Stethoscopes are used to diagnose the noises produced by the heart and lungs. Used in the conventional way, vibrations from the surface of the body are transmitted to a membrane in the chest piece and then to the user’s eardrum where they are perceived as sounds. Acoustic stethoscopes are comparatively inexpensive and have been used reliably for several decades but they have one drawback. The diagnosis of heart murmurs, such as the assessment of heart valve function, is carried out subjectively and is directly dependent on the experience of the doctor conducting the examination.
A procedure was developed for reliably detecting and diagnosing heart sounds using radar. Mobile radar devices could replace conventional stethoscopes and permanent, touch-free monitoring of patients’ vital functions could be possible using stationary radar devices.
Using a six-port continuous-wave radar system, vibrations on the skin caused by the heartbeat were measured. During this process, a radar wave is aimed at the surface of an object and reflected. If the object moves, the phase of the reflecting wave changes. This is used to calculate the strength and frequency of the movement of the chest. In contrast to radar systems for traffic monitoring, the biomedical radar system can detect changes in movement that measure a few micrometers, which is an important prerequisite to diagnosing even the smallest anomalies such as insufficiency, stenoses, or heart valves that do not close properly.
Test patients were examined in various states of activity such as while resting and after sports, and their heart sounds were detected. A direct comparison between the radar system and conventional standard instruments with a digital stethoscope and an electrocardiograph (ECG) showed a very high correlation — 83 percent in a direct comparison of the signal shapes with the digital stethoscope. The slight deviations are caused by the fact that measurements using the radar system and the reference systems cannot be carried out simultaneously on exactly the same place on the body. In addition, the radar system measures a surface area and not a single spot like the stethoscope, which is also a reason for the varying measurement values.
A significant advantage offered by radar is the fact that the values are recorded digitally and are thus not subjective, allowing human error to be increasingly ruled out during the diagnosis of anomalies or diseases. Using biomedical radar systems for automated prophylactic examinations — for example, in doctors’ waiting rooms, at work, or at home — is also feasible.
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