An external defibrillator comprises an energy storage module, a discharge circuit, electrodes, a measurement circuit to sense contemporaneously a first ECG signal from a first vector and a second ECG signal from a second vector, and a subsequent ECG signal, and a processor. The processor is configured to multiply values of the first ECG signal with values of the second ECG signal to derive a product waveform, detect, in the product waveform, peaks that exceed a detection threshold, measure durations of time intervals between pairs of successive detected peaks, compute a heart rate of the patient from the measured durations of the time intervals, determine from the subsequent ECG signal whether a shock criterion is met, and when met, control the discharge circuit to discharge a stored electrical charge to deliver a shock to the patient. A communication module is configured to transmit the computed heart rate.
