What characterizes fibrillation in terms of heart rate?

Fibrillation, particularly atrial fibrillation, is characterized by a rapid and disorganized electrical activity in the heart, leading to an irregular and often very fast heart rate. This chaotic rhythm makes it difficult to discern a consistent heartbeat, causing the heart rate to be so high that it becomes uncountable. The term "fibrillation" itself implies an ineffective and erratic contraction of the heart muscle, which aligns with the notion that the heart rate can exceed normal limits, rendering it too fast to be accurately measured.

In contrast, other options refer to slow or regular heart rates, which do not accurately reflect the condition of fibrillation. In summary, the hallmark of fibrillation is a heart rate that is rapid and irregular, often beyond a point that can be effectively counted.