Criteria to initiate and sustain reentry tachycardia include:

• A. Different conduction velocities
• B. Unidirectional block
• C. Two pathways
• D. All of the above

Answer: (D)

Reentry tachycardia needs a circulating impulse around a circuit that forms around an obstacle, with timing that allows the impulse to re-enter tissue that has recovered from refractoriness. Different conduction velocities create a path where one limb slows enough to let the other recover, helping the loop to persist. Unidirectional block is essential because it permits conduction in one direction around the circuit while blocking it in the opposite direction, creating the necessary loop. Having two pathways provides the substrate for such a circuit, offering distinct routes for the impulse to travel around the obstacle. Put together, these elements enable the initiation and maintenance of a reentrant tachycardia.