A microwave interferometer as a non-contacting cardio-pulmonary monitor

A coherent, phase-locked microwave interferometer radar system that records the mechanical vibrations of the surface of the chest cavity resulting from the cardiac activity within the chest is described. The instrument operates at 9.3 GHz, which corresponds to a free-space wavelength of 3.1 cm. With a phase resolution of about 0.01° of carrier phase shift, the system has the capability to detect target displacements of roughly 1 μm. With this resolution, the vibration of the anterior wall of the chest cavity of a cooperating supine subject is readily detected. The microwave energy passes through normal clothing; the subject can be fully clothed and no physical contact whatever between the subject and instrument is required. The analog data from the interferometer are digitized and subjected to digital signal processing on an IBM-PC/AT personal computer. The processed recording from a group of young male students reveals distinct peaks during each cardiac cycle that represent high-velocity displacements of the chest wall. Studies are currently in progress to relate these peaks to mechanical events in the cardiac cycle