Cardiac arrest
Cardiac arrest happens when the heart suddenly stops beating.  This can be caused by a heart attack in which the heart's demand for oxygen is not met and the heart muscle begins to die. With cardiac arrest, normal circulation of blood stops, because of a failure of the heart to contract. This results in the body not getting enough oxygenated blood, which  causes cells to start to die from oxygen starvation. Cerebral hypoxia (lack of oxygen to the brain) makes a person lose consciousness and stop breathing, and this makes the heart stop completely. Unless the person is in hypothermia, brain damage usually occurs after three to five minutes. (In some cases, the person is put into hypothermia on purpose, to help the person recover better with less damage.)