An electrical cardioversion, often referred to simply as a cardioversion, is a procedure used to treat an abnormal and rapid heart rhythm (also called a cardiac arrhythmia). The most commonly treated arrhythmia is atrial fibrillation. Another rhythm commonly treated with electrical cardioversion is atrial flutter. During a cardioversion procedure, an electrical "shock" is delivered to the chest to restore the heart rhythm to a normal pattern.
The following is a discussion of cardioversion and the situations in which it is used.
THE HEART'S CONDUCTION SYSTEM
To understand how electricity can be used to allow the heart to beat normally, it is helpful to first understand the heart's electrical system. The heart is made up mostly of muscle tissue. This muscle forms the walls of the four heart chambers. The upper chambers are called atria, with one on the right side and one on the left side of the heart. The two lower chambers are the right and left ventricles (figure 1).
Electrical impulses cause the heart muscle to contract. During a normal heart rhythm, each impulse moves in a standardized and orderly manner over nerves from the atria to the ventricles, with each impulse resulting in one heart beat (figure 2).
Under normal conditions, the electrical impulses are generated from an area of specialized pacemaker cells called the sinus node, located in the right atrium (the upper right chamber of the heart).
After leaving the sinus node, the impulse travels through and activates both atria. The impulse then travels through specialized heart fibers that transmit the impulse rapidly to various parts of the right and left ventricles; this causes the heart to contract. Each individual muscle cell of the ventricles is activated. If this pattern of conduction and activation is altered in any way, an abnormal heart rhythm (arrhythmia) may occur.
TYPES OF ARRHYTHMIAS
During a rapid cardiac arrhythmia, an abnormal electrical mechanism overrides the action of the sinus node. There are a wide variety of cardiac arrhythmias, some of which start in the atria and some in the ventricles.
In a normal heart rhythm, the electrical impulse moves from the top to the bottom of the heart, then dissipates. The next impulse begins independently in the sinus node (figure 2). Sometimes, cardiac tissue can form an electrical loop or short-circuit. If an electrical impulse enters this loop at the right time under the right conditions, it will cycle through the loop over and over again, repeating itself indefinitely. This is called a re-entrant loop. Each time the impulse cycles through the re-entrant circuit, it activates the heart, causing another contraction.
Arrhythmias that are generated by a re-entrant circuit are often regular and organized and can be treated with cardioversion. Examples of a single reentrant circuit include atrial flutter and ventricular tachycardia (VT). Under some circumstances, multiple reentrant circuits can develop, resulting in very rapid and chaotic arrhythmias, including atrial fibrillation. (See "Patient information: Wolff-Parkinson-White syndrome (Beyond the Basics)".)
Other arrhythmias, which may be regular or irregular, are caused by a single site (called an ectopic foci) firing repeatedly in the atria or ventricle. Cardioversion is generally not effective for this type of arrhythmia.
How cardioversion works in reentry arrhythmias — Cardioversion involves the delivery of a high-energy shock through the chest wall to the heart muscle. This high-energy impulse activates all of the cardiac muscle and conduction tissue simultaneously. Reentrant circuits are interrupted, breaking the repeating cycle and stopping the arrhythmia. When the reentrant circuit is broken and the arrhythmia stops, the sinus node begins to fire again and a normal heart rhythm is restored.
A cardioversion procedure is performed in a closely monitored hospital-based setting, such as an intensive care unit, an emergency department, or a specially equipped procedure room. The patient's heart rate and rhythm, blood pressure, breathing rate, and oxygen levels are monitored.
Because the shock would be painful for a patient who is awake, an intravenous medication is given to sedate the patient. Patients are asleep during the cardioversion and most do not remember the procedure. It is not usually necessary to have a breathing tube (endotracheal tube) placed before the procedure.
Two electrode pads or paddles are applied to the skin after the skin is shaved (figure 3). One pad is applied near the right upper chest, and the second is placed either on the lower left chest or the back, under the left shoulder blade. When the patient is sedated, a burst of electrical current is sent through these pads or paddles. Most of the electrical impulse is absorbed by the chest, but some makes it to the heart. The amount of energy used depends upon the arrhythmia. If the first attempt is unsuccessful at converting the heart rhythm to normal, subsequent higher energy shocks may be needed.
Once the setup and sedation are complete, the cardioversion itself takes only a few seconds. The patient usually awakens 5 to 10 minutes after the cardioversion when the anesthetic drug administered for sedation wears off.
Cardioversion in patients with implanted devices — Some patients who require a cardioversion procedure already have a pacemaker or implantable cardioverter-defibrillator (ICD). An ICD is a device that is surgically placed under the skin in the chest and constantly monitors the heart's rhythm. It delivers a cardioverting or defibrillating shock if a life-threatening arrhythmia develops. This device is discussed in detail in a separate topic. (See "Patient information: Implantable cardioverter-defibrillators (Beyond the Basics)".)
Several precautions are necessary when attempting external cardioversion in a patient with a permanent implanted device. Cardioversion can change the settings of the ICD or pacemaker, or may damage the ICD or pacemaker, the leads, or the heart's tissue. To reduce these risks, the paddles are placed at least 12 cm from the pacemaker or ICD and usually with the paddles in an anterior-posterior location. After cardioversion, the pacemaker or ICD should be evaluated to ensure that it still functions normally.
When a patient with an ICD undergoes cardioversion, the shock can be delivered using the ICD rather than in the standard fashion. When the shock is delivered from the ICD, it is sometimes referred to as an internal cardioversion. The advantage of using the ICD is that it avoids the risk of a skin irritation from an external shock and the small chance of damage to the ICD system from the shock. The disadvantage of using the ICD is that it consumes some of the battery in the device and does not always work for cardioversion of atrial arrhythmias.
