Which arrhythmia is characterized by a regular rhythm but varying heart rates?

Supraventricular tachycardia (SVT) is indeed characterized by a regular rhythm but can present with varying heart rates. In this condition, the electrical signals in the atria or the AV node fire rapidly, leading to a quick, regular rhythm. Although the rate may fluctuate, it usually remains consistent within a given episode, which is why it is described as having a regular rhythm.

This arrhythmia can occur suddenly and may also resolve abruptly, often resulting in heart rates that exceed 100 beats per minute, but the regularity of the rhythm is a defining feature. In contrast, atrial fibrillation, while irregular, does not maintain a regular rhythm. Sinus tachycardia features a regular rhythm with an increased rate due to physiological conditions, while ventricular tachycardia typically presents with a potentially life-threatening rapid and irregular rhythm that can be quite different from the regularity seen in SVT.