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Author: Philip J Podrid, MD
Editors: Bradley P Knight, MD, FACC; Leah K Moynihan, RNC, MSN; Gordon M Saperia, MD, FACC
CARDIOVERSION OVERVIEW — Cardioversion is a procedure used to treat abnormal heart rhythms (also called cardiac arrhythmias). The most commonly treated arrhythmia is atrial fibrillation During cardioversion, an electrical "shock" is delivered to the heart to restore its rhythm to a normal pattern. The electrical energy can be delivered externally, with electrodes placed on the chest or directly to the heart using paddles on the heart during an open chest surgery. Alternately, the energy can be delivered through the electrodes of a permanently implanted device called a cardioverter-defibrillator. (See "Patient information: Implantable cardioverter-defibrillators".)
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 alter the heart beat, 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 and left sides of the heart. The two lower chambers are the right and left ventricles (figure 1).
Electrical impulses cause muscle, including 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 nerve 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 arrhythmias, 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 reentrant loop. Each time the impulse cycles through the reentrant circuit, it activates the heart.
Some arrhythmias are 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".)
Other arrhythmias are irregular and 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 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.
CARDIOVERSION PROCEDURE — There are two ways of performing cardioversion: externally or internally.
External cardioversion — External cardioversion is much more common. It is performed in a closely monitored 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. One is applied near the breastbone, 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.
Internal cardioversion — One type of internal cardioversion uses a device called an implantable cardioverter-defibrillator (ICD). It is surgically placed 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".)
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. 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.
Embolism can occur after cardioversion if an embolus becomes dislodged from the atria as the heart begins to beat normally. Blood clots (emboli) can form within the atria 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. Some patients will be required to take an anticoagulant indefinitely. (See "Patient information: Atrial fibrillation".)
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 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.
Ventricular tachycardia — Ventricular tachycardia is an arrhythmia arising in the ventricles (lower chambers of the heart) and may be potentially life-threatening. Cardioversion is very effective (>90 percent) for stopping this arrhythmia.
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.
Cardioversion in patients with implanted devices — Several precautions are necessary when attempting external cardioversion in a patient with a permanent implanted device, such as a pacemaker or implantable cardioverter-defibrillator (ICD).
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. After cardioversion, the pacemaker or ICD should be evaluated to ensure that it still functions normally.
CARDIOVERSION COMPLICATIONS — There are several complications that can occur with cardioversion:
* Cardioversion can sometimes provoke other arrhythmias or disturbances of impulse conduction (heart block).
* Cardioversion can dislodge blood clots from the heart, which can travel to the lungs 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 such as warfarin 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. Pre-treating the skin with a steroid cream and taking a medication, such as ibuprofen, can reduce the severity of this pain.
Reference:
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