Which rate characteristics define Atrial Fibrillation?

Atrial fibrillation is primarily characterized by its "irregularly irregular" rhythm. This means that the heart's electrical signals are disorganized, causing the atria to quiver instead of contracting in a coordinated manner. As a result, the ventricular response can vary significantly, leading to a range of heart rates that are not consistent or predictable. Patients may experience a heart rate that fluctuates greatly, which is a hallmark feature of atrial fibrillation.

In contrast, the other characteristics outlined in the choices do not align with the nature of atrial fibrillation. A consistent and predictable rate would suggest a more organized electrical activity, such as seen in sinus rhythm or various other atrial arrhythmias. Additionally, a rapid and regular heart rate might be indicative of other conditions, such as atrial flutter or certain types of tachycardias, rather than the chaotic nature of atrial fibrillation. Lastly, while atrial fibrillation can lead to heart rates exceeding 150 beats per minute, it does not guarantee a constant heart rate at that level, as rates can vary considerably during the episode. Thus, the defining characteristic remains the irregularly irregular rhythm associated with varying rates.