Questions: Cardiac Arrhythmias: Reentry, Automaticity, and Triggered Activity

Question 1

A patient has a tachycardia that starts and terminates abruptly, is precisely regular, and is permanently cured by catheter ablation of a specific pathway. Which arrhythmia mechanism does this most strongly suggest?

• A. Reentry — a fixed anatomical circuit sustains the arrhythmia, and ablating the circuit eliminates it
• B. Abnormal automaticity — an ectopic focus fires at a fixed rate and can be destroyed
• C. Early afterdepolarizations — prolonged action potential duration creates triggered beats
• D. Delayed afterdepolarizations — calcium overload drives a transient inward current

Answer: (A)

The clinical signature of reentry is abrupt onset and termination (the circuit either sustains or doesn't), precise regularity (the circus movement fires at a fixed rate), and curability by ablating the circuit. Automaticity produces more gradual warm-up and cool-down. Triggered activity requires preceding beats and tends to cluster in bursts rather than sustained regular tachycardia. Catheter ablation targeting a specific pathway is the definitive treatment for reentry, not for automaticity or triggered activity.

Question 2

A region of myocardium damaged by scarring conducts impulses slowly but still conducts. An electrical wavefront reaches a fork where one path is healthy (fast conduction) and the damaged path is present. For reentry to be sustained, what must be true?

• A. The damaged path must block the wavefront antegrade but allow retrograde conduction — unidirectional block — and the healthy path must recover before the retrograde wave returns
• B. Both paths must block the wavefront, forcing it to restart from the SA node
• C. The healthy path must be permanently refractory so all conduction goes through the slow path
• D. The wavefront must split evenly between the two paths and collide at the far end

Answer: (A)

Reentry requires exactly two conditions working together: unidirectional block (the damaged path cannot conduct the wavefront in the normal direction) and slow conduction (the path conducts retrogradely slowly enough that the fast pathway has time to recover its excitability). If block were bidirectional, no retrograde conduction occurs. If conduction in the slow path were fast, the retrograde wave would arrive while the fast pathway is still refractory and extinguish. Both conditions must coexist.

Question 3

Triggered activity, unlike abnormal automaticity, cannot arise spontaneously from rest — it always requires a preceding action potential to initiate.

• T. True
• F. False

Answer: True

This is the defining mechanistic distinction. Triggered activity arises from afterdepolarizations — oscillations in membrane potential that follow an action potential. Early afterdepolarizations occur during the same action potential; delayed afterdepolarizations occur in phase 4 from calcium overload. Either way, they need a preceding beat to trigger. Automaticity, by contrast, is truly spontaneous: the cell depolarizes on its own during phase 4 without requiring a prior action potential, which is how the SA node normally initiates each heartbeat.

Question 4

Abnormal automaticity and reentry share the same underlying mechanism — both arise from circular conduction through a damaged circuit — and therefore respond to the same treatments.

• T. True
• F. False

Answer: False

These are completely different mechanisms. Reentry requires a circuit with unidirectional block and slow conduction; it is a property of tissue architecture and is cured by ablating the circuit. Abnormal automaticity is a property of individual cells that acquire spontaneous phase 4 depolarization — there is no circuit, just an ectopic pacemaker. Because the mechanisms differ, the treatments differ: catheter ablation targeting the circuit works for reentry; rate control, addressing the triggering metabolic state (e.g., correcting hypokalemia, stopping digoxin), or overdrive suppression addresses automaticity.

Question 5

Why are both unidirectional block AND slow conduction both required for reentry to sustain, and what would happen if only one condition were present without the other?

Model answer: Unidirectional block alone (without slow conduction): the wavefront would travel retrogradely through the blocked pathway and arrive at the far end before the fast pathway has recovered, finding it still refractory — the wavefront extinguishes. Slow conduction alone (without unidirectional block): the wavefront enters both paths simultaneously in the forward direction, they meet at the far end and collide, both extinguish because the entire circuit is refractory simultaneously. For reentry, unidirectional block ensures the circuit can be entered retrogradely, and slow conduction provides the time window for the previously activated tissue to recover excitability before the retrograde wave returns.

Think of it as a timing puzzle: the wavefront must arrive back at its starting point at exactly the right moment — after the tissue has recovered but before another sinus beat activates it. Unidirectional block creates the one-way entry; slow conduction provides the delay. Remove either and the timing fails.