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Direct cardioversion-defibrillation
Last reviewed: 04.07.2025
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Transthoracic direct cardioversion-defibrillation of sufficient intensity depolarizes the entire myocardium, causing immediate whole-heart refractoriness and recurrence of depolarization. The most rapid intrinsic pacemaker, usually the sinus node, then resumes control of the heart rhythm. Direct cardioversion-defibrillation is very effective in terminating reentry tachyarrhythmias. However, the procedure is less effective in terminating automatic arrhythmias because the restored rhythm is often an automatic tachyarrhythmia. For treatment of arrhythmias other than VF, direct cardioversion-defibrillation must be synchronized with the complex (called direct cardioversion), because a shock that occurs during the sensitive period (near the peak of the T wave) can lead to VF. In VF, synchronization with the complex is irrelevant and impossible to achieve. Direct cardioversion-defibrillation performed without synchronization with the complex is called direct defibrillation.
If cardioversion is the treatment of choice, the patient should fast for 6 to 8 hours before the procedure to prevent aspiration. Because the procedure may cause anxiety and is painful, brief general anesthesia or intravenous analgesia and sedation (eg, fentanyl 1 mcg/kg, then midazolam 1 to 2 mg every 2 minutes to a maximum of 5 mg) are given as needed. Personnel trained in mechanical ventilation should be available.
The electrodes (pads or fingers) used for cardioversion can be placed anteriorly and posteriorly (along the left sternal border in the third to fourth intercostal space and in the left subscapular region) or anteriorly and laterally (between the clavicle and the second intercostal space, along the right sternal border, and in the fifth to sixth intercostal space at the apex of the heart). After synchronization with the complex, confirmed on the monitor, the shock is delivered. The most effective level of shock depends on the type of tachyarrhythmia. The effect of cardioversion is increased by using biphasic shocks, in which the current polarity partially modifies the nature of the shock wave. Complications are usually few, mainly in the form of atrial and ventricular extrasystoles and muscle pain. Less frequently, mainly in patients with altered LV function or after the use of several shocks, cardioversion-induced myocyte death and electromechanical dissociation occur.
Direct cardioversion-defibrillation can be used directly on the heart during thoracotomy or when placing an intracardiac catheter, in which case much smaller shocks are required.
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