The most common mechanism of tachycardias is the result of which mechanism of activation?

• A. Reentry
• B. Automaticity
• C. Triggered activity
• D. Atrial fibrillation

Answer: (A)

Reentry is the mechanism most tachycardias use. A wavefront travels around a circuit and re-enters tissue that has recovered excitability, creating a self-sustaining loop that keeps the heart rate fast. This is common in many supraventricular tachycardias, such as AV nodal reentrant tachycardia and orthodromic AV reentrant tachycardia via an accessory pathway. Automaticity tachycardias come from ectopic pacemaker cells that fire spontaneously, and triggered activity tachycardias arise from afterdepolarizations; both can cause tachycardias but are less frequently the dominant mechanism for sustained fast rhythms. Atrial fibrillation, while very fast atrial activity, involves multiple wandering wavelets and chaotic atrial activation rather than a single reentrant circuit, so it’s not a single mechanism of activation.