How does an AED work?

Thirsty for knowledge

To understand the functioning of AED, we will first roughly look at the anatomy and physiology of the heart.

The structure and functioning of the heart:


The heart is a special muscle which lies in the chest. It consists of four chambers: two atria and two ventricles. The main function of the heart is to alternately contract and relax the chambers, thereby pumping blood through the body. Blood enters the heart through veins and leaves the heart through arteries. The heart establishes its own rhythm of contracting (normally 60 to 100 times per minute) by the so-called electrical conduction system of the heart, the heart’s own natural pacemaker. This system consists of special neuro-muscular fibers spread through the heart muscle, which conduct electrical impulses and thereby trigger contractions of the heart.

When something goes wrong

If the electrical conduction system is damaged, electrical impulses cannot spread through the heart correctly and it loses its rhythm. The most common cause of this is atherosclerosis, a disease of vessels which provide blood for the heart itself. Atherosclerosis can be caused by various factors, a major one being unhealthy lifestyle with smoking, stress, little exercise and unhealthy food. It is a process where fatty materials such as cholesterol accumulate in the vessel wall, thicken it and obstruct the blood flow. If a vessel is obstructed completely, the part of the heart it supplies is left without blood and hence without oxygen and nutrients. That part of the heart dies. That is called a myocardial infarction and can cause a malfunctioning of the electrical conduction system and the loss of the heart’s own rhythm, called arrhythmia.

There are many known arrhythmias, two of them being lethal:

  1. ventricular fibrillation: without any rhythm, parts of ventricles flicker randomly – such action is insufficient to pump blood through the body,
  2. pulseless ventricular tachycardia: the heart contracts so fast it cannot pump blood anymore.

Both of these conditions are also called a heart attack and can only be cured by delivering a therapeutic dose of electrical energy to the affected heart (defibrillation). Nothing else (chest compressions, medication…) is effective.

Why is this dangerous?

When the heart cannot pump blood through the body anymore, the whole body suddenly has no inflow of fresh blood carrying oxygen and nutrients. The first organ affected is the brain, which can only last without oxygen for a few minutes and after that irreparable damage is made. Consequently a person may be resuscitated after some minutes, but he or she may have brain damage. That is why we must begin resuscitation immediately.

Why perform cardiopulmonary resuscitation (CPR)?

Cardiopulmonary resuscitation consists of cycles of 30 compressions of the chest and 2 breaths by exhaling into the subject’s mouth. During chest compressions, one compresses and expands the heart, thereby pumping blood through the body. This pumping isn’t as effective as the heart’s own contractions, but fresh blood flow to the brain delays brain damage and increases chances of survival. With artificial respiration new oxygen is added to the blood. This action is a substitution of lung function.

And the purpose of AED

As already mentioned, ventricular fibrillation and pulseless ventricular tachycardia can only be stopped by delivering a therapeutic dose of electrical energy to the affected heart. Chest compressions may compress the heart, but it will not resume its normal rhythmical contractions. A strong electrical current basically “resets” the heart, allowing the heart to reestablish a normal and effective rhythm. The process is termed defibrillation exactly because it terminates fibrillation.

Let’s briefly summarize what we learned so far: (1) the heart lies in the middle of the chest, (2) the problem is a lethal arrhythmia and (3) the cure is a therapeutic dose of electrical energy. With that knowledge, the process of using an AED makes sense. First, we adjust one electrode pad under the right collar bone and one on the left lower side of the chest. The electrical axis of the heart lies in-between these two points. Second, we stop the lethal arrhythmia by triggering the electrical current.

Why should I resume CPR after defibrillation?

In the case of heart attack, the “exhausted” heart will not resume contracting with its full power immediately but only gradually. That’s why it’s important to “help” the heart and resume chest compressions after defibrillation (a state called “stunning”). One may only discontinue CPR after the clear presence of life signs such as breathing, moving, swallowing…

The perfect scenario

When a person isn’t responding and isn’t breathing, we must:

(1) immediately call the emergency telephone number (112 in Slovenia),

(2) order a person to get a nearby AED,

(3) begin CPR, alternating 30 compressions of the chest and 2 breaths by exhaling into the subject’s mouth.

When the other person brings the AED, he or she should turn it on and prepare the electrode pads. You should stop with CPR, adjust the pads to the chest and follow the AED’s instructions (which you can hear through the speakers and see on the display). Continue following its instructions and performing CPR until: (1) the subject begins to breathe, (2) a rescue team arrives, (3) for as long as you can.