A method for rapidly and accurately determining defibrillation thresholds. The method comprises the steps of delivering an initial shock series to a patient, the shock series comprising at least two shocks of differing energy levels, determining an estimated shock level adjustment based on the initial shock series, and delivering at least one adjusted shock of a predetermined energy level based on the estimated shock level adjustment. The technique uses optimized search criteria as opposed to the conventional step-wise decrease and increase techniques. An apparatus for implementing the method is also disclosed.