Final answer:
Ventricular fibrillation is the life-threatening arrhythmia that causes no cardiac output, leading to an absence of an effective pulse and necessitating immediate defibrillation to prevent brain death.
Step-by-step explanation:
The life-threatening arrhythmia that results in no cardiac output is ventricular fibrillation. Ventricular fibrillation is a severe disruption of the heart's electrical activity, where the ventricles beat in an uncoordinated and rapid manner, effectively preventing the heart from pumping blood. Without immediate treatment, such as defibrillation, this condition can lead to brain death within minutes due to the lack of oxygenated blood reaching the brain and other vital organs.
Given the total lack of normal electrical activity in ventricular fibrillation, there would be no effective heart rate (pulse) to detect. This is why defibrillation, the application of an external electrical shock, is crucial. It serves as an attempt to 'reboot' the heart's electrical system and restore a normal rhythm.