One of the most frequently misdiagnosed rhythms, atrial flutter with 2:1 conduction often masquerades as sinus tach.  Sinus tach usually has an obvious cause, such as exercise, severe hypovolemia, or age less than 6 months.  Atrial flutter usually produces flutter waves (P waves) at a rate of 250 - 350 per minute.  Therefore, a 2:1 conduction ratio would result in a heart rate of about 125 - 175 bpm).  Often, students are taught about atrial flutter using an electronic rhythm generator or a book with limited illustrations, and they become acustomed to seeing atrial flutter with 3:1 or 4:1 conduction.  The flutter waves are very easy to see in such a situation.  However, the AV node, if not affected by medication, is usually well able to conduct at a rate of 150 or more.  Therefore, the physiological block that protects us from extreme rates will keep the heart rate around 150 bpm in atrial flutter.

This ECG shows two instances where the conduction ratio slows to 3:1 momentarily, at beats no. 3 and 20.  This makes the atrial flutter more apparent.  Students should be taught to check multiple leads in any patient with a heart rate of between 125 bpm and 175 bpm, and look for flutter waves.  Flutter waves are continuous - they do not "pause" for the QRS.  For a rhythm strip with the flutter waves highlighted, see this patient's rhythm strip.