Determinants of defibrillation: prospective analysis of 183 patients.

Previous studies have suggested that a number of factors may influence the ability to defibrillate: the transthoracic resistance and resultant current flow, the paddle electrode size, the duration of preshock ventricular fibrillation (VF) and cardiopulmonary resuscitation, metabolic abnormalities, body weight, the shock energy selected, and whether the patient is receiving lidocaine. To examine the effect of these variables, a prospective study was conducted of 183 patients who received direct-current shocks for VF. Overall defibrillation rates approached 90%, even in patients with secondary VF, but rates of successful resuscitation and survival were much lower. Patients who never defibrillated despite multiple shocks had a prolonged duration of cardiopulmonary resuscitation preceding the first shock (21 +/- 14 minutes) and systemic hypoxia and acidosis. These conditions tended to occur in patients who initially had cardiac arrest from causes other than VF: asystole, severe bradycardia and electromechanical dissociation. In such patients, VF developed only as a late event, which was then often unresponsive to attempted defibrillation. The other factors examined were not major determinants of defibrillation.