Which of the following is classified as a lethal heart rhythm?

Ventricular fibrillation and pulseless ventricular tachycardia are both classified as lethal heart rhythms because they lead to ineffective heart contractions, resulting in severe impairment of blood flow and ultimately causing cardiac arrest if not treated promptly.

Pulseless ventricular tachycardia is particularly dangerous as it can rapidly deteriorate into more chaotic rhythms or lead to loss of consciousness due to inadequate cerebral perfusion. The absence of a pulse indicates that this rhythm is not generating effective cardiac output, requiring immediate intervention.

Ventricular fibrillation, although not the selected answer, also fits the lethal category by causing uncoordinated heart muscle contractions, failing to produce any effective heartbeat.

In contrast, supraventricular tachycardia is typically not lethal on its own, especially in a stable patient, as it often can be treated effectively with medications or vagal maneuvers. Bradycardia refers to a slower-than-normal heart rate which, while it can be serious depending on the context, is not classified directly as lethal without accompanying symptoms or complications.

Thus, pulseless ventricular tachycardia is recognized as a lethal heart rhythm that requires immediate medical response to restore effective circulation.