Final answer:
The potentially lethal cardiac arrhythmias are Ventricular fibrillation and Ventricular tachycardia. Ventricular fibrillation is particularly urgent and requires immediate treatment with defibrillation to prevent brain death.
Step-by-step explanation:
The potentially lethal cardiac arrhythmias are Ventricular fibrillation and Ventricular tachycardia. Both conditions result from problems in the electrical conduction system of the heart. Ventricular fibrillation is particularly dangerous as it causes the heart to beat erratically, preventing it from effectively pumping blood, which can lead to brain death if not treated within minutes. This condition is often treated with defibrillation, which attempts to reset the heart's rhythm to a normal state. Ventricular tachycardia, on the other hand, is a rapid but regular beating of the ventricles that can also be life-threatening if it persists. Sinus bradycardia, which is a slower-than-normal heart rate, is typically not lethal unless it is extreme and associated with other underlying conditions.