An external defibrillator includes a therapy delivery circuit, a sensor, and a processor. The therapy delivery circuit is configured to be electrically charged and to deliver electrical therapy to a patient. The sensor is configured to sense a physiological condition of the patient and generate data indicative of a probability that therapy will be delivered to the patient. The processor is configured to analyze data generated by the sensor to determine whether there is a threshold level of the probability that the therapy delivery will be delivered to the patient, if the probability is at least at the threshold level, charge the therapy delivery circuit, and determine whether therapy delivery is advisable based on the physiological condition of the patient after determining whether the probability is at least at the threshold level.