7 Differences Between Atrial Fibrillation and Atrial Flutter You Should Know About
Knowing these 7 differences will make you efficient in diagnosing and managing these arrhythmias!
The similarities between AFL and AF
Atrial flutter (AFL) and atrial fibrillation (AF) are similar rhythms, with AF sometimes transitioning to AFL (this is called AF being “organised” to AFL) and vice versa. [1] Although AFL can exist in isolation as an atrial arrhythmia, many patients will subsequently develop AF. [2]
They are both atrial reentry tachyarrhythmias, and they can lead to similar complications: [1]
Rapid ventricular response, potentially leading to rate-related myocardial ischaemia (i.e. MI) (due to oxygen supply-demand imbalance) or arrhythmia-induced cardiomyopathy (i.e. heart failure) (a form of dilated cardiomyopathy caused due to prolonged periods of rapid ventricular rates) [3]
Thromboembolism (stroke or other systemic embolism)
As a consequence of the similar complications, management is also similar:
Rate control
Rhythm control
Anticoagulation (prevention of thromboembolism)
But what is atrial flutter (AFL)?
Atrial Flutter
As mentioned above, AFL is an atrial reentry tachyarrhythmia.
The atrial rate is typically around 300 bpm (range from 240 to 340). [1]
Normal P waves are absent, but you can see the flutter “F” waves. They usually present in a “sawtooth” pattern that is most commonly visible in the inferior leads (II, III, aVF). [1]
The F waves appear regular with a consistent amplitude, duration, morphology, and reproducibility throughout the cardiac cycles. [1]
![Atrial flutter sawtooth pattern Atrial flutter sawtooth pattern](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F981dda6c-cee2-4721-add9-5e18fa7ffd6d_2000x661.jpeg)
AFL is typically seen as a narrow QRS complex tachycardia with a ventricular rate of (almost) exactly 150 bpm. Why?
There are a few different types of AFL, but in the commonest one, and in the absence of AV node dysfunction, there is 2:1 AV conduction. This means that, from the 300 F waves per minute, only half of them get conducted to the ventricles, producing a ventricular response of 150 bpm. [1]
Generally, even atrial-to-ventricular rate ratios (like 2:1 or 4:1 conduction) are much more common than odd ratios (like 3:1 or 5:1). [1]
Flutter waves may sometimes be hidden in the QRS complex in 2:1 conduction. [4]
![Atrial flutter with 2:1 AV conduction ratio Atrial flutter with 2:1 AV conduction ratio](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fb74f6484-9d10-4bec-bdf9-2da76f0e0251_1730x1016.jpeg)
However, it’s possible to see a variable conduction (for example, 2:1 and then 4:1), creating an irregular ventricular response.
Could you misdiagnose AFL with AF in this case because of the irregular ventricular response? I think it’s unlikely because once you start getting 3:1 or 4:1 or even more sparse ventricular conduction ratios, the F waves become prominent (because there is more time from one QRS to the next to see the underlying F waves).
So, would you think the following ECG is AF? Probably not.
![Atrial flutter with a variable AV conduction ratio Atrial flutter with a variable AV conduction ratio](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F9a504595-6280-4197-bf19-84c12ae83a23_858x491.jpeg)
Here’s a trick that you might find helpful when looking at a possible AFL:
Sometimes, the atrial electrical potential is too small, and you cannot see the F waves in the standard 12-lead transthoracic ECG. Try increasing the gain of the ECG (increase the voltage setting from 10 mm/mV to 20 mm/mV), and you might see the F waves better!
Despite the similarities between AF and AFL, there are some important differences.
The differences between AF and AFL
ECG features
1. Atrial rate
The atrial rate in AFL is lower (around 300 bpm), whereas in AF, it’s higher (350-600 bpm). [4][5]
2. Atrial wave morphology
In AFL, as mentioned above, the F waves are regular and have a consistent amplitude, duration and morphology. In AF, on the other hand, the f waves are all over the place: they have different morphologies, duration and amplitude.1
3. Ventricular response
The ventricular response in AFL is usually regular at 150 bpm, while in AF, it's characteristically irregularly irregular.
Even when there’s irregular ventricular conduction in AFL, an underlying pattern is present: each RR interval is going to be an integral multiple of the duration of a PP interval (or, more accurately, FF interval, as there are no P waves in AFL).
So, the duration of each RR interval is discretely spaced in units of one big square, as one big square is typically the space between two F waves.
This kind of “quantisation” is present in AFL but not in AF. In AF, each RR interval is completely random!
Clinical features
4. Prevalence
AFL is uncommon… whereas AF is the most commonly treated arrhythmia, as mentioned in a previous post.
5. Causes
When I was a medical student, a Cardiology SpR asked us which rhythm we would prefer to have: AF or AFL (“none” was not an option). Knowing that AF is more difficult to convert to sinus rhythm (see below), we thought AFL was the right answer.
That’s when he explained to us that AFL is uncommon in a structurally normal heart. [1]
So, he said, “If you have AFL, something is probably wrong with your heart (and you should definitely have an echo). Whereas you could develop AF even with a healthy heart.”
It was a trick question…
Management
6. Rate control
AFL is more difficult to rate control than AF… [1]
7. Rhythm control
…but, as mentioned earlier, cardioversion is usually more effective in AFL. [2]
Summary
Here’s a table summarising the 7 important differences between AF and AFL:
Key takeaways:
The main ECG difference between the two rhythms is the regularity of the ventricular response. In AF, you have the characteristic irregularly irregular ventricular response.
AFL indicates that there’s a structural cardiac abnormality. Whereas AF can occur in a structurally healthy heart (check out: Holiday Heart Syndrome).
AFL is much less common than AF.
Thanks for reading
References
Robert Phang, Jordan Prutkin. Overview of atrial flutter. Post TW, ed. In: UpToDate, Post, TW (Ed), UpToDate, Waltham, MA, 2022.
Gerhard Hindricks and others, 2020 ESC Guidelines for the diagnosis and management of atrial fibrillation developed in collaboration with the European Association for Cardio-Thoracic Surgery (EACTS): The Task Force for the diagnosis and management of atrial fibrillation of the European Society of Cardiology (ESC) Developed with the special contribution of the European Heart Rhythm Association (EHRA) of the ESC, _European Heart Journal_, Volume 42, Issue 5, 1 February 2021, Pages 373–498, [https://doi.org/10.1093/eurheartj/ehaa612](https://doi.org/10.1093/eurheartj/ehaa612)
Cynthia Tracy. Arrhythmia-induced cardiomyopathy. Post TW, ed. In: UpToDate, Post, TW (Ed), UpToDate, Waltham, MA, 2022.
Jordan Prutkin. Electrocardiographic and electrophysiologic features of atrial flutter. Post TW, ed. In: UpToDate, Post, TW (Ed), UpToDate, Waltham, MA, 2023.
Goodacre S, Irons R. ABC of clinical electrocardiography: Atrial arrhythmias. BMJ. 2002 Mar 9;324(7337):594-7. doi: 10.1136/bmj.324.7337.594. Erratum in: BMJ 2002 Apr 27;324(7344):1002. PMID: 11884328; PMCID: PMC1122515.
In AF, the f waves can have such small differences between each other that they may appear to be the same, leading to confusion with AFL. To differentiate between the two, it is crucial to check for the ventricular response pattern. If the ventricular response is irregularly irregular, then it is more likely that the rhythm is AF.