Atrial flutter refers to a very rapid atrial tachydysrhythmia that is caused by a macroreentry loop (greater than 2 cm), occurring at atrial rates of around 250 to 350 beats per minute (BPM), with no isoelectric segments separating them. (Atrial rates can extend to 200 to 400 BPM or more in many cases.) The lack of an isoelectric or baseline segment between the atrial waves, and the smooth, biphasic P-wave morphology, gives the atrial baseline a “saw-tooth” appearance on the ECG (Figure 19-1). That saw-tooth appearance is the most distinctive, and unique, feature of atrial flutter.
The saw-tooth appearance of the atrial rhythm is intermittently broken up by the QRS complexes of the ventricular response. The ventricular response rates are usually slower than the atrial rates, typically ranging from 75 to 175 BPM (most commonly between 140 and 160 BPM). The ventricular response can occur at either regular or variable intervals.
The slower ventricular rates develop when the atrioventricular (AV) node “blocks” conduction of the supraventricular impulses, to safeguard the ventricles from the very fast atrial rates. We say “block” because this is not a true AV block caused by a pathologic obstruction to conduction. It is, instead, a normal protective mechanism of the AV node to the constant bombardment it faces from the rapid supraventricular impulses, which makes it intermittently refractory to further conduction. More on this later.