Which arrhythmia is characterized by a heart rate under 40 bpm?

The arrhythmia characterized by a heart rate under 40 bpm is Idioventricular Rhythm. This rhythm originates in the ventricles, usually following a failure of the higher pacemaker systems, such as the sinoatrial (SA) node, to initiate impulses. When the heart rate falls below 40 bpm, it typically represents a slower ventricular escape rhythm, which is a direct indication of a compromised conduction system within the heart. This low heart rate can lead to symptoms such as dizziness, weakness, or even syncope due to inadequate cardiac output.

In contrast, Supraventricular Tachycardia typically involves elevated heart rates often exceeding 100 bpm, making it inconsistent with a heart rate below 40 bpm. Accelerated Idioventricular Rhythm, while originating in the ventricles, generally presents with a heart rate between 40 and 100 bpm. Ventricular Fibrillation is a chaotic and rapid rhythm, which is incompatible with a low heart rate as it results in a lack of effective heartbeat and leads to immediate loss of consciousness if not treated. Therefore, Idioventricular Rhythm is the correct choice as it specifically reflects a heart rate under 40 bpm.