Which heart rhythm is characterized by narrow QRS complexes and rapid beats?

The characteristic of having narrow QRS complexes and rapid beats is indicative of supraventricular tachycardia (SVT). SVT originates above the ventricles, typically in the atria or the atrioventricular node, which allows the electrical impulses to travel quickly through the conduction system of the heart. This results in the hallmark of a rapid heart rate, often exceeding 150 beats per minute, and narrow QRS complexes, which occur because the ventricular depolarization happens normally through the bundle branches.

In contrast, atrial fibrillation leads to an irregularly irregular rhythm with varied QRS complexes, and the heart rate can be rapid but the QRS may be wide if there is a bundle branch block present. Ventricular fibrillation is a chaotic rhythm without discernible QRS complexes, leading to ineffective heart contractions. Ventricular tachycardia features wide QRS complexes due to abnormal conduction originating in the ventricles and is typically much slower than SVT. Thus, the defining features of SVT make it the correct choice for this question.