Pulse oximetry/ principle

<pulse oximetry Pulse oximetry is used for non-invasive measurement of blood oxygenation, more precisely the oxygen saturation of hemoglobin in arterial blood. It uses two basic principles:


 * characteristic absorption spectra  of two  hemoglobin derivatives  present in the blood in the highest concentration, i.e. oxyhemoglobin and deoxyhemoglobin,
 * fluctuations in the volume of arterial blood in the tissue during the pulse wave.

Pulse oximeter
Pulse oximeters es that are attached to well-perfused areas - most often a patient's finger or earlobe. They have a built -in light source  that passes through the tissue and hits a  detector  that measures its intensity.

The light source consists of two electroluminescent diodes (LEDs) emitting light at  660 nm  (red light) and  940 nm  (near infrared light). These are the wavelengths at which the spectra of oxyhemoglobin and deoxyhemoglobin differ the most. Around 660 nm, deoxyhemoglobin  absorbs more significantly , while at 940 nm,  oxyhemoglobin  absorbs more. In addition, light of both wavelengths passes through tissues relatively well.

Measurement principle
The light that passes through the finger is absorbed by the skin, connective tissue, muscles, but also by arterial, venous and capillary blood. To determine the oxygen saturation of hemoglobin in arterial blood  (sO  2  ), it is necessary to recognize how much light has been absorbed by the arterial blood itself and how much by all other absorbing tissues. It makes use of the fact that the volume of arterial blood in the arteries of the finger changes during the pulse wave and thus also the absorption of light conditioned by the hemoglobin in it. The other absorbance components are practically constant.

The pulse oximeter therefore measures the so-called pulsating component of absorbance  (it makes up about 1-5% of the total absorbance). The difference between maximum and minimum absorbances during the pulse wave corresponds to light absorption by hemoglobin derivatives in the arteries. Since it is measured at two wavelengths, we obtain two differences ΔA 660  and ΔA  940  during the pulse wave. Their ratio corresponds to the ratio of the concentrations of oxyhemoglobin and deoxyhemoglobin, so it is possible to convert it to sO 2.

in addition to sO 2 Commonly available pulse oximeters measure heart rate. In practice, they are therefore used as one of the basic tools for monitoring vital functions.

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Source
Fialová L., Vejražka M.: Pulse oximetry