What characterizes atrial fibrillation?

Atrial fibrillation is characterized by a rapid and chaotic electrical activity, which results in an irregular heart rate. In this condition, the electrical signals in the atria become disorganized, preventing the atria from contracting effectively. Instead of smooth, coordinated contractions, the atria quiver, causing the heart rate to vary widely and unpredictably. This irregularity can lead to various complications, including an increased risk of stroke and heart failure.

Other options describe different cardiac conditions. The first option describes a regular atrial contraction, which is not typical in atrial fibrillation. The third option refers to "saw tooth" waveforms often seen in atrial flutter rather than atrial fibrillation. The fourth option describes inverted P waves, which relate to other arrhythmias, such as junctional rhythms, rather than the chaotic pattern seen in atrial fibrillation. Thus, the characteristic feature of atrial fibrillation is indeed its rapid and chaotic electrical activity, manifesting as an irregular heart rate.