Atrial Flutter
Atrial Flutter
A macro-re-entrant atrial circuit at 250 to 330/min, with a continuous sawtooth baseline and, most often, every second wave conducted.
On the trace
Read the strip in this order.
- Rate and regularity.
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150 bpm, R-R 400 ms on every beat. A regular narrow tachycardia near 150 should raise flutter before anything else. - Look at the baseline, not the complexes.
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It is never flat. A continuous sawtooth runs under the whole strip, 0.2 mV from peak to trough and dominantly downward in lead II: the troughs drop 0.13 mV below the line and the peaks rise only 0.07 above it. There is no P wave and nowhere to measure a PR interval from. - Look at one wave.
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It falls for 160 ms and snaps back in 40. That asymmetry is the lap itself: the long downslope is the wavefront crossing the cavotricuspid isthmus. - Time it.
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One wave every 200 ms, so 300 a minute, and it does not pause for the QRS. - Count the waves per beat.
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Two. One sits in the gap, the other is hidden inside the QRS and ST segment. That is what 2:1 means, and why the sawtooth is easy to miss here.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| Atrial rate | 250 to 330/min | Cycle length 180 to 240 ms. Slower with age and on antiarrhythmic drugs. |
| Ventricular rate | Atrial rate divided by the ratio | 150 here, from 300 at 2:1. |
| Rhythm | Regular, while the ratio holds | A changing ratio makes it irregular: see the variant below. |
| Atrial waves | Continuous sawtooth | Negative in II, III and aVF when the circuit runs counterclockwise; near flat in I and aVL. |
| Baseline | No isoelectric segment inferiorly | V1 is the exception: it does show a flat interval, with a positive or biphasic deflection. |
| Conduction ratio | 2:1 most often | Even ratios are commoner than odd. 4:1 gives 75, 3:1 gives 100. |
| QRS | Narrow, 76 ms here | Wide only with bundle branch block or pre-excitation. |
The strip
Mechanism
A single wavefront circles a fixed loop in the right atrium, passing each lap through the cavotricuspid isthmus. One lap takes about 200 ms. The loop stays open because a line of conduction block along the posterior right atrium stops the wavefront colliding with its own tail. Because it never stops, neither does the atrial activity, and that is why there is a sawtooth instead of discrete P waves.
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Reading the wave, segment by segment
In counterclockwise typical flutter the inferior leads show a gradual downsloping plateau, then a steep descent, then a sharp ascent with a small terminal positive component that runs straight into the next plateau. Each part is a piece of the lap: the plateau is activation of the isthmus, the steep descent is the atrial septum and left atrium, and the ascent is the lateral right atrial free wall. The same loop can run clockwise, which inverts the pattern and gives a bimodal negative wave in V1. Both are isthmus dependent and both are treated the same way.
Why 2:1 is the ratio that gets missed
At 4:1 there are three flutter waves in plain view between complexes and nobody misses it. At 2:1 there is one visible wave per beat and the other is buried under the QRS and ST segment, where it reads as part of the T wave. What is left looks like an ordinary regular narrow tachycardia at 150. A vagal manoeuvre or adenosine increases the block for a few seconds and exposes the sawtooth.
Drugs that do not work, and one that is dangerous
Class Ia and Ic antiarrhythmics are generally ineffective in flutter. Worse, a class Ic drug can slow the atrial circuit into a range the AV node can follow one for one, giving a ventricular rate near 300, which is why it is never given for atrial fibrillation without an AV nodal blocker alongside.
Flutter is a marker for fibrillation
This is the part that changes long-term management. New-onset typical flutter is associated with a five-fold rise in the ten-year incidence of atrial fibrillation, and fibrillation appears after isthmus ablation in about a quarter of patients who never had it before, rising much higher with long follow-up. Untreated, flutter itself recurs in roughly half of patients within five years.
Clinical impact
Rate control is harder than in fibrillation, because the node is already blocking: pushing it further moves the rhythm between whole-number ratios rather than slowing it smoothly.
Management
Unstable: synchronised cardioversion
Flutter cardioverts at low energy because the circuit is organised, and is a reasonable first step even in the stable patient, given how limited drug conversion is.
Stable: expose the rhythm before treating the rate
A vagal manoeuvre or adenosine increases AV block for a few seconds. It will not stop flutter, but it separates the complexes and shows the sawtooth.
Stable: rate control, knowing its limits
A beta-blocker, or diltiazem or verapamil, though rate control is often hard to achieve. IV ibutilide or dofetilide convert about half to three quarters within the hour, at the cost of QT prolongation and a small risk of polymorphic VT.
Definitive: isthmus ablation, and anticoagulate for life
Isthmus ablation is the most effective rhythm-control therapy and beats antiarrhythmic drugs in randomised trials. Anticoagulation follows the atrial fibrillation rule and continues regardless of current rhythm, because fibrillation so often follows.
Differential
The same circuit with a ratio that keeps changing. The sawtooth is identical; only the R-R intervals move.
Also regular, narrow and around 150 to 180, and the diagnosis 2:1 flutter usually gets given. AVNRT shows no atrial activity between complexes at all.
A focal atrial tachycardia has discrete P waves with a flat baseline between them, because one spot fires and then falls silent. A circuit never falls silent.
Fibrillation has a chaotic baseline with no repeating wave, and an irregularly irregular ventricular response. Flutter's baseline repeats exactly.
References
- Typical Atrial Flutter: A Practical Review — Journal of Cardiovascular Electrophysiology, 2025
- 2024 ESC Guidelines for the management of atrial fibrillation developed in collaboration with the EACTS — European Heart Journal, 2024
- 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020