Which phase is characterized by a rate exceeding 250 bpm?

Atrial fibrillation is characterized by rapid and irregular electrical activity in the atria, leading to an unpredictable heart rate that can exceed 250 beats per minute. This condition results from chaotic electrical signals, causing the atria to quiver rather than contract effectively. As a result, patients can experience a heartbeat that is both fast and irregular, which is a hallmark of atrial fibrillation.

In contrast, sinus bradycardia refers to a slower heart rate, typically defined as fewer than 60 beats per minute. Sinus arrhythmia involves a variation in heart rate that correlates with the respiratory cycle, and it typically does not involve rates exceeding 250 bpm. Atrial flutter, while it involves an organized, rapid atrial rhythm, usually presents at rates generally between 240 bpm and 340 bpm but does not reach the chaotic, irregular nature seen in atrial fibrillation. Thus, the characteristic of exceeding 250 bpm aligns specifically with atrial fibrillation.