Non contact monitoring of the respiration activity by electromagnetic sensing.

The aim of this paper is to present a novel measurement method for the detection of the respiratory activity (respiration rate and respiration period) based on the use of a continuous wave (6 GHz) microwave radar reflectometry technique. The paper aims, in particular, to explore the effect on the signal quality of distance D between the sensing apparatus and the patient. The measurement method proposed is based on the measurement of the phase variation of the reflection coefficient (S₁₁) signal measured by a vectorial network analyzer connected to a double ridge horn antenna. The S₁₁ signal has been compared with the synchronous acquisition made by means of a laser Doppler vibrometer (LDVi), measuring the thorax oscillations caused by the respiratory activity. Both signals have been filtered in order to eliminate the effect of high frequency disturbances (heartbeat) and noise. Results show an high correlation between respiration peaks measured with the proposed system and with LDVi; a reduction of the amplitude of the S₁₁ signal phase (as well as the SNR) is reported in correspondence to an increasing of the distance D (-0.11 dB/cm). Tests have been repeated for standing as well as for sitting condition of the subject confirming a better signal quality for the later. Despite the fact that S₁₁ phase variation and SNR are reduced by the distance D, in our experiments, it is still possible to correctly measure the respiration period up to 2.5 m. Data measured show that the reflectometeric approach can be used to monitor at distance with sufficient high SNR (18 dB at 2.5 m) the respiration activity of a subject without the need of a direct contact with the subject skin by means of electrods of sensing belts.