Atrial flutter is typically a regular rhythm. Unlike atrial fibrillation, which produces a chaotic, completely irregular heartbeat, atrial flutter follows a fast but consistent pattern. However, it can sometimes appear irregular depending on how the heart’s electrical signals are filtered on their way to the lower chambers.
Why Atrial Flutter Is Usually Regular
In atrial flutter, an electrical signal gets caught in a loop inside the right side of the heart. Instead of firing once and resetting, the signal circles continuously around a fixed track, passing through a narrow gap between two structures near the base of the heart. This looping circuit fires the upper chambers (atria) at a very fast, steady rate, often around 300 beats per minute.
That rate is far too fast for the lower chambers (ventricles) to keep up. So the connection point between the upper and lower chambers, called the AV node, acts as a gatekeeper. It blocks some of those signals and only lets a fraction through. Most commonly, it lets every second beat through (a 2:1 ratio) or every fourth beat (4:1), producing a ventricular heart rate of roughly 150 or 75 beats per minute. As long as that ratio stays fixed, the pulse you feel is perfectly regular.
When Flutter Becomes Irregular
Atrial flutter can look irregular when the AV node doesn’t maintain a consistent filtering ratio. Instead of blocking every other beat in a steady pattern, it might let two through, then block three, then let one through. This is called variable block, and it produces an uneven pulse that can be harder to distinguish from atrial fibrillation.
Sometimes the AV node cycles through a predictable pattern of gradually slowing conduction before dropping a beat entirely, then starting over. This creates what’s described as “regular irregularity,” a repeating cycle of unevenness rather than true chaos. The key distinction is that in atrial fibrillation, the irregularity is completely random. In atrial flutter with variable block, there’s often a detectable underlying pattern, even if the pulse feels uneven.
How Doctors Tell the Difference
The diagnosis of atrial flutter is based on what the upper chambers are doing, not the ventricular rate or rhythm you can feel at your wrist. On an ECG, atrial flutter produces a distinctive “sawtooth” pattern of sharp, repeating waves that look like the teeth of a saw blade, most visible in certain leads. Atrial fibrillation, by contrast, shows no organized atrial activity at all, just a chaotic, quivering baseline.
This matters because someone with atrial flutter and variable block could have an irregular pulse that mimics atrial fibrillation on a basic pulse check. The ECG resolves the question. The treatment approach overlaps significantly between the two conditions, but atrial flutter has a specific advantage: the looping circuit travels through that narrow gap in the right atrium, making it a good target for catheter ablation. This procedure has a high success rate and low recurrence for typical flutter.
Atrial Flutter vs. Atrial Fibrillation
The simplest way to think about it: flutter is organized, fibrillation is not. In flutter, the atria beat very rapidly but in a regular pattern. In fibrillation, the atria fire from multiple random spots with no coordinated rhythm, producing a completely irregular ventricular response.
The two conditions are closely related and often coexist. After successful ablation of atrial flutter, about one in three patients develops atrial fibrillation within the following 14 months. That number climbs to roughly half in patients who already had fibrillation before their flutter was treated. The stroke risk is also comparable. The rate of blood clots after cardioversion (restoring normal rhythm) is similar for both conditions, which is why anticoagulation guidelines apply to flutter just as they do to fibrillation.
What You Might Feel
When atrial flutter maintains a fixed conduction ratio, you’ll typically notice a heart rate that feels fast but steady. A 2:1 ratio often lands around 150 beats per minute, which can cause palpitations, shortness of breath, lightheadedness, or fatigue. Some people with a 4:1 ratio may have a resting heart rate closer to 75 and feel few symptoms at all.
If the conduction ratio shifts (during exercise, stress, or changes in medication), the rhythm can feel like it suddenly speeds up, slows down, or becomes uneven. This variability is one reason atrial flutter sometimes goes unrecognized or gets confused with other arrhythmias until an ECG confirms the diagnosis.