Entrainment
Take control of the circuit to find its exact location.
If resetting is a single, well-timed pebble thrown at a spinning wheel to bump it forward, entrainment is something entirely different. Entrainment is reaching out, grabbing the wheel, and forcing it to spin at exactly your speed, without ever stopping it.
It is continuous resetting.
When we map a tachycardia, a single reset proves the wheel exists. But entrainment proves exactly where the wheel is hiding.
The Concept
While the tachycardia is running, we pace from our catheter at a rate slightly faster than the tachycardia's own native speed.
Our artificial pacing impulses sneak into the excitable gap over and over again, beat after beat. The heart rate speeds up to perfectly match our pacing rate. We have taken complete control of the circuit.
We are driving the car. But the moment we let go of the steering wheel—the moment we stop pacing—the circuit immediately resumes its own native speed.
Constant Fusion
The hallmark of entrainment on the surface ECG is constant fusion. When we take over the circuit, our pacing impulses spread out through the heart muscle, blending with the wavefronts traveling through the normal conduction system.
Because we are entering the circuit at the exact same physical location every single beat, and driving it at a constant speed, the collision of wavefronts happens in the exact same way every time.
The resulting QRS complex is a fusion beat—a blend of our paced wave and the native circuit's wave. As long as we pace, every QRS looks completely identical.
The Magic Trick
The real magic of entrainment happens the millisecond we stop pacing. We watch the clock.
Our last pacing impulse travels from the catheter, enters the circuit, completes one full loop, and then exits the circuit to travel back to our catheter. It is recorded on our screen as the first spontaneous beat of the resumed tachycardia.
The time from our last pacing spike to that first returning electrical signal is called the Post-Pacing Interval (PPI).
The Tape Measure
The PPI is a tape measure. It tells us exactly how far our catheter is from the spinning wheel.
If the Post-Pacing Interval exactly matches the Tachycardia Cycle Length (TCL), our catheter is sitting directly inside the reentrant circuit. The pacing impulse didn't have to commute; it simply spun around the loop and was recorded immediately. We have found our target.
If the PPI is much longer than the TCL, our catheter is sitting far away. The pacing impulse had to commute all the way to the circuit, spin around it, and commute all the way back. The extra time is simply the round-trip commute.
When you map a scar-related ventricular tachycardia (VT), you look for the critical "slow zone"—a tiny, protected isthmus of surviving muscle snaking through dead scar tissue.
When you pace from inside this protected isthmus, a remarkable thing happens: the QRS morphology on the surface ECG matches the clinical VT perfectly. There is no fusion.
Why? Because your paced wave is trapped inside the scar. It can only exit the scar at the exact same breakout point that the native VT uses. The rest of the heart cannot tell the difference between your paced beat and the native arrhythmia. This phenomenon is called concealed entrainment. It is the absolute holy grail of VT mapping. When you see it, you burn there.
Key Takeaways
- Continuous Resetting: Entrainment is taking control of a reentrant circuit by pacing slightly faster than its native rate.
- Constant Fusion: During entrainment, the QRS complex is a consistent blend of the paced wavefront and the native tachycardia wavefront.
- The PPI: The Post-Pacing Interval is the time from the last pacing spike to the first returning native beat.
- The Tape Measure: A PPI equal to the Tachycardia Cycle Length (TCL) proves your catheter is directly inside the circuit.
- Concealed Entrainment: Pacing from a protected isthmus produces a QRS that exactly matches the clinical tachycardia, identifying the perfect target for ablation.