Heart pulse demodulation from Emfit mattress sensor using spectral and source separation techniques

Introduction: Unobtrusive bed mattress sensors such us the electromechanical film transducer (Emfit) record the heart's activity, breathing, and body movements with clinical applications ranging from epilepsy to sleep disorders. The mechanical activity of the heart recorded using these sensors is known as ballistocardiogram (BCG). However, BCG shape changes on position, patient, and mattress. Aim: In this study, we isolate a position- independent heart signal designed to retrieve the heart activity recorded using an Emfit mattress sensor using extended polysomnography (PSG) recordings as a reference. Methods: We used spectral and source separation techniques to infer the heart's Emfit Pulse (EP). We validated the resulting signal using representative 10- minute normal breathing epochs extracted from the PSG from 33 subjects and by estimating the heart rate (HR) from the EP and compared it against the electrocardiogram (ECG) using non-overlapping one- minute windows. Results: Results show a signal similar in shape to a photoplethysmogram (PPG) with different timings in relation to the ECG's R-peak. We found good agreement between HR with a mean absolute error (MAE) of 2.4 beats per minute (bpm) with standard deviation of 4.6bpm. Conclusions: A position independent heart pulse signal from the Emfit mattress sensor was obtained in a range of subjects and validated by means of HR analysis.