What exactly happens to the heart when a defibrillator is used?


Quick Overview:

1. Sudden cardiac death kills 350,000 Americans annually.

2. The presence of scar in the heart, most often from a prior heart attack, represents the greatest risk for sudden cardiac death.

3. Defibrillators provide life-saving therapy for sudden cardiac death, when then can be applied within seconds to minutes of the event.

4. The defibrillator shock provides an electrical reset for all the cells of the heart, such that life-threatening heart rhythms (VT/VF) stop instantaneously, allowing the normal heart rhythm to regain normal control of the heart.

5. Automatic external defibrillators (AEDs) are positioned strategically in public places, such as airports and schools, allowing bystanders to rapidly apply the AED to victims who suddenly collapse. The AED will determine if a shockable rhythm exists, and apply a life-saving shock if appropriate.

6. Certain high-risk individuals may receive an implanted defibrillator (ICD), providing constant defibrillator protection.



Elaborated Answer:

Defibrillators are a remarkable technology that have revolutionized our ability to rescue people from sudden cardiac death (SCD). SCD is caused by a life threatening abnormal cardiac rhythm (arrhythmia), which can occur unpredictably and instantaneously, and will result in death within several minutes unless a defibrillator is available. These lethal rhythms are called ventricular tachycardia (VT) or ventricular fibrillation (VF), during which the heart’s activity is exceedingly rapid (>200 beats per minute), with ineffective contractions that fail to pump blood sufficiently to the brain and the rest of the body, resulting in cardiovascular collapse an eventual brain death. Because the heart also pumps blood to supports its own function, SCD starves the heart of needed blood flow. The heart also becomes irreversibly damage during SCD and will die in a matter of minutes unless a defibrillator is applied.


In the United States, there are approximately 350,000 cases of SCD per year. The majority of these events occur in individuals who have developed some scar tissue within the heart muscle – such as those who have survived a prior heart attack. This scar tissue destabilizes the electrical activity of the heart, making the heart vulnerable to suddenly developing VT/VF, resulting in SCD.

To understand the function of a defibrillator, we must appreciate that the heart muscle contracts when a wave of electrical excitation propagates across the muscle. Under normal circumstances, the frequency of these electrical waves result in rhythmic contractions, and the frequency of these waves determines the heart rate. After each electrical wave, the muscle is refractory for further electrical activity for a period of 200-400 milliseconds. During this refractory period, the muscle will not support further electrical activity. During VT/VF, independent wavelets of electrical excitation propagate independently and chaotically in all directions within the muscle, making organized contractions impossible. The refractory periods are much shorter (~50 msec) during VT/VF, allowing the heart to support such dramatically faster activity


A defibrillator delivers a large external shock to the heart, which forces the entire heart muscle to become suddenly refractory. This immediately stops all electrical activity in the heart simultaneously. In this way, a defibrillator shock immediately stops the complex and chaotic activity of VT/VF, allowing the normal heart rhythm to regain control when the tissues recover excitability. When a defibrillator is applied before permanent damage has occurred, then the defibrillator shock can be lifesaving.

The most visible type of defibrillator is the Automatic External Defibrillator (AED). AEDs are frequently stored in public places such as airports, shopping malls, and schools. If someone suddenly collapses, a bystander may apply the AED to the victim. If VT/VF is detected, the AED will deliver a life-saving shock. Individuals who are high risk for SCD may receive an automatic implantable cardioverted defibrillator (ICD), which sits under the skin with leads/electrodes passing through the blood vessels into the heart. An ICD will detect when VT/VF occurs and may quickly deliver a life-saving shock. If the victim is awake at the time of the shock, the feeling of being shocked is described like being kicked in the chest by a horse. Frequently, SCD victims have lost consciousness before the shock is applied, and the shock is painless.

Everyone should be familiar with the purpose and function on an AED. Your ability to act quickly may save a life. One day, it may be your life that is saved!


Please see our YOUTUBE presentations related to this topic.


Treating sudden cardiac death with the first and only subcutaneous ICD


ICD therapy with no pacing indication: innovative technologies to improve care

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