Postprocedure care — After being monitored for several hours, most patients are able to go home, although a family member or friend should drive due to the residual effects of anesthesia. Patients who receive sedation usually cannot drive for 24 hours. Some patients have irritation of the skin in the area where the shock was delivered; an emollient cream such as Aquaphor® or Eucerin® may be applied to reduce the irritation.
CARDIOVERSION FOR SPECIFIC ARRHYTHMIAS
As mentioned above, cardioversion is useful for terminating arrhythmias that arise from a reentry circuit. Cardioversion is tailored for each type of arrhythmia.
Atrial fibrillation — Atrial fibrillation (AF) is a common arrhythmia that may be treated with cardioversion. The success rate of cardioversion with atrial fibrillation is generally better than 75 percent. Chances of success depend upon the duration of atrial fibrillation and size of the left atrium. The success rate is about 90 percent when atrial fibrillation is of less than one year's duration, compared to 50 percent when atrial fibrillation has been present for more than five years.
In general, there are two ways that a cardioversion procedure for AF can fail. In some cases, the energy required to restore the heart to a normal rhythm is higher than the maximum energy that can be delivered by the cardioversion equipment. When this occurs, newer generation equipment that can deliver a more effective type of energy waveform, known as a biphasic waveform, can be used if it was not used initially. Other potential solutions include applying pressure to the paddles during the shock, and administration of an intravenous medication called ibutilide. Ibutilide lowers the energy required to restore the heart to a normal rhythm and is highly effective in patients who can safely received the medication. When this drug is given, patients must be monitored for at least four hours.
A second way that a cardioversion procedure for AF can fail is when the AF recurs immediately after a normal rhythm has been restored. In some patients, administration of ibutilide and repeating the cardioversion is effective in restoring sinus rhythm.
Thromboembolism (blood clot traveling through the bloodstream) can occur after cardioversion if a thrombus (blood clot) becomes dislodged from the atria as the heart begins to beat normally. Blood clots can form within the atria during AF since blood flow is slowed. Most patients are given an anticoagulant (a blood thinner such as warfarin (Coumadin®)) for three to four weeks before and after cardioversion to prevent an embolism unless the AF has been present for less than a day or two. Some patients will be required to take an anticoagulant indefinitely. (See "Patient information: Atrial fibrillation (Beyond the Basics)".)
For a scheduled cardioversion, some patients will be given an antiarrhythmic drug starting 24 to 48 hours before the procedure to improve the chances of maintaining a normal rhythm following cardioversion. AF is most likely to recur within the first few hours after cardioversion; treatment before the cardioversion will allow the medication time to get into the bloodstream, which may prevent a recurrence. In addition, a normal rhythm may be restored with the medication alone in some patients, making electrical cardioversion unnecessary. Pretreatment with an antiarrhythmic drug may also improve the chances of a successful cardioversion and reduce the energy required for the procedure.
Atrial flutter — Atrial flutter is an atrial arrhythmia that can be treated with cardioversion. The success rate is 90 to 100 percent. Success is not related to the duration of this arrhythmia. Not uncommonly, anticoagulation is also given prior to and after cardioversion, although the risk of a thromboembolism is much less than with AF.
Cardioversion during pregnancy — Cardioversion can be performed during pregnancy without affecting the rhythm of the fetus. It is recommended, however, that the fetal rhythm be monitored during the procedure.
There are several complications that can occur with cardioversion:
- Cardioversion can sometimes provoke other arrhythmias or disturbances of impulse conduction (heart block). The shock is timed with the heartbeat to avoid provoking a dangerous arrhythmia. In an occasional patient, the sinus node may not work properly after cardioversion and a very slow heart rate may result. A temporary pacemaker may be necessary to correct this problem.
- Cardioversion can dislodge blood clots from the heart, which can travel to the brain or elsewhere throughout the body; this is less likely in patients who have had an arrhythmia for less than 48 hours. Use of an anticoagulant before cardioversion can reduce this risk. (See 'Atrial fibrillation' above.)
- Some heart tissue may be damaged or may actually die (called myocardial necrosis) as a result of high-energy shocks or repeated shocks. Significant damage is uncommon and there are generally no short or long term complications.
- The heart muscle may not function as well after cardioversion, although this is temporary and does not usually cause symptoms.
- The skin may be burned and/or painful as a result of the shocks. Taking a medication, such as ibuprofen, can reduce the severity of this pain.
WHERE TO GET MORE INFORMATION
Your healthcare provider is the best source of information for questions and concerns related to your medical problem.
This article will be updated as needed on our web site (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.
Patient level information — UpToDate offers two types of patient education materials.
The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.
Patient information: Wolff-Parkinson-White syndrome (The Basics)
Patient information: Heart failure and atrial fibrillation (The Basics)
Patient information: Ventricular tachycardia (The Basics)
Patient information: Atrial flutter (The Basics)
Patient information: Long QT syndrome (The Basics)
Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.
Patient information: Implantable cardioverter-defibrillators (Beyond the Basics)
Patient information: Wolff-Parkinson-White syndrome (Beyond the Basics)
Patient information: Atrial fibrillation (Beyond the Basics)
Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.
Basic principles and technique of cardioversion and defibrillation
Cardioversion for specific arrhythmias
Restoration of sinus rhythm in atrial fibrillation
The following organizations also provide reliable health information.
- National Library of Medicine
(www.nlm.nih.gov/medlineplus/ency/article/007110.htm, available in Spanish)
- National Heart, Lung, and Blood Institute
- American Heart Association