On a standard 12-lead ECG, the QT interval is prolonged when it exceeds which duration relative to the RR interval?

Explanation

## QT Interval Assessment ### Definition **Key Point:** The QT interval represents the total duration of ventricular depolarization and repolarization — from the onset of the QRS complex to the end of the T wave. ### Normal Duration and Heart Rate Correction Because the QT interval is **heart rate-dependent** (shorter at faster rates, longer at slower rates), it must be corrected for heart rate using the **Bazett formula**: $$QTc = \frac{QT}{\sqrt{RR}}$$ where QT and RR are measured in seconds. **High-Yield:** - **Uncorrected QT interval:** ~0.35–0.45 seconds at normal heart rates - **Corrected QT (QTc):** Normal <0.44 seconds in men, <0.46 seconds in women - **QTc >0.46 s (men) or >0.47 s (women) = prolonged QT** ### Rule-of-Thumb Method (Bedside) When a calculator is unavailable, use the **Bazett rule of thumb**: **QT interval is prolonged if it exceeds 50–60% of the RR interval** (depending on heart rate). At normal heart rates (~60–80 bpm, RR ~0.75–1.0 s): - QT should be <0.5 s - This approximates to **<60% of RR interval** ### Clinical Significance | Prolonged QT Cause | Example | |-------------------|----------| | Congenital | Long QT syndrome (LQTS) | | Acquired — Drugs | Antiarrhythmics (sotalol, amiodarone), antipsychotics (haloperidol), antiemetics (domperidone) | | Acquired — Electrolyte | Hypocalcaemia, hypokalaemia, hypomagnesaemia | | Acquired — Cardiac | Myocarditis, cardiomyopathy, bradycardia | **Warning:** Prolonged QT increases risk of **torsades de pointes** — a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation. **Mnemonic for QT-prolonging drugs:** **SADS** — **S**otalol, **A**ntipsychotics, **D**omperidone, **S**ertraline (and other SSRIs). [cite:Harrison 21e Ch 269]