A system for the detection of Atrial Fibrillation episodes based on the estimation of the heart rhythm irregularity, comprising: - a portable blood oxygenation monitoring device adapted to acquire a plethysmografic waveform of a patient over a period of time, - means to store in said device a computer program for processing said plethysmografic waveform; - a CPU configured to receive the plethysmografic waveform and to execute said computer program, causing the CPU to carry out the steps of a) processing the plethysmografic waveform so as to estimate the time-occurrence of each beat and to obtain the beat-to-beat interval series; b) analyzing said beat-to-beat interval series and extracting two values, namely a Coefficient of Variation value and a Shannon entropy value, • the Coefficient of Variation value being defined as σΔPP/μPP, i.e., the ratio between the standard deviation (σΔPP) of the series of the first derivatives (ΔPP) of the beat-to-beat interval series and the mean value (μPP) of the beat-to-beat interval series, • the Shannon entropy value being computed based on the probability of occurrence of beat-to-beat intervals. c) Classifying the heart rhythm based on the measure of the Mahalanobis distance of the extracted patient values (CV, EN), from values characterizing a population of patients having Atrial Fibrillation and of subjects having Normal Sinus Rhythm. - a portable blood oxygenation monitoring device adapted to acquire a plethysmografic waveform of a patient over a period of time, - means to store in said device a computer program for processing said plethysmografic waveform; - a CPU configured to receive the plethysmografic waveform and to execute said computer program, causing the CPU to carry out the steps of a) processing the plethysmografic waveform so as to estimate the time-occurrence of each beat and to obtain the beat-to-beat interval series; b) analyzing said beat-to-beat interval series and extracting two values, namely a Coefficient of Variation value and a Shannon entropy value, • the Coefficient of Variation value being defined as σΔPP/μPP, i.e., the ratio between the standard deviation (σΔPP) of the series of the first derivatives (ΔPP) of the beat-to-beat interval series and the mean value (μPP) of the beat-to-beat interval series, • the Shannon entropy value being computed based on the probability of occurrence of beat-to-beat intervals. c) Classifying the heart rhythm based on the measure of the Mahalanobis distance of the extracted patient values (CV, EN), from values characterizing a population of patients having Atrial Fibrillation and of subjects having Normal Sinus Rhythm. a) processing the plethysmografic waveform so as to estimate the time-occurrence of each beat and to obtain the beat-to-beat interval series; b) analyzing said beat-to-beat interval series and extracting two values, namely a Coefficient of Variation value and a Shannon entropy value, • the Coefficient of Variation value being defined as σΔPP/μPP, i.e., the ratio between the standard deviation (σΔPP) of the series of the first derivatives (ΔPP) of the beat-to-beat interval series and the mean value (μPP) of the beat-to-beat interval series, • the Shannon entropy value being computed based on the probability of occurrence of beat-to-beat intervals. • the Coefficient of Variation value being defined as σΔPP/μPP, i.e., the ratio between the standard deviation (σΔPP) of the series of the first derivatives (ΔPP) of the beat-to-beat interval series and the mean value (μPP) of the beat-to-beat interval series, • the Shannon entropy value being computed based on the probability of occurrence of beat-to-beat intervals. c) Classifying the heart rhythm based on the measure of the Mahalanobis distance of the extracted patient values (CV, EN), from values characterizing a population of patients having Atrial Fibrillation and of subjects having Normal Sinus Rhythm.