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Giving Shape to Ideas

Pulse wave sensor

Pulse oximeters are devices that measure the proportion of hemoglobin that is bound to oxygen in red blood cells in arterial blood by shining a light on an area such as a fingertip. Since they were first commercialized in 1977, they have undergone technological improvements at the hands of numerous medical device manufacturers, and today are essential devices in clinical settings including operating rooms, intensive care units (ICUs), and respiratory organ-related treatment.

Measurement principles

The arterial blood pumped out by the heart moves within the blood vessels in the form of a wave known as a pulse wave. Venous blood, however, flows slowly and does not exhibit a waveform (Fig. 1). It is therefore possible to obtain information on arterial blood simply by observing the variable components of light. The spectral characteristics of oxygenated hemoglobin (HbO2) and reduced hemoglobin (Hb) are shown in Fig. 2. As Hb exhibits higher absorbance in red light (R), blood that is low in oxygen appears dark red, while highly oxygenated blood is bright red. In the infrared region, there is little difference between the absorbances of HbO2 and Hb. The R/IR absorbance ratio therefore decreases when HbO2 levels are higher, and increases when Hb levels are higher. Pulse oximeters thus calculate the level of oxygen saturation from the ratio of the variable components of the amount of light at two different wavelengths.

In fact, the pulse components generated by the artery in the light passing through the body constitute a very weak signal, sometimes less than 1.0% of the amount of light passing through, which varies depending on the body and time. Technology for the accurate evaluation of fluctuating, very weak light is therefore required. Konica Minolta’s pulse oximeters utilize a lighting system to eliminate noise from commercial frequencies that override the pulse component, as well as signal processing in which the amount of red and infrared light emitted and photosensitivity are both optimized in accordance with measurement status, enabling reliable measurements to be obtained with low energy consumption. As a result, a comparison of oxygen saturation values obtained from blood samples with those measured by a pulse oximeter found that this represents a useful item in clinical terms.

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