Atrial flutter is an uncommon arrhythmia in the pediatric population, particularly in those without congenital heart disease. Atrial flutter is easily missed in this age group because of its rarity and the fact that it can masquerade as other rhythms depending on the extent of atrioventricular (AV) conduction. Atrial flutter is characterized on electrocardiogram by the appearance of flutter waves, which classically have a “saw tooth” pattern (Fig. 121.1). Flutter waves typically have an atrial rate of 240 to 360 beats per minute (bpm), but the rate can be slower. The ventricular response to atrial flutter can be highly variable depending on the conduction through the AV node. The ventricular response is typically described as the ratio of flutter waves to conducted QRS complexes (2:1, 3:1, 4:1, etc.). The most common AV conduction ratio is 2:1. In atrial flutter, the QRS complexes have a normal morphology but occur at a fraction of the atrial flutter rate depending on the degree of AV block.

When significant AV nodal block is present, the diagnosis of atrial flutter can be readily made because the flutter waves on electrocardiogram are more evident between the conducted QRS complexes. When there is less AV nodal block, such as 2:1 or 3:1 conduction, the flutter waves can be harder to differentiate (Fig. 121.2). In these cases, the rhythm may appear to be sinus in origin, with the flutter waves appearing to be the P and T waves of sinus tachycardia. In patients with 2:1 AV conduction, the second flutter wave is often concealed by the QRS complex. Alternatively, atrial flutter may also be confused with supraventricular or AV reentrant tachycardia because of the similarity in the electrocardiogram tracings. Infants who often have rapid AV conduction are frequently misdiagnosed as having either sinus tachycardia or supraventricular tachycardia. A key differentiating feature between sinus tachycardia and either sinus tachycardia or reentrant tachycardia is that the rate of sinus tachycardia is variable, whereas the rate of atrial flutter and re-entrant tachycardia is fixed. Thus, close observation of the patient’s heart rate over time, along with treatment of patient specific causes of sinus tachycardia
(e.g., fluid bolus, analgesics, or antipyretics if indicated), may help to clarify that sinus tachycardia is present.