By Ken Grauer, MD
Professor Emeritus in Family Medicine, College of Medicine, University of Florida
Dr. Grauer reports he is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.
The rhythm in the figure below was diagnosed as atrial flutter. Do you agree? If so, is there anything unusual about this rhythm strip?
Atrial flutter is characterized by a special pattern of regular atrial activity that in adults almost always occurs at a rate of 300/minute (250-350/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 usually will be close to 150/minute (i.e., 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 (i.e., 1:1: or 3:1 or 5:1) are possible but extremely uncommon unless the patient is on antiarrhythmic medication or has Wolff-Parkinson-White syndrome.
Atrial flutter typically manifests a sawtooth appearance that usually is best seen in the inferior leads. That said, flutter waves sometimes may be subtle and only seen in a handful of leads (if at all). Distinction between atrial tachycardia and atrial flutter 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.
The ventricular response in the figure is regular at a rate of ~85/minute. Regular atrial activity is seen, but instead of two P waves for each QRS, there are three P waves for each QRS complex. 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 (i.e., one out of every three P waves seen within each R-R interval is conducted to the ventricles).
Since we know that there are three 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 ~255/minute, which is above the usual range for atrial tachycardia. As a result, the rhythm in the figure most likely represents the unusual case of atrial flutter with the odd conduction ratio of 3:1.