  Wrist photoplethysmography (PPG) allows unobtrusive monitoring of the heart
rate (HR). PPG is affected by the capillary blood perfusion and the pumping
function of the heart, which generally deteriorate with age and due to presence
of cardiac arrhythmia. The performance of wrist PPG in monitoring beat-to-beat
HR in older patients with arrhythmia has not been reported earlier. We
monitored PPG from wrist in 18 patients recovering from surgery in the post
anesthesia care unit, and evaluated the inter-beat interval (IBI) detection
accuracy against ECG based R-to-R intervals (RRI). Nine subjects had sinus
rhythm (SR, 68.0y$\pm$10.2y, 6 males) and nine subjects had atrial fibrillation
(AF, 71.3y$\pm$7.8y, 4 males) during the recording. For the SR group, 99.44% of
the beats were correctly identified, 2.39% extra beats were detected, and the
mean absolute error (MAE) was 7.34 ms. For the AF group, 97.49% of the
heartbeats were correctly identified, 2.26% extra beats were detected, and the
MAE was 14.31 ms. IBI from the PPG were hence in close agreement with the ECG
reference in both groups. The results suggest that wrist PPG provides a
comfortable alternative to ECG and can be used for long-term monitoring and
screening of AF episodes.
