Makindo Medical Notes

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⚡ Key Point: Although a QT interval ≥500 ms is strongly associated with a higher risk of torsades de pointes, there is no absolute "safe" lower threshold. Even modest QT prolongation can be proarrhythmic, especially when combined with bradycardia, electrolyte imbalance, or QT-prolonging drugs.

📖 About

• The QT interval represents the total time for ventricular depolarisation and repolarisation — from the start of the QRS complex to the end of the T wave. It reflects the transmural dispersion of repolarisation (epicardial–mid–endocardial differences), which underlies susceptibility to re-entrant ventricular arrhythmias. QT duration shortens with faster heart rates and lengthens with slower ones. Hence, it should be corrected for heart rate (QTc) using formulae such as Bazett’s or Fridericia’s:

  QTc = QT / √(RR) (Bazett’s)

• Typical normal QTc: 350–440 ms.

🩺 ECG – QT Interval

• Prolonged QT reflects delayed ventricular repolarisation.
• Abnormal QTc: >450 ms in men and >460 ms in women.
• Excessive prolongation predisposes to Torsades de pointes (TdP) — a polymorphic ventricular tachycardia that may degenerate into ventricular fibrillation.

📏 Measuring the QT

• Measure manually in the limb lead showing the clearest T-wave termination.
• From the start of the QRS to the end of the T wave; average across 3–5 beats.
• U waves should be included only if they merge with the T wave.
• In atrial fibrillation, measure several beats with similar RR intervals.
• Preferably assess during peak drug plasma levels when evaluating medication effect.
• Always adjust for heart rate before interpreting QT values.

⚠️ Causes of Long QT

⬇️ Causes of Short QT

• Hypercalcaemia or hyperkalaemia ⚡
• Digoxin effect (“scooped” ST depression)
• β-blockers, phenytoin, or congenital short-QT syndromes

🧠 Teaching Commentary

QT prolongation represents disordered repolarisation across the ventricular wall. The mid-myocardial “M cells” are especially prone to delayed repolarisation, creating dispersion of recovery times and enabling early after-depolarisations — the substrate for torsades. Always review drugs, electrolytes, and heart rate before attributing it to a congenital cause. Even mild prolongation in a polypharmacy elderly patient can be dangerous, particularly if combined with bradycardia or hypokalaemia.

🔗 Reference

• What Clinicians Should Know About the QT Interval — JAMA