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Place the Defibrillator in Synchronous Mode when Cardioverting

Angela D. Shoher MD

Electrical cardioversion is used to manage patients with cardiac arrhythmias that involve re-entrant circuits, such as ventricular tachycardia or atrial fibrillation. In an emergent setting any patient with a ventricular rate greater than 150 who is unstable (i.e., chest pain, hypotension, unresponsive) should be treated with synchronized cardioversion. Cardioversion may also be performed on stable patients who have an arrhythmia that is refractory to pharmacologic intervention.

Cardioversion theoretically stops the arrhythmia by depolarizing the re-entrant circuit and making it refractory to propagation. Cardioversion delivers energy that is synchronous with the early part of the QRS complex. If the energy is delivered in an asynchronous manner, it can induce ventricular fibrillation. This occurs when the energy is delivered during the early phase of repolarization. When the cardioverter is placed in synchronous mode, it automatically discharges a current that coincides with a large R or S wave, thereby avoiding the period of repolarization when ventricular fibrillation can occur. In synchronous mode there is always a delay in energy delivery while the cardioverter searches for the R or S wave. It is important to switch the mode to asynchronous delivery if the patient develops ventricular fibrillation.

What to Do

When cardioverting, there are two possible placement options of the pads on the chest wall. In the anteroposterior position, one pad is placed to the right of the sternum anteriorly and the second is placed between the spine and the tip of the left scapula posteriorly. The second option is to place the paddles in the anterolateral position with one paddle in the fourth or fifth intercostal space in the left midaxillary line. The second paddle is then placed to the right of the sternum on the second or third intercostal space. The anterior-posterior approach is optimal in patients with implantable devices to avoid diverting current to the device. Pacemakers should be 10 cm from direct contact with the paddles. The paddles should be placed firmly against the skin to prevent arcing and skin

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burns. Conductive gel should be used to ensure good contact with the skin.

The amount of energy used to cardiovert is controversial. Prompt cardioversion prevents prolonged ischemia and multiple shocks. Excessive energy, however, can lead to myocardial damage. The optimal amount of energy also depends on the patient's arrhythmia. For a patient who is relatively hemodynamically stable, atrial fibrillation with rapid ventricular rate (RVR) that is refractory to pharmacologic therapy, for example, 50 joules may be adequate. For ventrical tachycardias, however, advanced cardiac life support guidelines now recommend starting with 360 joules (unlike the increasing amount of joules recommended in past guidelines). This number also depends on the type of defibrillator. If it ismonophasic (i.e., current travels in one direction between the paddles) more energy will be required. If the machine delivers biphasic current, consideration should be given to starting at 25 joules for a trial fibrillation and 150 joules for ventricular tachycardias.

Suggested Readings

Morgan GH, Mikhail MS, Murray MJ, et al., eds. Clinical Anesthesiology. 3rd Ed. New York: McGraw-Hill; 2002:472–473.

Podrid PJ. Basic Principles and Techniques of Cardioversion and Defibrillation. UpToDate. http://www.uptodate.com

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