Technique     Physiology     Quiz    

The heart is an electrical pump. Electricity is spontaneously generated in the Sinus Atrial node of the heart (in pacemaker cells) and propagated to all of the heart's muscle cells via the heart's conduction system. This electrical energy is then transformed by the heart's muscle cells into a mechanical contraction. Electricity first travels from the SA node to all the cells of the Atria (left and right) and produces the Atrial contraction. When the electricity reaches the Atrio-Ventricular node, it gets delayed for a second before being propagated to all the cells of the left and right ventricles via the Bundle of his to the right and left (anterior/posterior) bundle branches and finally to the Purkinje fibers. This produces the ventricular contraction and the ejection of blood from the heart to the lungs and body.

These waves of electrical contraction can be monitored by the use of an ElectroCardioGraph (ECG). The AED reads the electrical activity of the heart and decides whether or not to shock the heart. (Note AED records the ECG throughout the procedure so that it can be examined at the hospital.) Shocking the heart means that you administer enough electricity to STOP the heart's electrical activity. The SA node will set a new pace for electrical activity if the heart still has enough energy to beat. The heart uses phosphates for energy. A heart in ventricular fibrillation or ventricular tachycardia consumes a lot of energy and may deplete the heart's energy stores. Under such circumstances, advanced care is required.

Types of heart rhythms:

  • Ventricular Tachycardia
    The heart's electrical activity is fast because ectopic pacemakers arise within the ventricles, but the heart is still able to eject blood to the body. Victim has a rapid pulse and may be conscious and breathing. This type of victim must NOT be confused with pulseless ventricular tachycardia (see below). An AED cannot tell the difference between the two. That is why it is critical to check the pulse. Note ECG pattern normal but fast. Shock may be advised, but must NOT be given if the victim has a pulse.
  • Pulseless Ventricular Tachycardia
    When the heart's electrical activity is racing, the muscles may not be able to keep up and remain contracted. If they do not relax, the heart cannot fill with blood, and consequently no blood will be ejected. Note the ECG pattern may be normal but fast. Shock is advised because it will hopefully stop the ectopic pacemakers within the ventricles and the SA pacemaker also known as the "chef d'orchestre" will take over and regain control.
  • Ventricular Fibrillation
    The electrical activity of the heart is unorganized. Therefore the heart cells cannot work together to contract and relax. Note the ECG pattern is irregular and fast. Shock is advised.
  • Asystole
    There is no electrical activity in the heart. Note the ECG shows a straight line. Shock is not advised. Advanced care and drugs are required to revive the heart.
  • Bradycardia
    The heart's electrical activity is slow. Note the ECG pattern may be normal but slow. This may eventually lead to asystole. Shock is not advised. Advanced care and drugs are required to revive the heart.
  • Pulseless Electrical Activity (PEA) or Electro Mechanical Dissociation (EMD)
    The heart's electrical activity is normal, but the mechanical response cannot be generated. A massive generalized heart attack or a lack of blood in the system (severe hemorrhage) may be the cause. Note ECG pattern is normal. Shock is not advised. Advanced care required.

85% of pulseless victims are either in ventricular fibrillation or ventricular tachycardia. Only 15% of pulseless victims are in asystole or brachycardic asystole. PEA is rare.

Why is it important to defibrillate early? When defibrillation is done within the first minute, survival rate is close to 90%. Survival then decreases by 2-10% per minute. By 10 minutes, chances of survival are almost none. This rapid decrease is primarily due to the depletion of the heart's energy stores by the fibrillating or tachycardic heart. When CPR is performed during this time, survival rates improve. However, EARLY defibrillation remains the critical step to successful revival.

how well do you know your stuff?