Which of the following rhythms is classified by a heart rate of less than 60 bpm?

The rhythm that is classified by a heart rate of less than 60 beats per minute is known as a junctional escape rhythm. This specific rhythm occurs when the normal pacemaker of the heart, the sinoatrial (SA) node, fails to generate impulses at the normal rate, resulting in a slower heart rate. Junctional escape rhythms emerge from the junctional tissue (the area surrounding the atrioventricular node) and are typically characterized by a heart rate ranging from 40 to 60 bpm. This mechanism acts as a safety feature to ensure that the heart continues to beat when the higher pacemaker fails.

In contrast, sinus rhythm generally indicates a normal heart rate of 60 to 100 bpm driven by the SA node. Atrial fibrillation is marked by an irregular and often rapid heart rhythm, typically exceeding 100 bpm. Ventricular tachycardia represents a rapid heart rhythm originating in the ventricles, often exceeding 100 bpm as well. Thus, junctional escape is the only rhythm among the options that consistently presents with a heart rate that falls below 60 bpm, making it the correct choice.