Which of the following describes the heart rate when a person is experiencing atrial fibrillation?

Atrial fibrillation is characterized by an irregular and often rapid heart rate. This arrhythmia occurs when the electrical signals in the atria become chaotic, leading to a disorganized rhythm. As a result, the heart does not contract in a synchronized manner, which is what gives rise to the irregularity in the heartbeat. In many cases, this irregular rhythm can also lead to an increased heart rate, contributing to symptoms like palpitations or a racing heart.

The condition contrasts with a normal sinus rhythm, where the heart rate is regular and can be classified as either slow or fast depending on the person's physiology and other factors. In atrial fibrillation, the chaotic electrical activity not only makes the rhythm irregular but often elevates the heart rate as well. Hence, the correct description of heart rate during atrial fibrillation is that it is both irregular and fast.