When an electrical current is applied to the myocardium, electrons present at the outer surface of the myocytes stimulate voltage-dependent sodium channels present within the cell membranes causing rapid cellular depolarization. This wave of depolarization spreads nearly instantaneously through the myocardium, such that any remaining activation wave fronts present within the myocardium reach tissues while in the refractory phase, resulting in defibrillation. A dominant pacemaker can then resume an orderly depolarization-repolarization sequence, hopefully normal sinus rhythm. In order to achieve effective defibrillation, a critical mass of myocardium must be depolarized. If an insufficient amount of myocardium is depolarized, any remaining fibrillating wave fronts may not reach a sufficient amount of tissue in the refractory phase to be suppressed. The amount of energy delivered to the myocardium, transthoracic resistance, and paddle/pad position all influence the amount of current that traverses the myocardium, and thus how much myocardium is ultimately depolarized. There are two different waveforms by which energy may be delivered to the myocardium: monophasic and biphasic. Lower-energy biphasic waveform shocks are safe and have equivalent or higher success rates for termination of VF than monophasic waveform shocks of equivalent or higher energy. The optimal level of energy delivered by biphasic shocks has yet to be determined despite multiple prospective studies. The manufacturer’s recommended energy dose should be used for an initial shock (120-200 J). No human studies have demonstrated harm with biphasic shocks up to 360 J, therefore starting with 120-200 J and then escalating up to 360 J may be considered. Current flow through the heart causes myocardial depolarization and thus defibrillation. At a fixed level of energy, the amount of current delivered to the myocardium is inversely proportional to the transthoracic resistance, as stated by Ohm’s law (I=V/R, where V is voltage or electrical potential energy, I is current and R is resistance). Thus, lowering the resistance will help to ensure optimal current delivery. Proper contact between the patient’s skin and the paddle/pad can aid in lowering the resistance. Therefore, removing hair and/or moisture from the patient’s skin is critical. Conductive electrode gel reduces the transthoracic resistance by approximately 30%. A layer of conductive gel is incorporated into self-adhesive defibrillation pads during the manufacturing process. Also, application of 11 lbs of pressure with paddles lowers the transthoracic resistance by approximately 25%. Finally, paddle/pad size is inversely proportional to transthoracic resistance. 13 cm diameter paddles reduce transthoracic resistance by 21% as compared with 8.5 cm diameter paddles. Current recommendations by the AHA state electrode size of 8-12 cm is reasonable for use in adults. Four pad positions are reasonable and equally effective in terminating atrial and ventricular arrhythmias. These positions include the anterolateral, anteroposterior, anterior-left infrascapular, and anterior-right infrascapular. The most commonly used pad position is anterolateral due to ease of placement. Of note, pads or paddles should be placed under the breasts and hair should be shaved prior to placement of pads/paddles.
2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 2010;122(suppl 3):S706 –S719.
J H Truong, P Rosen Current concepts in electrical defibrillation. J Emerg Med: 1997, 15(3);331-8