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4 Measurement of Biosignals and Analog Signal Processing

may be the cause of reduced valve function or for a thromboembolic event⁶ can be.

Close monitoring by the patient opens up the possibility of detecting such accumula-

tion processes at an early stage and treating them with medication.

Fig. 4.21: Patient with handheld device for self-monitoring of implanted mechanical prosthetic heart

valve.

The method is again based on sound recording and analysis. Mechanical prosthetic

heart valves produce characteristic sound in the frequency range up to 20 kHz. In

the case of thrombus attachment, the high-frequency component of the valve sounds

changes in particular. Stethoscopes cannot be used as sound recorders at high fre-

quencies ⁷. The device shown in Figure 4.21 uses an electret microphone that registers

the airborne sound emitted from the body. The measurement position is in the sternal

axis and not above the heart, because there the high-frequency sound components

are largely absorbed by the lungs. The valve sounds are amplified, filtered and fed to a

digital signal processor for analysis. The analysis result is displayed on a small screen.

The subsection 6.3.2 presents the necessary analysis methods using the Matlab pro-

gramming language.

6 Thromboembolism: occlusion of a vessel by a thrombus (blood clot).

7 Good stethoscopes have a bandwidth up to approx. 600 Hz. The bandwidth is limited in particular

by the inertia of the diaphragm.