4.3 Transducer for Non-Electrical Biosignals

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4.3.2 Optical Sensors for Plethysmography and Determination of Oxygen

Saturation

Plethysmography (Gr. plethys = fullness) means the recording of volume variations

within the body. In the context of this book, we restrict ourselves to the volume change

in vessels due to pulsatile blood flow. The palpation of the pulses belongs to the oldest

diagnostic procedures and plays a major role especially in traditional Chinese medi-

cine. On surface (subcutaneous) vessels, the change in volume can also be measured

optically. In this case it is called photoplethysmography. The measuring principle is

that light is irradiated into the body and the transmitted or reflected radiation is meas-

ured with a light sensor. The sensor signal is variable in time as a result of the blood

flow. When the pulse wave passes through the irradiated vessel section, the vessel

dilates due to the larger blood volume. The larger blood volume then ensures greater

light absorption compared to the phase when there is only a small volume of blood

in the vessel section. As a result, the light sensor detects less light. In this way, con-

tinuous pulse curves can be recorded. The pulse curve contains information about

the condition of the vessels and the cardiovascular system. For a differentiated eval-

uation, pulse curves usually have to be measured at several points. It is particularly

easy to determine the heart rate with photoplethysmography. To do this, the duration

between the maxima of the pulse curve is determined. The reciprocal of this multiplied

by 60 gives the heart rate in the usual unit of beats per minute.

An extension of photoplethysmography is the SpO2 measurement⁸. It allows the

determination of the oxygen saturation of the blood. For this purpose, the different ab-

sorption of hemoglobin (Hb) and hemoglobin saturated with oxygen (oxyhemoglobin

HbO2) at different wavelengths is exploited. For the determination of oxygen satura-

tion, measurements at at least two different light wavelengths are necessary, so at least

two monochromatic light sources are required. The measurement result is expressed

as a percentage and corresponds to the ratio of HbO2 to total hemoglobin (HbO2 + Hb).

The procedure is presented in more detail in subsection 6.3.2.

Semiconductor light-emitting diodes (LEDs) are predominantly used as the light

source. These have a high intensity with comparatively low power consumption, have

a small design and are very inexpensive. When measuring SpO2 with two LEDs, care

must be taken to ensure that the emission spectra are far enough apart and do not

overlap. LEDs are available in the color spectrum from blue to infrared. Color selection

is based on the absorption spectra of Hb and HbO2. In order to distinguish between

the transmitted and reflected radiation of the respective LEDs at the detector, the two

LEDs must be operated in pulse mode and multiplexing mode. The accuracy of the

SpO2 determination depends crucially on the adherence to the emission wavelength,

as stated in subsection 6.3.2. From Table 4.3, it can be seen that the location of the

8 S denotes saturation, p pulse, O2 oxygen.