A patient with a re-entrant supraventricular tachycardia has a circuit involving the AV node and an accessory pathway. Which three conditions are required for re-entry to be sustained?
- A Enhanced automaticity, triggered activity, and sympathetic stimulation
- B Unidirectional block, slow conduction, and a closed circuit with excitable tissue ✓
- C Prolonged QT, early afterdepolarizations, and hypokalemia
- D Fibrosis, gap junction remodeling, and ion channel mutation
Correct answer: B. Unidirectional block, slow conduction, and a closed circuit with excitable tissue
Explanation
Classic re-entry requires three elements: a closed anatomical or functional circuit, unidirectional block in one limb so the impulse travels only one way, and slow conduction in the alternate path allowing the previously blocked tissue time to recover excitability. This sustains circus movement. Enhanced automaticity and triggered activity are distinct mechanisms of arrhythmia. Prolonged QT and EADs underlie torsades de pointes, not re-entry.
Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.
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