Cardiac Arrest Defined
The simple definition of cardiac arrest is that the heart has stopped beating normally. A heart beat is the contraction of the heart muscle. This contraction pushes (circulates) blood throughout the body. The circulation of blood is important because the blood will take oxygen and nutrients to the body cells. Without oxygen and nutrients the body cells will die which can potentially lead to the death of the casualty.
How The Heart Works
Muscle contraction is induced by an electrical current. The contraction of the heart is controlled by the heart's electrical conduction system. The sinoatrial node (SAN) of the heart normally generates the electrical impulses of the heart that cause the heart muscle to contract. The SAN is referred to as the "heart's pacemaker" because of this. The SAN usually generates an impulse 60 to 70 times a minute. We can feel this as the pulse or the "heart beat". Depending on a person's health, fitness, and other stimuli, this rate can increase. The heart beat can be shown on the electrocardiogram (EKG) as a graph ("heart rhythm").
The Science of Cardiac Arrest
Problems with the impulse conduction can occur because of disease or abnormalities to the heart's conduction system. These abnormally conducted impulses (also called arrhythmia) will affect the heart's normal beating. This when a casualty goes into cardiac arrest.
A common electrical abnormality associated with cardiac arrest is ventricular fibrillation (VF). VF occurs when the various parts of the heart muscle is moving chaotically and not in sync with each other. This lack of coordinated movement means the heart is not contracting properly to circulate blood. Rapid stimulation to the heart, called ventricular tachycardia (VT), causes VF.
Defibrillation and AED
The definitive treatment for cardiac arrest is defibrillation. Defibrillation means sending a therapeutic dose of electrical current to the heart muscle using a defibrillator. The electrical current will depolarize the critical mass of the heart muscle, stops the arrhythmia (abnormal heart beat), and allows the SAN to re-establish a normal heart beat.
There are various types of defibrillators: external, wearable, and implanted. All defibrillators work on the same principle. A VT or VF abnormal heart rhythm is detected by the device's monitoring circuitry. The device's capacitor is charged up to the appropriate level of voltage and a shock is initiated (by the rescuer pushing a button or automatically by the device) to send an electrical current to the casualty's heart muscle.
The automated external defibrillator (AED) is the common type of defibrillator used by professional and lay rescuers. AED models range from basic devices (detects VT or VF, and if detected sends shock to casualty) to feature-rich devices (provide CPR coaching, CPR feedback, high ingress protection rating, etc.).
Reference:
http://www.resuscitationcentral.com/defibrillation/history-and-science