Atrial Fibrillation and Atrial Flutter

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Teaching Points:

  • All you need to know about the ECG diagnosis of atrial fibrillation and atrial flutter

Atrial fibrillation

Atrial fibrillation is the most common form of arrhythmia. It may be intermittent or permanent.

So, what do we see in the ECG in atrial fibrillation?

As mentioned before, atrial fibrillation involves a completely chaotic spread of excitation. Regular P waves are missing. Instead, one can usually see small waves of different sizes, the so-called fibrillatory waves. These are most obvious in lead V1. However, remember that these waves sometimes look like regular P waves or the depolarizations that occur in a regular atrial tachycardia.

Two findings indicate real fibrillatory waves:

1. You see a few waves that might be P-waves or fibrillatory waves. In the case of atrial fibrillation, no clear iso-electric line between any two of these waves can be defined

2. In general, if there is a P-Wave, you can measure the PQ or PR interval. In case of a misinterpretation of fibrillatory waves as a real P, these PR intervals would be completely irregular. Completely irregular PR intervals are suggestive of fibrillatory waves.

Rarely, the amplitude of the fibrillatory waves is so low that they cannot be seen. In this event, distinguishing between these waves and artifact-related fluctuations of the isoelectric line can be difficult.

Fortunately, another criterion can be used to help us diagnose atrial fibrillation: the absolute irregularity of the distances between the QRS complexes.

Depending on the current status of the AV node, the spread of excitation into the ventricles may occur with a normal heart rate, bradycardia, or tachycardia.

There is one pitfall in patients with atrial fibrillation and severe tachycardia. A so-called pseudo-regularity may be found, starting at heart rates of above 160-180 bpm, which is usually attributable to measurement inaccuracies. When just looking at the ECG, the R-R-intervals appear to be regular. Particularly in the case of very high ventricular rates, the R-R intervals must be measured precisely. In the case of tachycardia, fibrillatory waves are rather difficult to distinguish, because the T wave fills almost the entire period of diastole. The most important differential diagnoses for the regular narrow complex tachycardias are atrial tachycardias, an AVNRT, and an AVRT. We will talk about these extensively in a later chapter. For now, just remember that apart from atrial fibrillation, when assessing a patient with tachycardia, you must keep in mind these three important differentials: atrial tachycardia, AVNRT, and AVRT.

Now back to atrial fibrillation: Let’s have a look at one of the most tricky cases of atrial fibrillation: this is if the heart rate is tachycardiac, and there is a bundle branch block or aberrancy.

In this case, the QRS complex is wide, that is, it is wider than 120 ms. If the heart rate is very fast, the R-R-intervals may seem regular. In the event of a regular tachycardia with wide QRS complexes, we must instantly consider the life-threatening possibility of a ventricular tachycardia. There is no easy way to rule this out, and in clinical practice, you may well come upon cardiologists discussing such an ECG without reaching any quick consensus. We’ll consider the diagnosis of a VT in detail in the next chapters.

In case of an emergency, there is one simple rule: if your patient is hemodynamically unstable, it doesn’t really matter if you are dealing with a VT or rapid atrial fibrillation: the rhythm has to go. So you go cardiovert it!

In case of a normal heart rate, atrial fibrillation with aberrancy has such irregular R-R intervals that fibrillatory waves are usually visible. Here, both criteria are met, and the diagnosis atrial fibrillation can be made with a high degree of certainty.

Special case: cardioconversion

If a conversion from atrial fibrillation back to sinus rhythm actually happens, the sinus node normally resumes its role as the major pulse generator. However, if the sinus node needs a little more time to re-start following conversion, so-called pre-automatic pause can occur, together with some rather scary periods of asystole. Depending on the nature and duration of the symptoms, this may be an indication for pacemaker implantation. More information on this is provided in the chapter on sinus node dysfunction.

Special case: atrial fibrillation with complete heart block

If third degree AV block and atrial fibrillation occur at the same time, the ECG criteria of the two pathologies merge. Unfortunately, there are also some that disappear.

Remember, there are two criteria for the diagnosis of atrial fibrillation: fibrillatory waves and irregular R-R-intervals. In third degree AV block, no conduction of atrial impulses to the ventricles occurs. Instead, a regular ventricular rhythm originates from a position below the AV node. This ventricular rhythm is slower, and has a wider QRS complex, than a regular atrial rhythm.

Third degree AV block is defined by AV dissociation, meaning there are regular P-waves and regular QRS-complexes but without any connection to each other.

This ECG sign is, of course, missing in atrial fibrillation, because there are no regular P-waves in the first place.

In many cases, an implanted pacemaker will be found in this constellation, which stimulates the ventricles continuously.

It is then impossible to determine in a 12-lead ECG tracing whether total AV block exists, or whether atrial fibrillation is present but at a rate that is slower than that provided by the pacemaker. Nevertheless, in terms of patient management, it is still important to know if the patient is in atrial fibrillation, since this can influence treatment. For example, if atrial fibrillation is present, the patient may require anticoagulation. It is very easy to solve this question: ask your pacemaker specialist. In most cases, they will be able to tell you what the rhythm is after a quick look at the pacemaker software.

Atrial flutter

Let’s look more closely now at  atrial flutter. In general, atrial flutter is much rarer than atrial fibrillation. However, patients with both atrial fibrillation and atrial flutter are seen relatively frequently.

Around 25% to 35% of patients with atrial flutter also experience atrial fibrillation at some point! Since these are two different heart rhythms, only one can be present at any given time, of course.

Atrial flutter may be intermittent or permanent.

In contrast to the chaotic excitation that occurs in atrial fibrillation, atrial flutter is usually characterized by a regular rhythm. Atrial flutter is a macro-reentry tachycardia. Typical atrial flutter of the so-called "common type" or of the "reversed common type" always involves a circling of excitation around the tricuspid valve. Circulating excitations around other atrial structures, such as the mitral valve, are called atypical atrial flutter.

In the ECG, we see the classic flutter waves instead of a P wave. If you look at these closely, you will notice several typical characteristics:

1. There is no isoelectric line! This makes sense, because the pre-excitation never stagnates at any time. There are always portions of the atria which are only just depolarizing.

2. The flutter waves look like the blade of a saw: each shaft forms a so-called "saw-tooth".

3. In typical flutter of the so-called "common type", the waves in leads II, III, and aVF are negative. In the "reversed common type" , the waves are positive in leads II, III and aVF.

4. Atrial flutter waves have a rate of approx. 250-300 per minute.

5. The conduction of atrial excitation to the ventricles is slowed down by the AV node. Ratios of 2:1, 3:1, or more, are usually observed, meaning that 2, 3, or more atrial excitations are followed by one ventricular excitation. You can determinate the conduction ratio by counting the P-waves that you see in front of each QRS complex. Sometimes the conduction rate changes from one QRS complex to another. This alternating conduction can be of a 2:1 and 3:1 type, meaning that there are two atrial flutter waves in front of the first, and three atrial flutter waves in front of the second QRS-complex. Take care: this pattern can be misinterpreted as supraventricular premature beats or even as atrial fibrillation, since the intervals of the QRS complexes seem to be irregular. If you suspect atrial flutter, it is always worth measuring the distances of the QRS complexes to look for a possible underlying regular pattern.