Accurate ECG-Free Heartbeats Localization in Long-Lasting SCG Recordings

Seismocardiography (SCG) is a non-invasive technique able to record the weak mechanical vibrations induced by the beating heart via accelerometers placed onto the subject’s thorax. SCG signals are characterized by cyclic oscillations related to heartbeats. SCG provides information about cardiac mechanical performance, e.g. opening/closure of heart valves, blood ejection, etc. Heartbeats localization on SCG signals is usually performed by using a simultaneous Electrocardiography (ECG) recording, thus limiting the potentiality of the sole SCG patient monitoring. In this study, a template matching algorithm for ECG-free heartbeats localization was tested on long-lasting SCG recordings. SCG and ECG signals were simultaneously acquired from 20 healthy subjects while listening to music for 50 min. A statistical comparison between inter-beat intervals from SCG and ECG signals was carried out. A sensitivity of 97.5% and a positive predictive value of 95.4% were achieved for heartbeat detection. Linear regression, correlation and Bland-Altman analyses showed high accuracy in inter-beat intervals estimation. R2 resulted greater than 0.999, while the limits of agreement were confined within ±4.7 ms. The results suggest that the SCG signal alone is able to provide very accurate, continuous and non-invasive heart rate monitoring. This paves the way to unobtrusive, pervasive, long-term patient monitoring by means of small and lightweight accelerometers.