A wave cannot turn back
When a heart cell fires, it cannot fire again straight away. For a few hundred milliseconds it is refractory: its sodium or calcium channels are inactivated and no stimulus can restart them. Only when the cell has repolarized far enough do those channels recover.
That rule decides where an impulse can go. A wavefront can only advance into resting tissue, and it leaves refractory tissue behind it. So it can never turn round and run back the way it came. When it runs out of resting tissue in front of it, it stops. That is why a normal beat happens once: the impulse sweeps through the heart, everything it has touched is refractory, and there is nowhere left to go until the sinus node fires again.
Watch it on a ring of tissue. A stimulus at the top sets off two waves, one each way round.
A stimulus at the top fires the tissue there. The wave can only move into resting tissue, so it sets off in both directions.
Three things a loop needs
For an impulse to keep going it has to find its way back to tissue that has recovered. That takes three things at once:
Two paths that join into a loop. The ring itself: two routes from one point to another and back.
A block in one direction. If the impulse sets off both ways round, the two halves meet and cancel, as above. It has to be stopped from going one way, so that it travels round the loop in a single direction. The blocked path must still conduct the other way, so the impulse can come back through it. This is unidirectional block.
A trip long enough for the tissue to recover. By the time the impulse gets back to where it started, that tissue must have finished its refractory period. Slow conduction somewhere in the loop buys the time.
Try each one on its own, then both together.
Fire the stimulus and watch where the wave goes.
The wavelength rule
The wavelength of an impulse is how far it travels during one refractory period: conduction speed multiplied by refractory period. It is the length of the refractory tail behind the front. A loop can only sustain itself if it is longer than that tail, so that a stretch of recovered tissue, the excitable gap, is always waiting ahead of the front. Anything that slows conduction or shortens refractoriness shrinks the wavelength and makes re-entry easier. Anything that lengthens refractoriness makes the tail catch up with itself, which is how several antiarrhythmic drugs stop re-entry.
What re-entry explains
A re-entrant rhythm is one beat per lap, so it is fast and regular: the lap time sets the rate and barely changes from beat to beat.
It starts suddenly, usually with a premature beat. A beat that comes early finds one path still refractory, which is exactly the one-way block the loop needs. Arriving later, the same beat would have gone down both paths and cancelled itself. That is why electrophysiologists can start these rhythms on purpose with carefully timed extra beats, and why they are called paroxysmal.
It stops suddenly, because blocking a single lap anywhere in the loop is enough. If the circuit runs through the AV node, a vagal maneuver or adenosine can do it. Electrical cardioversion makes all the tissue in the loop refractory at once. Catheter ablation cuts the loop for good.
Where re-entry lives
A wave circles the right atrium around the tricuspid valve, about five times a second, drawing the sawtooth flutter waves. Atrial flutter
A small loop between a fast and a slow pathway into the AV node. The commonest regular supraventricular tachycardia in adults. AVNRT
Down the AV node and back up an accessory pathway, a strand of muscle bridging atria and ventricles. This is the commonest re-entrant supraventricular tachycardia in dogs. WPW syndrome
Surviving channels of muscle through an old infarct scar conduct slowly and form the loop. Ventricular tachycardia
Key takeaways
Tissue behind a wavefront is refractory, so an impulse cannot turn back and normally dies out once it has activated everything.
Re-entry needs a loop, a block in one direction, and a lap that takes longer than the refractory period of the tissue in the loop.
The wavelength (conduction speed times refractory period) must be shorter than the loop. Slow conduction makes re-entry easier; longer refractoriness makes it harder.
Re-entrant rhythms are fast, regular, start with a premature beat, and stop when one lap is blocked.
Sources this page draws on
The ring experiment and the wavelength idea go back more than a century.
- Mines GR. On dynamic equilibrium in the heart. Journal of Physiology. 1913;46(4-5):349-383.The ring of tissue, unidirectional block, and the first statement of the conditions for circus movement.
- Allessie MA, Bonke FIM, Schopman FJG. Circus movement in rabbit atrial muscle as a mechanism of tachycardia. III. The "leading circle" concept. Circulation Research. 1977;41(1):9-18.Re-entry without an anatomical obstacle and the relation between wavelength and circuit size.
- Wit AL, Rosen MR. Pathophysiologic mechanisms of cardiac arrhythmias. American Heart Journal. 1983;106(4 Pt 2):798-811.Re-entry alongside automaticity and triggered activity as the three arrhythmia mechanisms.
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