Method for monitoring cardiac arrest during physical exercise and consequently activating a rescue request, comprising the steps of: continuously measuring a user's heart rate to determine a heart rate in each of a sequence of time intervals having predetermined duration, by means of a heart rate measurement device worn by the user; generating a signal representative of the heart rate (Sfc) measured in each of the time intervals, by means of the heart rate measurement device; continuously detecting a user's motion condition and generating a motion signal (Sm) representative of the motion detected, by means of at least one motion sensor with which the user is provided; receiving said signal representative of the heart rate (Sfc) and said motion signal (Sm) by means of a user portable processor device, communicating with the heart rate measurement device and with the at least one motion sensor; processing the signal representative of heart rate (Sfc), by means of the portable processor device, to recognize a possible cardiac arrest condition based on a real time analysis of the heart rate measured; - processing the motion signal (Sm) to detect a possible stillness condition of the user, by means of the portable processor device; - if, in said processing steps, a cardiac arrest condition and, at the same time, a stillness condition of the user have been recognized, activating a rescue request by the portable processing device; wherein the step of processing the signal representative of the heart rate (Sfc) comprises: - continuously measuring the difference between the heart rates measured in consecutive time intervals; comparing said difference with one or more predetermined variation thresholds, defining normal values associated with the considered physical exercise; recognizing a cardiac arrest condition when the heart rate difference exceeds said one or more variation thresholds for a predetermined number of times along the duration of a predetermined first number of consecutive time intervals. continuously measuring a user's heart rate to determine a heart rate in each of a sequence of time intervals having predetermined duration, by means of a heart rate measurement device worn by the user; generating a signal representative of the heart rate (Sfc) measured in each of the time intervals, by means of the heart rate measurement device; continuously detecting a user's motion condition and generating a motion signal (Sm) representative of the motion detected, by means of at least one motion sensor with which the user is provided; receiving said signal representative of the heart rate (Sfc) and said motion signal (Sm) by means of a user portable processor device, communicating with the heart rate measurement device and with the at least one motion sensor; processing the signal representative of heart rate (Sfc), by means of the portable processor device, to recognize a possible cardiac arrest condition based on a real time analysis of the heart rate measured; - processing the motion signal (Sm) to detect a possible stillness condition of the user, by means of the portable processor device; - if, in said processing steps, a cardiac arrest condition and, at the same time, a stillness condition of the user have been recognized, activating a rescue request by the portable processing device; wherein the step of processing the signal representative of the heart rate (Sfc) comprises: - continuously measuring the difference between the heart rates measured in consecutive time intervals; - processing the motion signal (Sm) to detect a possible stillness condition of the user, by means of the portable processor device; - if, in said processing steps, a cardiac arrest condition and, at the same time, a stillness condition of the user have been recognized, activating a rescue request by the portable processing device; - continuously measuring the difference between the heart rates measured in consecutive time intervals; comparing said difference with one or more predetermined variation thresholds, defining normal values associated with the considered physical exercise; recognizing a cardiac arrest condition when the heart rate difference exceeds said one or more variation thresholds for a predetermined number of times along the duration of a predetermined first number of consecutive time intervals.