A method of delivering electrical therapy to a patient by a medical device includes activating the medical device and performing a first analysis of a first set of data signals sensed by the medical device. If the first analysis shows the first set of data signals meets a first criterion, then charging of an energy delivery circuit is commenced upon completion of the first analysis. A second analysis of a second set of data signals from the patient is performed, and if the second analysis determines that the second set of data signals meet a second criterion, the therapy is delivered. The steps of performing the first analysis and performing the second analysis may be begun at substantially the same time. The step of charging may overlap in time with the step of performing a second analysis. The medical device may be an external defibrillator and the therapy may be a defibrillating shock.