A defibrillator comprising: one or more capacitors for delivering a defibrillating shock to a patient; one or more electronic ports for receiving signals from at least one sensor for obtaining indications of an electrocardiogram for the patient; a patient treatment module comprising one or more processing devices configured for: receiving (402, 422) data representing an ECG signal, processing (404, 424) a variable-length time segment (252, 254, 256) of the ECG signal comprising an initial time period, wherein the processing comprises identifying one or more features of the variable-length time segment based on an initial rule set corresponding to the initial time period, for determining whether an ECG rhythm represented by the ECG signal is shockable or non-shockable, based on the initial rule set, determining (308, 310) whether the ECG rhythm represented by the ECG signal is shockable or non-shockable or whether to extend the variable-length time segment for further analysis, based on a determination to extend the variable-length time segment for further analysis, extending the initial time period of the variable-length time segment by one or more additional time periods, identifying one or more features of the variable-length time segment based on an adjusted rule set corresponding to an adjusted time period, for updating the determination of whether the ECG rhythm is shockable or non-shockable; and an output circuit configured to provide an output based on the determination. one or more capacitors for delivering a defibrillating shock to a patient; one or more electronic ports for receiving signals from at least one sensor for obtaining indications of an electrocardiogram for the patient; a patient treatment module comprising one or more processing devices configured for: receiving (402, 422) data representing an ECG signal, processing (404, 424) a variable-length time segment (252, 254, 256) of the ECG signal comprising an initial time period, wherein the processing comprises identifying one or more features of the variable-length time segment based on an initial rule set corresponding to the initial time period, for determining whether an ECG rhythm represented by the ECG signal is shockable or non-shockable, based on the initial rule set, determining (308, 310) whether the ECG rhythm represented by the ECG signal is shockable or non-shockable or whether to extend the variable-length time segment for further analysis, based on a determination to extend the variable-length time segment for further analysis, extending the initial time period of the variable-length time segment by one or more additional time periods, identifying one or more features of the variable-length time segment based on an adjusted rule set corresponding to an adjusted time period, for updating the determination of whether the ECG rhythm is shockable or non-shockable; and receiving (402, 422) data representing an ECG signal, processing (404, 424) a variable-length time segment (252, 254, 256) of the ECG signal comprising an initial time period, wherein the processing comprises identifying one or more features of the variable-length time segment based on an initial rule set corresponding to the initial time period, for determining whether an ECG rhythm represented by the ECG signal is shockable or non-shockable, based on the initial rule set, determining (308, 310) whether the ECG rhythm represented by the ECG signal is shockable or non-shockable or whether to extend the variable-length time segment for further analysis, based on a determination to extend the variable-length time segment for further analysis, extending the initial time period of the variable-length time segment by one or more additional time periods, identifying one or more features of the variable-length time segment based on an adjusted rule set corresponding to an adjusted time period, for updating the determination of whether the ECG rhythm is shockable or non-shockable; and identifying one or more features of the variable-length time segment based on an initial rule set corresponding to the initial time period, for determining whether an ECG rhythm represented by the ECG signal is shockable or non-shockable, based on the initial rule set, determining (308, 310) whether the ECG rhythm represented by the ECG signal is shockable or non-shockable or whether to extend the variable-length time segment for further analysis, based on a determination to extend the variable-length time segment for further analysis, extending the initial time period of the variable-length time segment by one or more additional time periods, identifying one or more features of the variable-length time segment based on an adjusted rule set corresponding to an adjusted time period, for updating the determination of whether the ECG rhythm is shockable or non-shockable; and an output circuit configured to provide an output based on the determination.