Which phenomenon is a known mechanism for generating tachyarrhythmias via reentry?

• A. Reentry circuits
• B. Bradycardia
• C. Normal sinus rhythm
• D. Atrioventricular block

Answer: (A)

Reentry circuits explain tachyarrhythmias because an impulse can continuously travel around a loop of conducting tissue, re-exciting the heart after the tissue ahead has recovered from refractoriness. For a reentrant tachycardia to sustain, there must be a pathway with a unidirectional block in one limb and slow conduction in another, creating a temporal window in which the previously excited tissue becomes ready to be re-entered. Structural or functional heterogeneity, such as scar tissue or distinct conduction pathways, often provides the substrate for these circuits. This mechanism underlies many common reentrant arrhythmias, like atrial flutter or certain ventricular tachycardias, where the circulating wavefront propagates around a loop.

Bradycardia, normal sinus rhythm, and AV block do not describe this reentrant looping process. Bradycardia is simply a slow heart rate, normal sinus rhythm is regular atrial and ventricular activation without a self-sustaining loop, and AV block denotes impaired conduction rather than a circuit that self-perpetuates tachycardia.