Interference and Removal of Respiration Harmonics on Noncontact Seismo-cardiogram Signals

Abstract

The microwave radar technique is a promising noncontact method for detecting seismocardiogram (SCG) signals, which can provide valuable information regarding the mechanical aspects of cardiovascular health. The noncontact method takes the measurement without directly attaching sensors to the skin, therefore providing a less obtrusive and more convenient solution for the patient. Our preliminary investigation has demonstrated the ability of this noncontact approach to capture the timing features of the SCG waveform. However, since the chest wall motion measured by the noncontact microwave radar sensor contains both respiration and heartbeat patterns of a human subject, higher order harmonics of the respiration signal might severely contaminate the noncontact SCG signals. Since the respiration harmonics are discretely spread in the full frequency band of heartbeat signals, comb filters are adopted to remove the respiration harmonic interference. To achieve the goal of reducing interference of respiration harmonics and at the same time preserving as much cardiac information as possible, the comb filters are designed to have a steep slope and a narrow bandwidth. The position of the respiration and its harmonics are obtained with a complex demodulation method and are used as the input parameter of the comb filter. A comb filter with a shelving filter order of 10 and a bandwidth of 0.024 Hz, which can filter out the respiration harmonics up to the 10th order, is demonstrated to be effective in reducing the interference of respiration harmonics and in improving the capability of extracting the change of systolic time intervals from the noncontact SCG signals in the presence of physiological perturbations.