Atrial Flutter with Variable Block

Atrial flutter with variable block is the same circuit as ordinary flutter, running at the same rate, but the AV node no longer settles on one conduction ratio. It conducts two waves, then three, then four, so the ventricular rhythm is irregular while the atrial rhythm is as metronomic as ever. This is the presentation most often mislabelled atrial fibrillation, and the mistake matters: the two rhythms have different ablation targets.

On the trace

Read the strip in this order.

1. The ventricles are irregular. R-R intervals here run from 400 ms to 800 ms, a doubling. Mean rate about 100 bpm, but the mean hides the swing.
2. The atria are not. The sawtooth underneath is unchanged: one wave every 200 ms, 300 a minute, marching straight through the irregular complexes without pausing or varying. Two rhythms, and only one of them is irregular.
3. Measure the R-R intervals. 400, 600 or 800 ms. Nothing in between, ever. Each one is a whole number of 200 ms flutter cycles, because the node can only conduct a wave or block it.
4. Count the flutter waves in each gap. Two, three or four, and the count is exactly the interval divided by 200. The number changes from beat to beat; the wave itself never does.
5. Compare with fibrillation. This is the test that matters. An irregular rhythm whose intervals land on multiples of one number is flutter. One whose intervals are smeared across a continuum is fibrillation.

How to recognise it

The strip

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Mechanism

Nothing has changed in the atrium. The same circuit runs the same lap at the same rate. What varies is the AV node, whose refractoriness drifts with vagal tone, circulating catecholamines and rate-slowing drugs. When it recovers in time it conducts the next flutter wave; when it does not, that wave is blocked and the ventricle waits another full cycle. The ratio is therefore always a whole number, and so is every R-R interval.

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Why this gets called atrial fibrillation

An irregular narrow-complex rhythm at about 100 fits the pattern most readers reach for first, and at faster ventricular rates the gaps are short enough that the sawtooth is hard to resolve. The tell is not the irregularity but its structure. Research modelling AV conduction has shown the two can be told apart from the ventricular response alone, because flutter's R-R sequences follow rules while fibrillation's are absolutely irregular, though the separation is imperfect and better at slower rates. On a real strip, looking at the baseline is faster and more reliable than analysing the intervals.

Why the distinction is worth the effort

Anticoagulation is assessed the same way for both, so that decision is safe either way. The ablation target is not. Typical flutter is cured by a line across the cavotricuspid isthmus, a short and highly effective procedure. Fibrillation needs pulmonary vein isolation. A patient labelled with the wrong rhythm is offered the wrong procedure.

Wenckebach inside the node

Variable block is often not random. The node can go through Wenckebach cycles of its own, conducting with progressively longer delay until one wave drops, which produces repeating groups of beats rather than an even scatter of intervals. Group beating in an irregular narrow rhythm is another pointer away from fibrillation.

What this strip does not show

The ratios here are drawn independently for each beat, so the strip shows the quantisation cleanly but not the grouped, repeating patterns that Wenckebach periodicity produces in a real patient.

Clinical impact

The atrial circuit runs unchanged at about 300/min

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AV nodal refractoriness drifts, so the conduction ratio changes beat to beat

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Irregular and narrow reads as atrial fibrillation, and the sawtooth goes unnoticedMisdiagnosis

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The patient is offered pulmonary vein isolation instead of an isthmus lineWrong Target

Rate assessment is also harder here than in fixed flutter: a mean rate of 100 can be made of beats at 150 and beats at 75, and neither number is the one the patient feels.

Management

1

Settle the diagnosis first

Look at the baseline before treating the rate. If the gaps are too short to read, a vagal manoeuvre or adenosine will lengthen them and expose the sawtooth.

2

Unstable: synchronised cardioversion

As for any flutter, and at the same low energies, because the underlying circuit is organised whatever the node is doing.

3

Stable: rate control, and look for the cause of the drift

A beta-blocker, or diltiazem or verapamil. Check for the things that move nodal conduction: drug changes, sepsis, thyroid disease, digoxin.

4

Definitive: ablate the circuit, not the node

The variable block is a property of the AV node, not a second arrhythmia. Cavotricuspid isthmus ablation treats the flutter itself, and anticoagulation is assessed as it is in atrial fibrillation.

Differential

Atrial Fibrillation (AFib)

The rhythm this is mistaken for. Fibrillation has a chaotic baseline with no repeating wave, and its R-R intervals fall anywhere rather than on multiples of a single number.

Atrial Flutter

The same circuit with a ratio that holds steady, so the ventricular rhythm is regular. The baseline is identical.

Focal Atrial Tachycardia

A focal atrial tachycardia can also conduct with varying block, classically in digoxin toxicity. Its atrial waves are discrete, with a flat baseline between them.

References

1. Atrial Flutter, Typical and Atypical: A Review — Arrhythmia & Electrophysiology Review, 2017
2. Discriminating atrial flutter from atrial fibrillation using a multilevel model of atrioventricular conduction — Heart Rhythm, 2014
3. 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020