ECG Blog #137 (SVT – AV Block – Atrial Flutter – Atrial Tachycardia – Atrial Fibrillation)

The rhythm in Figure-1 was diagnosed as AFlutter (Atrial Flutter). Do you agree? If so — Is there anything unusual about this rhythm strip?

Figure-1: Lead MCL1 rhythm strip. Is this typical atrial flutter? NOTE — Enlarge by clicking on the Figure.

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Interpretation: Atrial flutter is characterized by a special pattern of regular atrial activity that in adults almost always occurs at a rate of 300/minute (250-to-350/minute range). The most common ventricular response to atrial flutter (by far! ) — is with 2:1 AV conduction. As a result, the ventricular rate with untreated atrial flutter will usually be close to 150/minute (ie, 300÷2) — although the ventricular rate may be slower if the patient is taking antiarrhythmic drugs.

• Less commonly with atrial flutter — there is 4:1 AV conduction (ventricular rate ~75/minute) — or a variable ventricular response.
• Odd conduction ratios (ie, 1:1: 3:1; 5:1) are possible, but extremely uncommon unless the patient is on antiarrhythmic medication or has WPW (Wolff-Parkinson-White) Syndrome.
• Atrial flutter typically manifests a sawtooth appearance that is usually best seen in the inferior leads. That said, flutter waves may sometimes be subtle and only seen in a handful of leads (if at all).
• Distinction between ATach (Atrial Tachycardia) and AFlutter may be difficult. This is especially true when the characteristic sawtooth appearance of flutter is missing, and the rate of atrial activity is at least slightly below the usual range for flutter.
• NOTE — Practically speaking, distinction between ATach and AFlutter matters little in the acute setting — since initial treatment of these 2 entities is similar (ie, use of AV nodal rate-slowing medication). There is also an area of "overlap" between atypical forms of AFlutter and ATach — for which distinction lies in the realm of the EP cardiologist.

The ventricular response in Figure-1 is regular at a rate of ~85/minute. Regular atrial activity is seen — but instead of 2 P waves for each QRS, there are 3 P waves for each QRS complex (Figure-2). Note that the PR interval preceding each QRS complex is the same! This tells us that there is conduction — in this case with a 3:1 ratio (ie, only one out of every 3 P waves seen within each R-R interval is being conducted to the ventricles).

• Since we know that there are 3 times as many P waves as QRS complexes in this example — the easiest way to accurately calculate the atrial rate is to multiply the ventricular rate (85/minute) by 3. This yields an atrial rate of ~255/minute, which is above the usual range rate for atrial tachycardia (which generally doesn’t exceed 240/minute). As a result, the rhythm in Figure-2 most probably represents the unusual case of AFlutter with an odd conduction ratio (here with 3:1 AV conduction).

Figure-2: We have labeled Figure-1 by adding red arrows for the regular atrial activity. There are 3 P waves for each QRS complex (See text).

PEARL: There will usually be “something else” going on medically with the patient when you encounter AFlutter with an odd conduction ratio. Many such patients will be receiving one or more antiarrhythmic drugs (that may slow the flutter rate and affect the AV conduction ratio) — or perhaps the patient has already undergone ablation for one or more previous episodes of AFlutter.

• The unusual situation of AFlutter with 1:1 AV conduction (ventricular rate close to 300/minute) — should lead one to inquire IF the tracing is from a child who may have congenital heart disease and/or a patient of any age with WPW. Bottom Line: Non-cardiologists will probably only rarely see AFlutter with an odd conduction ratio, if at all.

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For More on this Subject:

• See also Section 14.4 (from our ACLS-2013-ePub) — on Atrial Flutter.

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