intubation difficult and may suggest the need for the presence of an anesthesiologist during the procedure.
TABLE 59.1 Indications and Contraindications of Cardioversion | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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TABLE 59.2 Indications for Cardioversion in Patients with Atrial Fibrillation | |||||||
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Electrical Cardioversion
Electrical Cardioversion
Sandeep Duggal
I. INTRODUCTION.
Delivery of electrical countershock to terminate cardiac arrhythmias is a safe and effective technique that is routinely performed in most hospitals. Cardioversion is defined by delivery of energy synchronized to the QRS complex, whereas random delivery of shock during the cardiac cycle (usually done for terminating ventricular fibrillation) is termed defibrillation.
II. MECHANISM.
Although it has long been recognized that application of an electrical shock to the myocardium can restore a normal rhythm, knowledge of the fundamental mechanism underlying defibrillation remains incomplete. A rapidly delivered electric shock depolarizes the myocardial cells and creates a zone of myocardium with an extended refractory period. Activation fronts encountering tissue with a prolonged refractory period will not be able to propagate, thus terminating both macro- and micro-reentrant circuits. Atrial fibrillation and ventricular fibrillation are generally agreed to be more electrically stable rhythms and thus require higher current delivery for termination. This is likely because only regional depolarization in the path of an advancing wave front is required. The most common waveform shapes used in external defibrillation are the monophasic and biphasic waveforms. In biphasic waveforms, the polarity at each electrode reverses partway through the defibrillation waveform. The use of a biphasic waveform in cardioversion and defibrillation has been shown to be associated with an increased efficacy and may reduce the development of postshock arrhythmias.
III. INDICATIONS AND CONTRAINDICATIONS.
The indications and contraindications of cardioversion are listed in Tables 59.1 and 59.2. Cardioversion should not be performed in patients in whom the rhythm is sinus or the abnormal rhythm is secondary to increased automaticity (e.g., multifocal atrial tachycardia and junctional tachycardia). If the presenting rhythm is ventricular fibrillation or ventricular tachycardia with hemodynamic compromise, the only clear contraindication to defibrillation is clear expression of the patient’s (or patient’s surrogate’s) informed wish not to be resuscitated.
IV. PROCEDURE
A. Patient preparation
1. Informed consent should be obtained from the patient or surrogate (if the patient is unable to comprehend and give meaningful informed consent).
2. In elective cases, patient should fast for a minimum of 6 to 8 hours.
3. A review of the patient’s medical history and a focused physical examination should be performed. Special attention should be paid to the airway. Inability to visualize the uvula, inability to open the mouth with at least 2 cm between the teeth, or difficulty in extending the neck are factors that may make potential
intubation difficult and may suggest the need for the presence of an anesthesiologist during the procedure.
intubation difficult and may suggest the need for the presence of an anesthesiologist during the procedure.
4. The patient’s medication and anticoagulation status (for patients in atrial fibrillation or flutter) should be confirmed. Because patients may not always have symptoms with arrhythmias such as atrial fibrillation and atrial flutter, convincing historical or electrocardiographic evidence of the tachycardia initiating within 48 hours of cardioversion should be documented before cardioverting a patient with atrial fibrillation or atrial flutter without adequate anticoagulation due to the risk of thromboembolism.
5. Anticoagulation is a key factor for patients in atrial fibrillation or flutter (Table 59.3) to prevent thromboembolism. The two key oral anticoagulants that may be used for anticoagulation are Coumadin and the newly approved dabigatran, which is used for nonvalvular atrial fibrillation or flutter. Therapeutic levels of anticoagulation for Coumadin and dabigatran differ in timing. For Coumadin, there is frequent laboratory monitoring that needs to be done with adjustment of dosing to reach a target international normalized ratio of 2 to 3, which on average takes 3 to 5 days. However, in regard to dabigatran, there is no laboratory monitoring needed, with therapeutic levels being achieved in about 12 hours. The dosing is fixed at 150 mg bid for patients with creatinine clearance (CrCl) > 30 mL/min and 75 mg bid with CrCl of 15 to 30 mL/min.
6. A 12-lead electrocardiogram (ECG) should be obtained to confirm the presenting rhythm, as well as to discern any suggestion of electrolyte abnormality (hypo- or hyperkalemia) or drug toxicity (digitalis). If any of these is suspected, appropriate blood levels should be checked. Routine measurement of digoxin levels is not recommended.
7. Peripheral venous access should be obtained for elective cases.