Cardiac Arrest: Cardiac arrest is the sudden loss of cardiac function when the heart abruptly stops beating. A person whose heart has stopped will lose consciousness and stop breathing normally. The pulse and blood pressure will be absent. Unless resuscitative efforts are begun immediately, cardiac arrest leads to death within a few minutes. This is often referred to by doctors as "sudden death" or "sudden cardiac death".
Ventricular fibrillation is a heart rhythm problem that occurs when the heart beats with rapid, erratic electrical impulses. This causes pumping chambers in your heart (the ventricles) to quiver uselessly, instead of pumping blood. During ventricular fibrillation, your blood pressure plummets, cutting off blood supply to your vital organs. Ventricular fibrillation is frequently triggered by a heart attack and causes cardiac arrest.
Permanent brain damage and death can occur unless the flow of blood to the brain is restored within five minutes. Less common causes of cardiac arrest include respiratory arrest (loss of breathing function), choking, trauma, electrocution and drowning.
CPR: Cardiopulmonary Resuscitation.
When someone's blood flow or breathing stops, seconds count. Permanent brain damage or death can happen quickly. If you know how to perform cardiopulmonary resuscitation (CPR), you could save a life. CPR is an emergency procedure for a person whose heart has stopped or is no longer breathing. CPR can maintain circulation and breathing until emergency medical help arrives.
Even if you haven't had training, you can do "hands-only" CPR for a teen or adult whose heart has stopped beating ("hands-only" CPR isn't recommended for children). "Hands-only" CPR uses chest compressions to keep blood circulating until emergency help arrives. If you've had training, you can use chest compressions, clear the airway, and do rescue breathing. Rescue breathing helps get oxygen to the lungs for a person who has stopped breathing.
To keep your First Aid skills up to date, you should repeat the training every two years. If you do not know about CPR it will be most useful to take First Aid lessons.
Medical Emergency: Cardiac Arrest is a very serious condition. It is important to seek medical help immediately as any delay in treatment such as CPR can result in death or brain damage. 
