The heart is a muscular pump, approximately the same size as its owner's fist, and is located behind and slightly to the left of the breastbone. Its function is to pump oxygen-rich blood from the lungs to various parts of the body, and to pump the de-oxygenated blood from the tissues back to the lungs to take on more oxygen.

Views of the Heart - Front, Inside, and Rear.

Pumping blood through the lungs removes carbon dioxide and re-supplies the blood with oxygen. The newly oxygenated blood is then pumped around the body to provide oxygen and nutrients and remove waste products.

The heart's mechanical action

The heart has four chambers with one-way flaps called valves between the upper and lower chambers. The atria are the upper chambers and they receive blood that is being returned to the heart.

When the atria pump (contract), they push the blood through valves into the relaxed ventricles (lower chambers). When the ventricles contract, the right ventricle pumps blood into the lungs. It is the pressure of the blood reflected on the walls of the arteries which is felt as a pulse.

A normal healthy adult heart beats rhythmically at a rate of about 60 to 100 beats a minute when at rest. During strenuous exercise, the heart can increase the amount of blood it pumps up to four times the amount it pumps at rest within only a matter of seconds.

The heart's electrical system

The heart's electrical system causes the heart to beat, controls the heart rate (the number of beats per minute).

Given an electrical signal, the upper heart chambers (atria), contract (pump) and then relax and then passes down the separating tissue to the ventricles, causing them to contract and pump blood to the body.

After the ventricles complete contracting, the electrical impulses cease, and the heart muscle relaxes.

Sudden Cardiac Arrest (SCA)

Sudden cardiac arrest is the unexpected collapse of a casualty whose heart has ceased to function due to an electrical malfunction of the heart, disrupting that muscle's normal rhythm. SCA is not a heart attack, which is a problem with the plumbing of the heart. In a heart attack one or more of the arteries delivering blood to the heart is blocked, so oxygen in the blood cannot reach the heart muscle and the heart muscle is damaged.

During sudden cardiac arrest, the electrical signals to the pump suddenly become erratic. The ventricles may flutter or quiver (ventricular fibrillation), and so blood is not delivered to the body. Blood flow to the brain is reduced and the casualty loses consciousness. Death will follow unless emergency treatment is begun.

Cardiac arrest is closely linked with sudden chest pain. During SCA the heart twitches irregularly, most often in adults due to ventricular fibrillation (VF), and cannot pump oxygenated blood efficiently to the brain, lungs, and other organs. The casualty quickly stops breathing and loses consciousness.

There are no prior symptoms of sudden cardiac arrest. However, risk factors have been identified as increasing the potential for sudden cardiac arrest. Risk factors include:

• Family history of sudden cardiac arrest
• Heart failure (heart pumps poorly)
• History of heart rhythm disorders
• Previous heart attack
• Previous sudden cardiac arrest episode

Successful resuscitation of such a casualty depends on quick decisive action taken in sequence, like the links in a chain.

Defibrillation pads

Defibrillation pads are soft, thin foam about the size of a hand. The pads are adhesive and have a layer of gel which helps pick up the electrical signals. They normally have a cable attached to each pad. Normally a set is packaged in a special sealed pouch. As they can ‘dry out', they have an expiry date printed on the package.

Pads must adhere firmly to the chest, so it is important to press the pads on firmly, including the edges of the pads.

Pads may not stick securely for several reasons. These include moisture and excessive hair on the chest. Moisture can be wiped away with a towel or cloth. You may need to clip or shave excessive hair off the chest around the pad area.

Each AED manufacture will have the instructions for applying the pads on the pad packaging and on the pads themselves. Follow these instructions as they will apply directly to the AED that is being used.

Procedure for administering an AED

• send someone for the AED if not already done
• ensure safety
  
  • If two rescuers are present, assign tasks for each rescuer
• turn on the AED
• attach the electrode pads to the casualty's bare skin
  
  • if multiple rescuers, continue CPR while the pads are attached
• follow the voice/visual prompts of the AED
• ensure that nobody touches the casualty while the AED is analysing the rhythm

If a shock is indicated:

• ensure that nobody touches the casualty
• say ‘Stand Clear'
• check nobody touching casualty
• push the shock button as directed
  
  • fully-automatic AEDs will deliver the shock automatically
• continue to follow the voice/visual prompts of the AED

If no shock is indicated:

• immediately resume CPR using a ratio of 30 compressions to 2 rescue breaths.
• continue to follow the voice/visual prompts of the AED

Safety

An AED, just like any electrical appliance, has safety precautions to prevent injury. The AED operator is responsible for keeping all persons from touching the casualty when a shock is delivered.

• State a ‘Clear' message. For example, say loudly "don't touch the casualty" or "Stand Clear"
• Look to ensure that no one is touching the casualty before pressing the shock button

Beware of implants

If a casualty has an electronic pacemaker or cardioverter implanted under the skin, ensure that the AED chest pad is positioned at least 1Ocm away from the pacemaker.

Ensure that the pads are not placed over a Glyceryl Trinitrate patch (GTN).

Beware of gaseous or oxygen-rich environments

Defibrillators can cause sparks. If using oxygen to supplement resuscitation, ensure that there is no concentration of oxygen around the casualty's chest. Ensure that there is no build up of other gases around the scene.

Beware of water

Do not use an AED if the casualty or the surroundings are saturated with water. Water is an effective transmitter of electricity and the shock may be transmitted to the AED operator.

Separate the electrodes

Don't put or place the electrodes or connected pads together, or allow them to touch if the AED is ‘on'. This may complete a circuit and cause an electrocution.

The AED should never be connected to anyone other than a casualty in cardiac arrest, nor should an AED be attached to a person for training or demonstration purposes.

A sign should be used to indicate the location of the AED.