A photoplethysmography or PPG
signal processing device configured to detect
atrial flutter, includes a processor and memory including
computer program code The processor and memory are configured to cause the PPG
signal processing device toobtain (101) at least one PPG
signal (200) for a human or animal;detect (102) heartbeats (201, 202, 203, 204, 205, . . . ) in the at least one PPG signal (200); anddetermine (103) interbeat intervals (301, 302, 303, 304, . . . ) for pairs of consecutive heartbeats (201-202, 202-203, 203-204, 204-205, . . . ) in the at least one PPG signal (200).The processor and memory cause the PPG
signal processing device to detect (104) interbeat interval baselines, each interbeat interval baseline (302-304) corresponding to a plurality of consecutive interbeat intervals (302, 303, 304) that are constant within a predefined tolerance value;determine (105) respective average interbeat interval values (401, 402, . . . ) for the interbeat interval baselines (302-304); anddetect (110) a relationship between the average interbeat interval values (401, 402, . . . ; 501, 502, . . . ).