A 52-year-old man with a 10-year history of major depressive disorder is on amitriptyline 75 mg at bedtime. He presents with palpitations, dizziness, and syncope on exertion. Physical examination shows an irregular pulse. An ECG is ordered and shows a prolonged QTc interval (480 ms). Before attributing this to amitriptyline toxicity, which investigation is most appropriate to exclude a secondary cause of QT prolongation?

Explanation

## Investigating QT Prolongation in a TCA-Treated Patient **Key Point:** QT prolongation is a known adverse effect of tricyclic antidepressants (TCAs), especially amitriptyline, due to their class Ia antiarrhythmic properties (sodium and potassium channel blockade). However, **electrolyte abnormalities are the most common and reversible secondary cause of QT prolongation** and must be excluded first. ### Why Serum Electrolytes Are the Investigation of Choice | Electrolyte | Mechanism of QT Prolongation | Clinical Significance | |---|---|---| | **Hypokalaemia** | Increases repolarization time; prolongs action potential duration | Most common cause; easily reversible | | **Hypomagnesaemia** | Impairs K⁺ channel function; enhances QT prolongation | Often coexists with hypokalaemia | | **Hypocalcaemia** | Prolongs ST segment and QT interval | Less common but important | | **Hypernatraemia** | Mild QT prolongation; less clinically significant | Rarely sole cause | **High-Yield:** The **"Hypokalaemia + TCA" combination is a high-risk scenario for torsades de pointes and sudden cardiac death**. This must be ruled out before attributing QT prolongation solely to the drug. ### Clinical Context in This Case 1. **Amitriptyline is a known QT-prolonger**: Tricyclic antidepressants have anticholinergic and cardiac effects, including QT prolongation at therapeutic and supratherapeutic doses. 2. **Syncope and palpitations suggest arrhythmia risk**: These symptoms, combined with prolonged QTc, raise concern for torsades de pointes. 3. **Electrolyte abnormalities are common in depression**: Patients on TCAs may have: - Diuretic use (hypertension, heart failure) → hypokalaemia - Poor nutrition, chronic illness → hypomagnesaemia - SIADH (depression-related or drug-induced) → hyponatraemia (less QT-relevant) **Mnemonic:** **"CHAMPS"** — Causes of QT prolongation: **C**alcium ↓, **H**ypomagnesaemia, **A**ntiarrhythmics (and antidepressants), **M**edicines (other drugs), **P**otassium ↓, **S**tructural heart disease. ### Investigation Hierarchy 1. **First: Serum electrolytes** (cheap, fast, reversible if abnormal). 2. **If electrolytes normal: Serum TCA level** (to assess drug exposure and toxicity). 3. **If both normal: Echocardiography or cardiac MRI** (to exclude structural disease, cardiomyopathy, or channelopathy). **Clinical Pearl:** In Indian clinical practice, hypokalaemia is frequently overlooked in psychiatric patients on TCAs, especially those with concurrent diuretic use for hypertension. A simple serum K⁺ check can be life-saving. [cite:Harrison 21e Ch 297; KD Tripathi 8e Ch 12]