What is defibrillation?

Indications and Procedures

Heart tissue beats in a rhythmic manner as a result of electrical signals generated by the heart tissue. Sometimes, the normal heart rhythm, known as "normal sinus rhythm," goes awry. Many different abnormal heart rhythms can develop from a variety of causes. These abnormal heart rhythms, known as "arrhythmias," can be deadly since the heart cannot effectively pump blood when the heart is not in normal sinus rhythm. "Cardioversion" refers to any procedure that seeks to correct an arrhythmia and return the heart to normal sinus rhythm. Defibrillation, or electrical cardioversion, delivers short bursts of direct electrical current across the chest to the heart.

Although cardioversion is used for a variety of abnormal and dangerous heart rhythms, defibrillation treats two very deadly arrhythmias, known as "ventricular fibrillation (VF)" and "pulseless ventricular tachycardia
(VT)." The heart has four chambers: two atria and two ventricles. The ventricles pump blood to the lungs and body. In VF, the heart’s electrical activity is chaotic and disorganized. This electrical chaos results in the ventricles fibrillating (quivering like a bag of wiggling worms) and unable to pump blood. If blood is not pumped, the body will die in a manner of minutes. Therefore, defibrillation is of urgent and lifesaving importance.

In VT, the ventricles are beating very fast. If the patient is unconscious and has no pulse (this can happen if the ventricles are beating too fast to fill up with enough blood to circulate), then VT treatment includes defibrillation.

A person with VF is unconscious because that person will have no cardiac output and no pulse. If the patient is placed on a cardiac monitor, a distinctive abnormal electrical pattern is displayed. The patient will shortly die if the fibrillation is not resolved quickly. The electrical current delivered during defibrillation shocks the heart tissue. This electrical shock stops all electrical heart activity, a process called "depolarization," and resets the heart’s electrical activity. Once the heart resets with defibrillation, normal sinus rhythm can take over.

Electrical cardioversion is also used as an elective, nonemergency procedure for rhythm control in patients with atrial fibrillation (an arrhythmia originating in the atrium) and in cases of VT with a pulse.

Uses and Complications

Advanced cardiac life-support ambulance units and hospitals use devices equipped with electrical paddles and heart rhythm monitors for defibrillation. An unconscious patient with no pulse, showing the characteristic pattern of VF or VT on the cardiac monitor, will be urgently shocked with specified doses of electricity. The paddles are placed firmly on the patient’s chest wall. Firm pressure, along with electrode paste, gel, or saline pads, helps ensure good electrical contact between the paddles and the chest wall.

Before discharging the electricity through the paddles, health care providers check the area surrounding the patient to ensure that no one is in physical contact with the patient or anything touching the patient. The electricity will shock anyone in direct contact with the patient or any metal touching the patient. This shock can change normal sinus rhythm into an abnormal rhythm. The health care provider delivering the shock must quickly look around the area and shout “clear” and must avoid touching metal objects (such as the stretcher frame) with any body part. Hands in the paddles are safely insulated from the delivered electricity.

Defibrillation protocols specify the sequencing and amount of electricity delivered during defibrillation. These protocols are quite detailed, involving cardiopulmonary resuscitation (CPR) and various medications.

Complications of defibrillation include damage to the heart muscle, which is rare unless repeated high energy shocks are used; blood clots dislodging from the heart and traveling to the body and lungs, causing problems when the clots block off blood supply; other abnormal heart rates emerging after defibrillation; fluid and blood backing up into the lungs, resulting in pulmonary
edema (swelling); and low blood pressure that usually resolves in three to four hours.

Perspective and Prospects

Automated external defibrillators (AEDs) are lifesaving devices that have gained in prominence and availability since the late twentieth century. AEDs are portable machines often found in public areas, such as airports, sports arenas, shopping malls, or office buildings. They differ from the larger cart-based defibrillators found in health care facilities in several ways, so that they are easier and simpler to use. AED electrodes are attached to the patient with adhesive pads, rather than pressed down by hands and paddles, allowing hands-free operation. AEDs have microprocessors built into the system that detect ventricular fibrillation or ventricular tachycardia. If the device senses VF or VT, then the device advises the operator to deliver a shock.

The portable and automated nature of AEDs saves lives. Defibrillation is needed to reset the heart’s electrical activity within minutes of the patient losing consciousness as the result of VF or VT. When defibrillation is indicated, earlier treatment is better. The increasing availability of AEDs, along with the ability of people trained to use the automated device in the right circumstances, promises to save even more lives in future.

An automatic implantable cardioverter defibrillator (ICD) has been developed for those with unpredictable fibrillation. An ICD incorporates a pacemaker component and, much like an external defibrillator, can deliver large electrical jolts directly to the heart to restore normal rhythm.

Bibliography

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"Cardioversion." Health Library, November 26, 2012.

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