What is the definition of reentry in cardiac physiology?

Reentry in cardiac physiology refers to a specific phenomenon where an electrical impulse travels along a circuit that allows it to depolarize a specific area of the heart more than once. This occurs when the impulse reenters an area of the myocardium after having previously depolarized it.

For reentry to happen, certain conditions must be met, including the presence of a unidirectional block and a slow conduction pathway that allows the impulse to circle back to its point of origin, thus enabling the impulse to reactivate the same myocardial area. This mechanism is significant because it can lead to arrhythmias, where the heart beats in an irregular pattern due to this looping electrical activity.

The other options describe mechanisms that do not align with the definition of reentry. The first option discusses a unidirectional impulse, which does not imply the capability of returning to its point of origin. The second option indicates an inability to reenter the same area, contradicting the core concept of reentry. The last option concerns impulses generated by ectopic pacemakers, which involves a different context of arrhythmogenesis and does not involve the reentry phenomenon.