A 55-year-old woman with chronic kidney disease on dialysis has a baseline ECG showing prolonged QTc of 520 ms. Telemetry shows a premature wide-complex beat that lands on the T wave of the preceding beat, followed by a polymorphic ventricular tachycardia with twisting of the QRS axis. What is the initiating mechanism of this arrhythmia?
- A Automaticity enhancement in the His-Purkinje system
- B Re-entry around a fixed anatomical obstacle in the right ventricle
- C Triggered activity from early afterdepolarizations during prolonged repolarization ✓
- D Enhanced normal automaticity of the ventricular myocardium
Correct answer: C. Triggered activity from early afterdepolarizations during prolonged repolarization
Explanation
The R-on-T phenomenon initiating torsades de pointes occurs because prolonged repolarization (long QT) predisposes to early afterdepolarizations (EADs) during phase 3 of the action potential. When a premature beat occurs during the T wave (vulnerable period), it encounters heterogeneous repolarization and triggers polymorphic VT. This is triggered activity, not automaticity or fixed re-entry.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.