ECG Level 2 Tutorial: Rhythm Diagnostic Criteria
Tutorial: Rhythm Diagnostic Criteria
This module will list the basic rhythms, their diagnostic criteria, and their clinical significance .
How to level up?
Develop your skills by completing our Practice Cases!
Tutorial: Rhythm Diagnostic Criteria Atrial Flutter
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Atrial Flutter
Atrial Flutter


 Diagnostic criteria: (boldfaced with * indicates an important feature)
  • Rate: any (usually fast)
  • * Rhythm: regular or irregular
  • * P waves: more P waves than QRS, non-stop "flutter" waves
  • * PR interval: sometimes normal, sometimes appears random
  • QRS: narrow


The origin of atrial flutter is the atria. That is why the word "atrial" is in the title. This rhythm does not come from the sinus node. Atrial flutter is typically caused by a re-entry circuit that is contained within the atria. These re-entry circuits are very fast. They are typically cycling 300 times a minute. If you can see the just the P waves (with the QRS removed), the P waves will look like the teeth of a saw and sometimes called "sawtooth".


Below is an ECG strip that shows the QRS's in the beginning of the strip, but due to a drug (adenosine) being given, they disappear and we see the atrial activity demonstrating many many P waves. These P waves are also called flutter waves or F waves. They are also called sawtooth waves.

sawtooth pattern

The P waves are not always this big. Usually they are smaller. Here is an example of smaller P waves seen in between the QRS complexes:

Can you see the P waves? They are upside down (inverted) in this example. There is something very important to understand: the P waves are like clockwork. They are regular, just as we see in the image above in the blue highlighted region. However, when a QRS complex occurs at the same time as a P wave, the much larger QRS obliterates the P wave so that we cannot see it. The P wave is still there, but it is not easily seen. In the image below, the blue arrows point to P waves and the red arrows point to where we would EXPECT to see a P wave, but due to the QRS complexes, we cannot see them. Notice that the spacing between the red and blue arrows is all constant, indicating a regular rhythm of P waves. 

many P waves, red arrows are hidden P waves

So here is a question for you: if the P waves have a regular rhythm, then why do the QRS's not have a regular rhythm?

The answer relates to the AV node. The P waves are occuring at a rate of close to 300/minutes. The AV node cannot conduct this fast; this is a good thing, because a ventricular rate that fast would not have time to fill, so the heart would be "beating empty", which is very bad. The AV node has a refractory period, during which it cannot conduct impulses. If a P wave hits the AV node when the AV node is refractory, that P wave will be blocked and not conducted to the ventricles.

Sometimes, the AV node is very consistent and blocks the P waves in a pattern. For example, it might block 1 P wave and conduct 1 P wave. This is called 2:1 conduction. It could block 2 P waves and conduct 1 (3:1 conduction) or block 3 P waves and conduct 1 (4:1 conduction). It can also block even more P waves (5:1, 6:1 etc).

Sometimes, the AV node has trouble deciding between 2:1, 3:1 or 4:1 block and will switch between the different blocks. That is what is happening in the ECG strip above. We call this atrial flutter with variable block.

Therefore, atrial flutter could have a regular ventricular rhythm or it could have an irregular ventricular rhythm.

Another abnormality with atrial flutter is the PR interval. There is probably a complicated explanation why the PR interval is sometimes not constant, but sadly, I don't have a good understanding of it. What is important to see is that the PR interval can be variable. In the ECG strips above, the PR interval is constant. However, in the ECG strip below, can you see that the relationship of the QRS is changing relative to the timing of the P waves? In other words, the PR interval is not the same for every QRS:

abnormal PR interval

Therefore, some examples of atrial flutter have a PR interval that is difficult to describe and so could be called "not applicable".

The ventricular conduction in atrial flutter is normal. Therefore, the QRS is narrow.

Clinical Significance:

Atrial flutter is common and sometimes patients with atrial fibrillation can also have episodes of atrial flutter. Causes of atrial flutter are similar to the causes of atrial fibrillation.

Atrial flutter usually results in high heart rates and this is one of the main problems with atrial flutter. The ventricular rate depends on the degree of AV node block. Assuming an atrial rate of 300, a 2:1 block gives a ventricular rate of 150. A 3:1 block gives a rate of 100, and 4:1 block gives a rate of 75.

Patients might present with palpitations (the sensation of their heart beating fast), weakness or reduced physical activity (because their heart rate is very high and the ventricles do not have time to fill properly), feeling of almost passing out (called pre-syncope) due to low blood pressure from the inappropriately high heart rate, or sometimes chest discomfort (angina) because the heart is beating so fast that it is becoming ischemic.

Treatment principles for atrial flutter are similar to the treatment of atrial fibrillation:
  • convert back to sinus rhythm with anti-arrhythmic drugs or electrical cardioversion
  • reduce the ventricular heart rate if too fast, using drugs that reduce AV node conduction (beta blockers or calcium channel blockers)
  • anti-coagulate because the atria do not contract very well and therefore blood can pool and clot in the atria

Here are some examples of atrial flutter:

Note that you might have to look in many different leads before you can clearly see the P waves. Leads II and V1 are usually best to show the P waves, so save some time and look there first.
atrial flutter

atrial flutter

atrial flutter

This one below might be more difficult to see P waves because the conduction is 2:1. One "clue" of atrial flutter is a regular rhythm at a rate of 150. This is because the atrial rate is commonly 300 and 2:1 conduction gives regular ventricular rate of 150.
atrial flutter