Biosignal Processing Fundamentals and Recent Applications with MATLAB (Stefan Bernhard, Andreas Brensing etc.)

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6 Applications and Methods in Biosignal Processing

and frequency domain is a very sensitive method for assessing the integrity of mechan-

ical Prosthetic heart valves [17, 91]. Differentiation of the respective impact sounds of

both valve leaflets requires a time resolution of less than 1 ms, implying a frequency

resolution of 1 kHz. This relatively poor frequency resolution is, however, still suffi-

cient to be able to detect conspicuous changes in the spectrum.

6.3.3 Determination of Oxygen Saturation and Photoplethysmography

As already announced in section 4.3, the oxygen saturation of the blood can be de-

termined by measuring the light absorption at two different wavelengths. In this sec-

tion, the relationship between the measured quantity and the oxygen saturation will

be derived mathematically. The measured quantity is the light intensity I, which is

measured at the transmission on the opposite side of the irradiation point by the pho-

todiode (cf. Figure 6.45). In addition to absorption in the tissue, the light is also atten-

uated by scattering from internal finger structures. The scattering effect is not taken

into account in the SpO2 calculation.

270k

22n

Photodiode

Fig. 6.45: Arrangement of LED and photodiode for transmission measurement at the fingertip (left):

A transimpedance amplifier with low-pass filter is used to amplify the signals of the photodiode.

(see electronic circuit diagram in the middle). A measurement of the pulse curve can also be per-

formed with a reflection sensor on any part of the body (here on the finger) (right).

For a simple homogeneous layer, the intensity according to the Lambert-Beer law de-

pends exponentially on the substance-specific and wavelength-dependent absorption

coefficient α and the layer thickness d:

I = I0e⁻^αd,

(6.32)

where I0 is the intensity at the irradiation point. If the light path is composed of sev-

eral layers i with different absorption coefficients αi and layer thicknesses di, then the

intensity after the passage of all partial paths is

I = I0e⁻^∑^αⁱ^dⁱ.

(6.33)