A wearable cardioverter defibrillator system (100, 200), comprising: a support structure configured to be worn by an ambulatory patient; an energy storage module configured to store an electrical charge; a discharge circuit coupled to the energy storage module; electrodes configured to sense a first Electrocardiogram signal of the patient, and a second ECG signal after sensing the first ECG signal; and a processor configured to: detect peaks occurring sequentially within the first ECG signal, establish pairs of the detected peaks, at least one of the pairs being established by peaks not occurring sequentially, in particular wherein all possible pairs of the detected peaks are established, measure durations of time intervals defined by the established pairs, identify, from the measured durations, a representative duration that best meets a plausibility criterion, compute a heart rate of the patient from the representative duration, determine from the second ECG signal whether or not a shock criterion is met, and control, responsive to the shock criterion being met, the discharge circuit to discharge the stored electrical charge through the patient while the support structure is worn by the patient so as to deliver a shock to the patient; and a memory configured to store the computed heart rate. a support structure configured to be worn by an ambulatory patient; an energy storage module configured to store an electrical charge; a discharge circuit coupled to the energy storage module; electrodes configured to sense a first Electrocardiogram signal of the patient, and a second ECG signal after sensing the first ECG signal; and a processor configured to: detect peaks occurring sequentially within the first ECG signal, establish pairs of the detected peaks, at least one of the pairs being established by peaks not occurring sequentially, in particular wherein all possible pairs of the detected peaks are established, measure durations of time intervals defined by the established pairs, identify, from the measured durations, a representative duration that best meets a plausibility criterion, compute a heart rate of the patient from the representative duration, determine from the second ECG signal whether or not a shock criterion is met, and control, responsive to the shock criterion being met, the discharge circuit to discharge the stored electrical charge through the patient while the support structure is worn by the patient so as to deliver a shock to the patient; and detect peaks occurring sequentially within the first ECG signal, establish pairs of the detected peaks, at least one of the pairs being established by peaks not occurring sequentially, in particular wherein all possible pairs of the detected peaks are established, measure durations of time intervals defined by the established pairs, identify, from the measured durations, a representative duration that best meets a plausibility criterion, compute a heart rate of the patient from the representative duration, determine from the second ECG signal whether or not a shock criterion is met, and control, responsive to the shock criterion being met, the discharge circuit to discharge the stored electrical charge through the patient while the support structure is worn by the patient so as to deliver a shock to the patient; and a memory configured to store the computed heart rate.