DC CARDIOVERSION AND DEFIBRILLATION



Direct current (DC) electrical cardioversion or defibrillation is the method of choice for terminating tachyarrhythmias that result in hemody­namic deterioration and those unresponsive to pharmacological termination. Cardioversion re­fers to the delivery of a DC shock, usually of rel­atively low energy, synchronized with the QRS complex of an organized tachyarrhythmia. QRS synchronization is important to avoid delivering a shock during ventricular repolarization (T wave) that may precipitate ventricular fibrillation. De­fibrillation refers to an asynchronously delivered, relatively high energy shock to terminate ventric­ular fibrillation. Most supraventricular and ven­tricular tachyarrhythmias terminate with DC shock, although rhythms due to abnormally in­creased automaticity, especially if associated with digitalis intoxication, may not.

Prior to elective cardioversion the procedure should be explained to the patient and a physical examination, including palpation of all pulses, performed. Metabolic parameters (i.e., blood gases, electrolytes) should be normal, and ideally the patient should have fasted for 6 to 8 hours prior to the procedure. Digitalis should be with­held on the morning of cardioversion. Patients re­ceiving digitalis without clinical evidence of tox­icity are at very low risk for digitalis-induced complications. A short-acting barbiturate or di­azepam can be used for anesthesia. Intravenous access should be available and resuscitation equipment at hand. Paddles should be lubricated with an electrolyte jelly and placed firmly to con­tact with the chest wall, either one paddle in the left infrascapular region and the other over the upper sternum at the third interspace, or one pad­dle to the right of the sternum at the first or second interspace and the other in the left midclavicular line at the fourth or fifth interspace. Shocks of 25 to 50 joules terminate most tachyarrhythmias ex­cept for atrial fibrillation that may require 100 to 200 joules and ventricular fibrillation that may require 100 to 400 joules. If the first low-energy shock fails to terminate the arrhythmia, the energy should be titrated upward.
Tachycardia that produces complications of hy­potension, congestive heart failure, or angina and does not respond promptly to medical manage­ment should be terminated electrically. DC shock should be avoided if possible in patients with tachyarrhythmias caused by digitalis toxicity be­cause of the risk of precipitating life-threatening refractory ventricular tachyarrhythmias. The ad­ministration of an antiarrhythmic drug prior to electrical termination of the arrhythmia may help maintain sinus rhythm after cardioversion. Many arrhythmias, especially chronic atrial fibrillation, commonly recur, and maintenance of sinus rhythm is sometimes a difficult problem.

Ventricular fibrillation due to an improperly synchronized or at times a properly synchronized shock is a complication of DC cardioversion. Im­mediate electrical defibrillation is mandatory. Systemic emboli occur, and patients with atrial fibrillation, especially those with a high risk for emboli (e.g., mitral stenosis, atrial fibrillation of recent onset, a history of emboli, a prosthetic mi­tral valve, enlarged left ventricle or left atrium, or congestive heart failure) may require anticoagu­lation for two weeks before cardioversion to lower the risk of emboli. Anticoagulation should be con­tinued for several weeks after cardioversion. El­evation of myocardial enzyme fractions after car­dioversion is not common.