A Tricky Conversion (by Arnel C.)

A patient admitted with atrial fibrillation (AF) was later noted to have this. Do you see conversion to sinus rhythm?

Image 1 – ECG case

Image 2 - ECG case marked with arrows to highlight PR interval variation

This is a regular narrow QRS complex rhythm with a ventricular rate of about 80’s. Variation in the PR interval can be observed as marked with arrows. So, is this sinus rhythm with variable PR interval? This pattern cannot be explained by dual AV node conduction or concealed conduction. In dual AV node conduction, there are 2 pathways in the AV node which are designated as slow pathway (SP) and fast pathway (FP). In dual AV node conduction with dual AV node physiology, we should see 2 PRI’s (one short and one long) and the PRI change is usually sudden (Image 3).

Image 3- Sinus rhythm with dual AV node physiology. After QRS #5, there is sudden prolongation of PRI (~520 ms).

So can this be atrial flutter or atrial tachycardia (AT)?

Atrial flutter (AFL) and atrial fibrillation (AF) can occur in the same person. They can appear on the same electrocardiogram as atrial flutter-fibrillation or “impure atrial flutter”. According to Braunwald’s Heart Disease – A Textbook of Cardiovascular Medicine ( 10^th ed) -

“Atrial fibrillation (AF) is a supraventricular arrhythmia characterized electrocardiographically by low-amplitude baseline oscillations (fibrillatory or f waves) and an irregularly irregular ventricular rhythm. The f waves have a rate of 300 to 600 beats/min and are variable in amplitude, shape, and timing. In contrast, flutter waves have a rate of 250 to 350 beats/min and are constant in timing and morphology. In lead V1, f waves sometimes appear uniform and can mimic flutter waves (Image 4). The distinguishing feature from atrial flutter is the absence of uniform and regular atrial activity in other leads of the electrocardiogram.”

Image 4 – An example of atrial fibrillation with prominent f waves in V1 that mimicked atrial flutter. The typical f waves can be seen in lead II.

Treatment of atrial flutter with digitalis (digoxin) shortens the atrial refractory period and often converts atrial flutter to atrial fibrillation. Conversely treatment with sodium channel-blocking drugs (quinidine or procainamide) often converts atrial fibrillation to atrial flutter as transitional stage before restoration to sinus rhythm. During transition the flutter cycle tends to be irregular and the flutter morphology is variable.

Patients with markedly enlarged atria (and massive dilatation) tend to have slower rate or atrial flutter that could have rates of less than 200 beats per minute. Patients on antiarrhythmics can also decrease the atrial flutter rates. The resulting decrease in the atrial flutter rate will reveal the isoelectric interval we typically see in focal atrial tachycardia.

Atrial tachycardia (AT) is defined as a regular atrial rhythm originating from the atrium at 100 bpm to 240 bpm. As mentioned above, atrial flutter can look like atrial tachycardia if patients are on antiarrhythmics or with atrial myopathy. Atrial tachycardia in a scarred atrium can be rapid and mimic atrial flutter. So, it is a matter of semantics to define AT or AFL based on surface ECG features.

How to prove that this is not sinus rhythm but either atrial flutter vs atrial tachycardia?

1.      Heart Rate Histogram or Heart Rate Trend

Image 5 – Heart Rate Histogram of the case

The Heart rate histogram is the graphical representation of the heart rate over time. It has a number of uses in cardiac telemetry. It can guide us that a rhythm could be atrial tachycardia or atrial flutter rather than sinus rhythm. Atrial tachycardia or atrial flutter will have a flat histogram (Image 5 and 6).

Image 6 – A “flat” histogram (from a GE system) from a patient with atrial flutter. The heart rate was 120’s for several hours. A histogram of sinus rhythm will show variations in heart rate.

2.      The Role of a premature ventricular complex (PVC) in arrhythmia diagnosis

A PVC can unmask a “hiding P wave”. In the case, the “hidden” P (arrows) wave was revealed by a properly time PVC. The atrial rate was about 187 bpm (Image 7).

Image 7 – After a wide QRS beat, 2 distinct P waves can be seen at a rate of about 187 bpm.

3.      The drop in rate

If you follow the heart rate histogram where the heart rate decreased, you can see the strip below. This further supports that case is not sinus rhythm but can either be atrial tachycardia vs. atrial flutter (Image 8). The atrial rate is about 187 bpm.

Image 8 – The arrow shows distinct P waves in leads II and V with a rate of about 187 bpm.

Back to the case

So, in the case presented is not sinus rhythm (no conversion) but could either be atrial flutter (slow) vs. atrial tachycardia with 2:1 AV conduction.

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