Electrical cardioversion, or simply cardioversion, is a medical procedure that uses electrical
shocks to treat abnormal heart rhythms (arrhythmias), in particular tachycardias.
The procedure is done by delivering a brief current across the chest, through a pair of
manual paddles or adhesive “hands-free” patches.
Cardioversion is different from defibrillation.
Defibrillation is an emergency procedure that uses non-synchronized, high-energy shocks
to treat cardiac arrest patients.
Cardioversion, on the other hand, is often an elective procedure delivering a lower-energy
shock that is synchronized to a certain stage of the cardiac cycle.
The heart is essentially a muscle that contracts and pumps blood.
Contractions of the heart muscle are coordinated by electrical impulses that are initiated
in the heart’s natural pacemaker - the SA node.
The firing frequency of the SA node determines the heart rate.
The signals from the SA node spread through the cardiac conduction system in a synchronized
manner to produce regular heartbeats.
Arrhythmias occur when abnormal electrical circuits override the normal conduction.
Common abnormal rhythms are either initiated from ectopic sites outside the SA node, or
caused by an electrical impulse that travels around in a localized self-perpetuating loop,
called a re-entrant pathway.
Cardioversion depolarizes the bulk of cardiac muscle simultaneously, disrupting the abnormal
electrical circuits, allowing the heart’s pacemaker to regain control and restore normal
rhythms.
The delivery of the shock is synchronized to the QRS complex, which represents ventricular
depolarization.
This timing is critical to prevent the R-on-T phenomenon, which could put the patient into
cardiac arrest.
Electrical cardioversion is most effective in treating arrhythmias that arise from a
single reentrant circuit such as atrial flutter, AV nodal reentrant tachycardia (AVNRT), or
atrioventricular reentrant tachycardia (AVRT).
Arrhythmias that result from multiple circuits typically require higher-energy shocks to
be treated successfully.
Cardioversion is often scheduled in advance as an elective procedure, but it can also
be used as an emergency treatment for patients who still have a pulse but are hemodynamically
unstable.
Stable patients with arrhythmias are usually treated with medications first, in a process
known as chemical or pharmacological cardioversion.
During electrical cardioversion, the patient is sedated.
The defibrillator is set to “synchronize” mode and R-waves are tracked.
A brief electrical current is sent, during the time of an R-wave, through two electrode
patches or paddles placed on the chest, one on each side of the heart.
The result should be immediate.
If the first attempt is unsuccessful, subsequent higher-energy shocks may be performed.
The procedure itself takes only a few minutes.
Though uncommon, several complications may occur with cardioversion:
- The procedure can either cause a new blood clot to form, or a pre-existing clot, such
as in patients with atrial fibrillation, to dislodge and travel to the brain, resulting
in stroke.
Use of a blood thinner before cardioversion can reduce this risk.
- Cardioversion can sometimes lead to development of a new arrhythmia.
This typically occurs a few minutes after the procedure, in which case medications or
additional shocks can be given as a remedy.
- Other complications are related to sedation, skin burns at the site of contact with the
electrodes, and rarely, heart tissue damage due to high-energy or repeated shocks.