How Heart Responds to Defibrillation

Last updated: February 18, 2021


Whenever someone suffers a sudden cardiac arrest, the person's heart beats in a rhythm commonly known as Ventricular Fibrillation (VF). When the heart is beating in such a manner, the chambers of the heart contract in a confused manner and the heart is unable to supply blood to other parts of the body. If ventricular fibrillation is not addressed within eight to ten minutes, it can prove to be fatal. Defibrillation is the process of applying a specially designed electrical charge to the heart to stop VF and hopefully allow the return of a normal cardiac rhythm.

Medical science has proven that the sooner a fibrillating heart is defibrillated, the greater are the chances of survival for the victim. Every passing minute will reduce the odds of survival by 10%. If defibrillation can be completed within one minute of the cardiac arrest, the chances of survival are almost 90%. After five minutes, the chances are about 50%. In the ten-minute range, there is little hope of life left. 1

Defibrillation process

Cardiac defibrillation is a process of supplying a transthoracic electrical current to a person undergoing one of the two destructive ventricular dysrhythmias, Ventricular Fibrillation (VF) or pulseless Ventricular Tachycardia (VT). Under Advanced Cardiac Life Support (ACLS) guidelines, pulseless VF and VT are treated in the same way.

As per the Centers for Disease Control and Prevention (CDC), 610,000 deaths occur every year in the United States due to heart disease. Heart disease is the number one source of death in both genders. In grown-up patients, VF is the most familiar source of sudden cardiac arrest. The ultimate treatment for VF is to carry out electrical defibrillation. Defibrillation proves to be very efficient in ending VF when carried out as soon to the initiation of VF as possible. 2

VF/tachycardia defibrillation is done by passing enough electrical current to the heart to depolarize a certain mass of the myocardium. The quantity of the current applied to the heart is dependent on the energy of the shock and transthoracic impedance. 3

It produces nearly concurrent depolarization of a critical mass of the myocardium, causing cessation of all cardiac activity for a short time. Under ideal conditions, a workable site within the heart’s intrinsic electrical conduction system will automatically start an electrical impulse that can restore normal propagation of the heart’s cardiac cycle, with a resultant restoration of ventricular contraction and, hence, a pulse.2

In the course of transthoracic defibrillation in humans, as little as 4% of the supplied transthoracic current passes through the heart because parallel pathways (thoracic cage, lungs) divert the current away from the heart. Elements that decide transthoracic impedance include:

  • Time to defibrillation
  • Electrode size
  • Contact
  • Pressure
  • Distance from the heart
  • Previous shocks
  • Ventilatory phase

The defibrillation's success rate depends not only on the threshold but also on chances. At high energy levels, defibrillation can be proarrhythmic and cause functional and morphological damage to the myocardium. Electrical shocks below the defibrillation threshold are of no use and lethal as they may reignite VF by stimulating parts of the myocardium during a vulnerable period.

Prospective clinical studies have shown that the success of defibrillation and following hospital discharge rates were almost the same in patients experiencing the first shock of 175 or 320 J. Thus, the energy selected for the initial shock is a settlement between chances of success and risk of damage.

According to this scientific proof, the following recommendations have been accepted widely for defibrillating an adult facing the VF:

  • First shock - 200 J
  • Second shock - 200-300 J
  • Following shocks 360 J

The weight-related initial energy of 2 J kg1 is instructed for children. The average transthoracic impedance of a human being is 70 to 80 but this can vary from as high as 150 and as low as 15. The energy of initial shock can be estimated and modified based on automatic measurement of impedance, and ill-judged low energies can be avoided in patients with higher impedance. Modern defibrillators are equipped with this facility. 3


It is necessary to integrate the concept of early defibrillation into an effective emergency cardiac care system 3 . VF can be successfully treated when defibrillation is carried out soon after the onset of VF. After 10 minutes of VF, the odds of successful resuscitation drop to nearly zero. For this reason, many communities have developed early defibrillation programs, whereby certain members of the public are trained to perform defibrillation by using AEDs. 2


This page is last reviewed and updated by on Feb 16, 2021.