Which type of atrial fibrillation is characterized by a heart rate of less than 100?

The correct choice, characterized by a heart rate of less than 100, is referred to as controlled atrial fibrillation. In controlled atrial fibrillation, the ventricular rate remains less than 100 beats per minute at rest, which indicates that while the atria are in fibrillation, the heart's rhythm is effectively managed, allowing for adequate hemodynamics and reduced symptoms.

In this context, maintaining the heart rate below 100 beats per minute helps prevent complications associated with rapid ventricular rates, such as decreased cardiac output and increased risk of thromboembolic events. Patients with controlled atrial fibrillation often require less aggressive treatment strategies, as the heart's function remains relatively stable compared to uncontrolled variants.

The other types of atrial fibrillation either involve a heart rate above this threshold or have other characteristics that differentiate them. For instance, uncontrolled atrial fibrillation is typically described by a ventricular rate consistently greater than 100 beats per minute, which can lead to more significant cardiac compromise. Understanding these distinctions is crucial for effective management and treatment planning in patients presenting with various forms of atrial fibrillation.