What is the hallmark sign of atrial flutter?

The hallmark sign of atrial flutter is the presence of saw-toothed flutter waves, often referred to as "F-waves." These flutter waves can typically be observed on an electrocardiogram (ECG) in a distinct pattern, usually best seen in the inferior leads (II, III, and aVF). This characteristic pattern arises due to the reentrant circuit that commonly occurs in the right atrium, creating these repetitive, organized electrical impulses that result in the saw-tooth appearance.

In contrast, the other options, while they may appear in various cardiovascular conditions, do not uniquely identify atrial flutter. For instance, flat T-waves can be indicative of other issues such as ischemia or electrolyte imbalances but are not specific to atrial flutter. A prolonged PR interval is associated more with first-degree atrioventricular block rather than atrial flutter. Lastly, irregular R-R intervals are typically seen in atrial fibrillation, which is a different arrhythmic condition altogether. Thus, the saw-toothed flutter waves are definitively what distinguishes atrial flutter from other arrhythmias.