What defines atrial fibrillation?

Atrial fibrillation is characterized by an "irregularly irregular" heart rhythm. This term refers to the chaotic and disorganized electrical activity in the atria of the heart, leading to ineffective contractions and an irregular pulse. In this condition, the heart does not beat in a synchronized and controlled manner, which is distinctly different from other arrhythmias that may present with regularity. This irregular rhythm can lead to fluctuations in heart rate, but the key feature is the unpredictability of its timing, hence the descriptor "irregularly irregular."

In contrast, other choices reflect characteristics that do not align with atrial fibrillation. Fast heart rates could occur in other conditions or even during episodes of atrial fibrillation, but they do not define the arrhythmia. Similarly, a slow heart rate does not relate to atrial fibrillation, as the hallmark feature is the irregularity rather than speed. Finally, consistent rhythmic heartbeats are characteristic of a normal sinus rhythm rather than the erratic nature of atrial fibrillation. Thus, the defining aspect of atrial fibrillation is its irregularly irregular rhythm.