Resetting
Prove the circuit exists by sneaking inside it.
Imagine a spinning wheel. If you throw a pebble at the wheel from far away, it might bounce off. But if you throw it at just the right time, it might bump the wheel forward, advancing its rotation, so the next spoke arrives earlier than expected.
The wheel hasn't stopped spinning; you just reset its clock.
This same principle applies to the heart. When we map a tachycardia, we are looking for a spinning wheel—a reentrant circuit. To prove that it truly is a wheel, and not just a single spot firing rapidly, we throw a well-timed electrical pebble at it.
The Excitable Gap
During a reentrant tachycardia, an electrical wave spins continuously in a circle. But it is not a solid ring of fire. There is a "tail" of recovering tissue right behind the wave front. We call this the excitable gap.
Resetting only works if this gap exists. The tissue here is fully recovered and waiting for the next wave. If you pace from a catheter and your artificial impulse can sneak into that excitable gap, you enter the circuit.
Advancing the Tachycardia
To reset the circuit, you deliver a single premature extra-stimulus (a pacing beat) while the tachycardia is running. The timing has to be perfect.
If your paced beat enters the circuit, travels around it, and causes the next beat of the tachycardia to happen earlier than it normally would have, you have "reset" the tachycardia.
You didn't stop it. You didn't start a new one. You just bumped the spinning wheel forward. The morphology of the tachycardia remains identical; you simply shifted its clock.
Proving Reentry
This simple maneuver is incredibly powerful. Resetting is the absolute proof that an arrhythmia is a reentrant circuit, not an automatic focus.
Think about an automatic focus—a cluster of cells firing on their own like a rogue metronome. If you try to pace an automatic focus early, you typically reset its internal pacemaker completely, or you turn it off entirely through overdrive suppression. You cannot reliably and predictably "advance" the focus cycle by cycle.
But a reentrant circuit? A reentrant circuit can always be reset, as long as you can reach the excitable gap. The wave is forced to adopt the new timing of the premature beat.
When you deliver a premature beat that resets the tachycardia, the time it takes for the next spontaneous beat to arrive is called the return cycle.
If you pace from far away, the return cycle is long. The impulse had to travel from your catheter to the circuit, spin around, and travel back out to your recording catheter.
But if you pace from inside the circuit, the return cycle perfectly matches the tachycardia's own native speed (its cycle length). This concept is the foundation of mapping: the closer your return cycle is to the tachycardia cycle length, the closer your catheter is to the heart of the circuit.
Key Takeaways
- The Concept: Resetting is like bumping a spinning wheel forward—you advance the timing of the tachycardia without stopping it.
- The Excitable Gap: Resetting requires an excitable gap, a segment of fully recovered tissue within the reentrant loop.
- The Proof: Advancing the tachycardia with a premature beat is the definitive proof that the mechanism is reentry, not automaticity.
- The Method: You deliver a single perfectly timed premature extra-stimulus that sneaks into the circuit.
- The Return Cycle: The time it takes for the next spontaneous beat to arrive tells you how far your pacing catheter is from the circuit itself.