defibrillation: experimental studies

In defibrillation, current flow depends on the energy selected and the transthoracic impedance. If transthoracic impedance is high, current flow may be inadequate to defibrillate. We developed a method by which high transthoracic impedance is automatically compensated for by an increase in operator-selected energy when impedance is high. Transthoracic impedance was predicted in advance of the first shock by passing a low-level current between the defibrillator electrodes during the defibrillator charge cycle; a microprocessor monitored current flow and determined impedance. In 28 mongrel dogs we manipulated transthoracic impedance by placing glycerin-soaked gauze pads between the paddle electrodes and the chest. If the predicted impedance exceeded a preset value, the delivered energy was automatically increased by 40% or 100%. Using this impedance-based energy adjustment technique, we found significant improvements in current flow and success rate of shocks when energy was automatically increased to compensate for high transthoracic impedance. The use of transthoracic impedance as a basis for energy adjustment appears a promising technique to minimize the hazards of high electrical energy; it allows low-energy shocks in most patients while avoiding inappropriate low energies in patients with high impedance. Clinical trials are justified. Circulation 71, No. 1, 136-140, 1985. DEFIBRILLATION is accomplished by passage of sufficient electrical current through the heart to depolarize a critical mass of myocardium. 'The amount of current that flows therefore becomes critical. Current flow is dependent on two factors: the energy selected by the operator and the transthoracic impedance. Impedance can be reduced to some degree by firm operator pressure on the electrodes, and this will enhance current flow.3 If pressure is already maximal, the energy selected and the inherent transthoracic impedance then become the major determinants of whether current flow sufficient to defibrillate will be achieved. High electrical energies can be used for defibrillation in most patients,4'6 but have been shown to cause histologic abnormalities in animals7 and atrioventricular block in humans.5 Gascho et al.4 found that when total delivered energy exceeded 240 J, the defibrillation rate fell significantly. Ideally, one should administer the lowest possible energy shock that will minimize damage but still achieve defibrillation.8-0 From the Cardiovascular Center, University of Iowa, Iowa City, and Hewlett-Packard Corporation, McMinnville, OR. Supported in part by NHLBI grant No. HL-14388 and in part by a grant from the Hewlett-Packard Corporation. Address for correspondence: Richard E. Kerber, M.D., Department of Medicine, University of Iowa Hospital, Iowa City. IA 52242. Received Aug. 22, 1984; accepted Sept. 27. 1984. 136 Unfortunately, the energy and current requirements for defibrillation appear to vary considerably between patients,6 12 making it impossible to identify a single energy level that could accomplish both aims in all patients. At any given energy impedance is the major determinant of current flow. It is therefore predictable that in patients with high transthoracic impedance, a lowenergy shock might fail to achieve adequate current flow to defibrillate. We have recently confirmed this in patients in ventricular fibrillation who had a high transthoracic impedance. In these patients 100 J shocks had a success rate of only 22%, as opposed to 68% success when impedance was low or average. 12 Thus, if it were known in advance that a patient had high transthoracic impedance, selection of a low-energy shock would be inappropriate since it would be unlikely to achieve defibrillation; a high-energy shock would be necessary. Conversely, if impedance were low or average, a low-energy shock would be preferable since it would probably be adequate to defibrillate and would be less likely to cause myocardial toxicity. We now have the ability to accurately predict transthoracic impedance before any shocks are given.'2 This should facilitate appropriate energy selection based on impedance. The hypotheses of this study CIRCULATION by gest on N ovem er 8, 2017 http://ciajournals.org/ D ow nladed from LABORATORY INVESTIGATION-DEFIBRILLATION were two: (1) The success of a shock of any given energy with regard to defibrillation is related to the transthoracic impedance; shock success falls as impedance rises. (2) High impedance can be automatically compensated for by increasing the shock energy, which will improve the success rate of the defibrillation procedure.