Evaluation of Wearable Head BCG for PTT Measurement in Blood Pressure Intervention

This study evaluates the usability of wearable head ballistocardiography (BCG) in providing accurate pulse transit time (PTT) measurements during blood pressure (BP) interventions. Head BCG is a new technique enabling measurement of proximal aortic blood ejection from sensors placed at distal sites, which envisions PTT measurement from single integrated device for cuff-less BP estimation. However, due to its low signal-to-noise ratio and sensitivity to motion artifacts, accurate beat selection is crucial to ensure the integrity of PTT calculation. In this paper, using inertial measurement unit (IMU) sensors integrated in a prototype earbud, we investigate whether the wearable head BCG signal is aligned with the ground-truth proximal reference acquired from the synchronously-recorded impedance cardiography (ICG) signal, to assess the usability of head BCG as the proximal indicator for PTT measurement at different stages of BP intervention. Wearable BCG signals showed highest reliability during resting states, with 63% of detected j-peaks aligned with ground-truth ICG signals. Beat selection via removal of IBI outliers improves the ratio of reliable peaks, at rest (68%) and during exercise (63% at intervention and 52% at plateau). Other methods, such as template matching or rejecting amplitude outliers, only improve the ratio at rest. Overall, this study reveals characteristics of distortions in the head BCG signal during intervention, as a first step toward robust solutions for PTT-based BP tracking on integrated wearable devices.