Final answer:
Ventricular fibrillation and ventricular tachycardia are two heart rhythms that can lead to cardiac arrest, which is often treated using a cardiac defibrillator to 'restart' the heart's normal rhythm.
Step-by-step explanation:
Two heart rhythms that can cause cardiac arrest are ventricular fibrillation (VF) and ventricular tachycardia (VT). Ventricular fibrillation is a condition where the heart beats in a rapid, erratic electrical rhythm, preventing it from pumping blood effectively. Ventricular tachycardia is a fast but regular heart rhythm that can also degrade into ventricular fibrillation if not treated. Both conditions can be life-threatening and may lead to cardiac arrest if the heart cannot pump blood adequately to the body's vital organs. These heart arrhythmias are often treated with a cardiac defibrillator, which delivers an electrical shock to help 'restart' the normal rhythm of the heart.