cardioversion

the delivery of a direct current countershock synchronized with the QRS complex to the myocardium as an elective treatment to end tachydysrhythmias. (For emergency treatment using a nonsynchronized current to terminate arrhythmia, see defibrillation.) The goal of cardioversion is to restore sinoatrial control of the heart rhythm by depolarizing the entire myocardium at the moment of shock. The depolarization interrupts reentry circuits, thus ending myocardial fibrillation and some other types of dysrhythmias. The electric shock can be delivered directly to the myocardium in an open chest procedure, or through externally applied paddles placed on the chest.

Cardioversion is most effective in terminating arrhythmias due to continuous reentry, including atrial flutter, atrial fibrillation, paroxysmal supraventricular tachycardia, ventricular tachycardia, and ventricular fibrillation. Patients who have had a recent myocardial infarction and resultant atrial, nodal, or ventricular tachycardia are the most frequent candidates for cardioversion. Those with severe, longstanding arrhythmias due to chronic extensive heart disease usually do not benefit from this procedure.

Cardioversion should be done only by trained physicians in a setting where resuscitation equipment and respiratory support are readily at hand. Serum potassium levels must be within normal limits at the time of procedure because hypokalemia increases the patient's chance of developing deadly postconversion dysrhythmias. If necessary, potassium salts can be given prior to the procedure. digitalis toxicity predisposes the patient to life-threatening dysrhythmiasduring cardioversion and the drug should be withheld several days prior to the anticipated procedure. hypoxia and acidosis may decrease the chances of successful cardioversion.

The restoration of normal sinus rhythm by chemical or electrical means. When performed medicinally, the procedure relies on the oral or intravenous administration of antiarrhythmic drugs. Electrical cardioversion relies instead on the delivery of synchronized shock of direct electrical current across the chest wall. It is used to terminate arrhythmias such as atrial fibrillation, atrial flutter, supraventricular tachycardia, and well-tolerated ventricular tachycardia. Unlike defibrillation, which is an unsynchronized shock applied during dire emergencies, electrical cardioversion is timed to avoid the T wave of cardiac repolarization to avoid triggering malignant arrhythmias. A patient will almost always require sedation and analgesia before the procedure. See: illustration

CAUTION!

Electrical cardioversion should not be used in patients who have recently eaten (because of the risk of regurgitation of stomach contents), in patients with severe electrolyte abnormalities, in patients with some drug overdoses, or in patients unable or unwilling to give informed consent. Patients need to be advised of the risks of cardioversion, including the rare precipitation of ventricular fibrillation and ventricular tachycardia, the development of bradyarrhythmias or heart blocks, and the possibility of embolic stroke.

Patient care

The procedure, expected sensations, complications, and risks are explained to and clarified for the patient. Emotional support is provided throughout the procedure and at its conclusion. The patient's medication history is reviewed, and cardiac glycoside use is reported to the health care provider, along with the patient's electrolyte levels. Emergency equipment (including ACLS drugs, a bag mask device, supplemental oxygen, suction, laryngoscope and appropriate size ET tube, defibrillator, and supplies for intravenous injection) are assembled at the bedside. In the hospital setting, emergency personnel (respiratory technicians, anesthesiologists, nurses, and paramedics) may assist the attending physician. The patient's vital signs are checked, an intravenous infusion is started, and the patient is connected to a continuous ECG monitor. Dentures are removed from the mouth, and necklaces or pendants, as well as nitroglycerin patches, are removed from the chest and neck. Chest electrodes are placed to facilitate recording of tall R waves without interfering with paddle placement. A 12-lead ECG is obtained and the patient is given enriched oxygen to breathe. The patient is placed in a supine position, and adequate ventilation and oxygenation are ensured by observation and oximetry. A sedative, such as diazepam, is provided as prescribed unless the patient is profoundly hypotensive. The defibrillator leads are attached to the patient. The cardioverter/defibrillator is set to synchronize with the patient's QRS complex, and the recording is checked to ensure that each R wave is marked. The control is set to the energy level prescribed by the health care provider or by protocol. The defibrillation pads for hands-free operation (or manual paddles) are placed in prescribed positions on the chest wall. All personnel in attendance are cleared from direct contact with the patient or his or her bed. After this is carefully verified, the electrical current is discharged. The monitor is immediately analyzed to ensure that the dysrhythmia has resolved. If it has not, the procedure is repeated, usually with a higher energy setting. After successful cardioversion, health care personnel monitor the posttreatment rhythm and vital signs until the patient's stability is assured. The patient's skin is inspected for burns. See: defibrillation