Some tell tale signs are:
- Flutter waves, in this case best noted in lead III
- Regular rhythm. Unless a patient is in flutter with variable block, patients with flutter characteristically increase or decrease their ventricular rate by stepwise fractions of the atrial rate, classically from 300 beats/min (1:1 conduction), 150 (2:1), 100 (3:1), or 75 (4:1). Of note, the atrial depolarization rate of atrial flutter may be substantially less than 300 beats/min especially in patients on antiarrhythmic drugs or with structural heart disease (as was the case with this patient who had 2:1 conduction and a rate of 123).
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| Atrial flutter. Flutter waves noted with red line. Click image to enlarge. |
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| Once the tachyarrhythmia was slowed down, it was much easier to appreciate the flutter waves. Click image to enlarge. |
Ectopic atrial tachycardia with block (albeit rare) can look very similar to atrial flutter with block. One way to distinguish the two is to examine whether there is an isoelectric interval between P waves in all leads. If there is, this suggests ectopic atrial tachycardia rather than flutter which is caused by a macroreentry mechanism which creates one F-wave immediately after another with no isoelectric interval. Furthermore, with ectopic atrial tachycardia there is generally beat to beat and long term variability in the rhythm which excludes a reentrant mechanism such as flutter.
Source
Goldberger: Clinical Electrocardiography: A Simplified Approach, 7th ed.
the ecg blog. Ectopic Atrial Tachycardia with Variable AV Block. http://ecgblog.wordpress.com/2009/02/03/ectopic-atrial-tachycardia-with-variable-av-block/