Which of the following is NOT considered a lethal arrhythmia?

Atrial flutter is not classified as a lethal arrhythmia because it often does not lead to immediate cardiac instability or cessation of effective circulation, unlike other listed options. Atrial flutter can result in rapid heart rates, but it generally does not produce the same level of chaos in heart rhythm that is associated with lethal arrhythmias.

Lethal arrhythmias, such as ventricular fibrillation and asystole, are more critical conditions as they can lead to sudden cardiac arrest and require immediate medical intervention. Ventricular fibrillation is a disorganized rhythm that prevents the heart from effectively pumping blood, while asystole indicates a complete stoppage of electrical activity in the heart, making it impossible to sustain life without immediate resuscitative efforts.

Atrial fibrillation, although it can have serious consequences like stroke and can lead to hemodynamic instability, is also not typically classified under lethal arrhythmias when compared to the acute dangers presented by ventricular fibrillation and asystole. Thus, it is the unique properties of atrial flutter that categorize it as less critical in this scenario.