects total ventricular depolarization + repolarization; QRS is only depolarization. Hypokalaemia prolongs QT, not QRS.