ECG Level 2 Tutorial: Rhythm Diagnostic Criteria
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Tutorial: Rhythm Diagnostic Criteria
This module will list the basic rhythms, their diagnostic criteria, and their clinical significance .
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Tutorial: Rhythm Diagnostic Criteria Atrial Fibrillation
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Atrial Fibrillation
Atrial Fibrillation (also known as "AF" or "a. fib")

Diagnostic criteria: (boldfaced with * indicates an important feature)
  • Rate: any
  • * Rhythm: irregular with no pattern
  • * P waves: none - a wavy chaotic baseline is present
  • * PR interval: no P waves
  • QRS: narrow
Do not confuse atrial fibrillation with ventricular fibrillation. They are very very different. 


The origin of atrial fibrillation is the atria. That is why the word "atrial" is in the title. This rhythm does not come from the sinus node. One way to think of atrial fibrillation is that there are numerous little chaotic electric circuits within the atria, each forming a random pacemaker:
multiple atrial re-entry circuits

Each of these pacemakers is firing off signals at random down to the AV node. Because these signals are all random, the "baseline" of the ECG appears to be a wavy chaotic baseline. There is no isoelectric baseline in atrial fibrillation.

wavy baseline

Also, there are no P waves. At random locations, it might appear like the occasional P wave can be seen, but these are not P waves; they are random changes in the chaotic baseline that simply mimic a P wave. Do not be tricked. You are smarter than that!

The number of electrical signals being generated in the atria is very high; as high as 300-500 per minute. This is far more than the AV node can conduct, so many of these impulses are blocked by the AV node. This is a good thing; if the AV node conducted all of them, the ventricular rate would be very high and would not have enough time to fill with blood between heartbeats. This would result in an empty beating heart with no blood being ejected.

Frequently, the heart rate with untreated atrial fibrillation is high.

Because these chaotic impulses from the atria bombard the AV node in a random pattern, the AV node conducts a fraction of these impulses also in a random pattern. This is why the ventricular rhythm in atrial fibrillaiton is irregular with no pattern.

Clinical Significance:

Atrial fibrillation is very common. It is more common in older patients.

Some conditions associated with an increase in atrial fibrillation:
  • any structural heart abnormalities, especially valve diseases and enlarged atria
  • hypertension
  • acute myocardial infarction
  • hyperthyroidism
  • alcohol consumption
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, calcium channel blockers, and digoxin)
  • anti-coagulate because the atria do not mechanically contract and therefore blood can pool and clot in the atria


atrial fibrillation

In this example, the rate is slower (probably with drugs) and the wavy baseline is easily seen.
Do you see P waves? No, you do not. Even if you think you do, you still do not.
atrial fibrillation

In this example, the wavy baseline is much less pronounced, but still present. The rhythm is irregular with no pattern so if you thought this looked like a junctional rhythm, the rhythm is a very important detail to help you differentiate them.
atrial fibrillation

This is very fast atrial fibrillation.
atrial fibrillation

So it this. The rhythm almost looks regular, but it is not when you look closely.
atrial fibrillation

The QRS is wide in this example of atrial fibrillation. There are also T wave and ST segment abnormalities. Note that a wide QRS is not a requirement for the diagnosis of atrial fibrillation, but a second problem that causes a wide QRS can co-exist with atrial fibrillation. Therefore, atrial fibrillation does not always have a narrow QRS, but when the QRS is wide, it is caused by a second diagnosis.
atrial fibrillation