Physiological Motion Artifacts in Capacitive ECG: Ballistocardiographic Impedance Distortions

Abstract

Capacitive electrocardiogram (cECG), a tried and tested technique for more than 50 years, can replace the gold standard electrocardiogram (ECG) only in certain applications, where unobtrusiveness is aimed at the expense of reduced signal quality. This inferiority is due, among other reasons, to the fragility of the capacitive coupling between the body and the electrode, observed as motion artifacts in the presence of time-variant coupling capacitance. Attempts were made to remove these artifacts in earlier studies using adaptive noise cancellation by assuming statistical independence between motion and ECG signals. This article considers the mechanical vibrations of the human body itself as the source of time-variant coupling capacitances, which can cause motion artifacts. Ballistocardiography (BCG) measurements on cECG electrodes were recorded and analyzed to investigate how these mechanical vibrations influence the coupling impedance of the clothing between the body and the electrode. A novel sensor has been proposed and tested in a test bench for four different nominal pressure values (15, 20, 25, and 30 mmHg) with minute-increasing steps for the measurement of the coupling capacitance. Subsequently, simultaneous measurements of BCG and the coupling capacitance were conducted on three healthy subjects for two different electrode positions. A significant correlation between the coupling capacitance and measured ballistocardiograms was found both in time-domain shapes and time–frequency distributions in all measurements.