Cardioversion
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Synchronized electrical cardioversion is the process by which an abnormally fast heart rate or cardiac arrhythmia is terminated by the delivery of a therapeutic dose of electrical current to the heart at a specific moment in the cardiac cycle.
Pharmacologic cardioversion uses medication instead of an electrical shock to convert the cardiac arrhythmia.
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[edit] Synchronized electrical cardioversion
To perform synchronized electrical cardioversion two electrode pads are used, each comprising a metallic plate which is faced with a saline based conductive gel. The pads are placed on the chest of the patient, or one is placed on the chest and one on the back. These are connected by cables to a machine which has the combined functions of an ECG display screen and the electrical function of a defibrillator. A synchronizing function (either manually operated or automatic) allows the cardioverter to deliver a reversion shock, by way of the pads, of a selected amount of electrical current over a predefined number of milliseconds at the optimal moment in the cardiac cycle which corresponds to the R wave of the QRS complex on the ECG. Timing the shock to the R wave prevents the delivery of the shock during the vulnerable period (or relative refractory period) of the cardiac cycle, which could induce ventricular fibrillation. If the patient is conscious, various drugs are often used to help sedate the patient and make the procedure more tolerable. However, if the patient is haemodynamically unstable or unconscious, the shock is given immediately upon confirmation of the arrhythmia. When synchronized electrical cardioversion is performed as an elective procedure, the shocks can be performed in conjunction with drug therapy until sinus rhythm is attained. Multiple electrical shocks may cause burns of the epidermis at the pad sites. After the procedure, the patient is monitored to ensure stability of the sinus rhythm.
Synchronized electrical cardioversion is used to treat hemodynamically significant supraventricular (or narrow complex) tachycardias, including atrial fibrillation and atrial flutter. It is also used in the emergent treatment of wide complex tachycardias, including ventricular tachycardia, when a pulse is present. Pulseless ventricular tachycardia and ventricular fibrillation are treated with unsynchronized shocks referred to as defibrillation. Electrical therapy is inappropriate for sinus tachycardia, which should always be a part of the differential diagnosis.
[edit] Pharmacologic cardioversion
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Various antiarrhythmic agents can be used to return the heart to normal sinus rhythm. Pharmacological cardioversion is an especially good option in patients with fibrillation of recent onset. Drugs that are effective at maintaining normal rhythm after electric cardioversion, can also be used for pharmacological cardioversion. Drugs like amiodarone, cardizem, and metoprolol are frequently given before cardioversion to decrease the heart rate, stabilize the patient and increase the chance that cardioversion is successful.There are two classes of agents that are most effective for pharmacological cardioversion.
Class I agents: Procainamine, quinidine and disopyramide are Class Ia agents, while flecainide and propafenon are Class Ic agents.
Class III agents: Amiodarone and sotalol are effective Class III agents.
If the patient is stable, Adenosine may be administered first, as the medicine performs a sort of "chemical cardioversion" and may stabilize the heart and let it resume normal function on its own without using electricity.
