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The Working Principle of Pulse Oximeter

Updated: Dec 5, 2022

Spectral absorption characteristics of biological tissues

When light shines on biological tissues, the effects of light on biological tissues are primarily classified as absorption, scattering, reflection, and fluorescence. If the scattering factor is ignored, the distance traveled by light in biological tissue is primarily determined by absorption. When light passes through a transparent material (solid, liquid, or gas state), the intensity of the light decreases noticeably due to the targeted absorption of some specific frequency components, which is the material to light absorption phenomenon. The amount of light absorbed by a substance is referred to as its optical density, also known as absorbance.

The amount of light energy absorbed by the material during the entire process of light propagation is proportional to three elements: light intensity, light path distance, and the number of light-absorbing particles on the cross-section of the light path. On the basis of uniform matter, the number of absorbent particles on the optical path section can be regarded as the number of absorbent particles per unit volume, i.e., the material's absorbent particle concentration, and then Lambert Beer's law can be obtained:

When the concentration of the substance and the length of the optical path are both unit quantities, it can be interpreted as the optical density. It can reflect the substance's light absorption ability and indicate the substance's light absorption characteristic. In other words, the absorption spectrum curve of the same substance has the same shape. The absolute position of the absorption peak will change due to the concentration difference, but the relative position will not change. The absorption process occurs in the same cross-sectional volume, the absorption substances are uncorrelated, there are no fluorescent compounds, and there is no phenomenon in which the properties of the medium are changed by light radiation. As a result, the optical density A has additivity for the solution with n absorption components. The additivity of optical density provides a theoretical foundation for quantitatively measuring the content of each absorbent component in mixtures.

The spectral region in the band of 600 1300nm is commonly referred to as "the window of biological spectroscopy" in biological tissue optics. The light in this band is important for a variety of known and unknown spectral therapy and spectral diagnosis. Water becomes the dominant light-absorbing substance in biological tissue in the infrared region, so the wavelength used by the system must avoid the absorption peak of water to better obtain light absorption information of the target substance. As a result, the main components of human fingertip tissues with light absorption ability in the near-infrared spectral range of 600 950nm include water in the blood, O2Hb (oxygenated hemoglobin), RHB (reduced hemoglobin), and skin melanin in the peripheral tissues.

As a result, by analyzing the emission spectrum data, we can obtain effective information about the component concentration to be measured in the tissue. And once we have the concentrations of O2Hb and RhB, we can calculate the oxygen saturation. The percentage of the volume of oxygen-bound hemoglobin (HbO2) in the blood to the total binding capacity of hemoglobin (Hb, hemoglobin) is referred to as blood oxygen saturation (SaO2).

The Pulse

Here's a new concept: blood volume pulse wave.

During each cardiac cycle, the contraction of the heart raises the blood pressure in the aortic root vessels, causing the vessel walls to expand outwards. In contrast, diastole of the heart causes the blood pressure in the aortic root vessels to fall, causing the vessel walls to contract. As the cardiac cycle continues indefinitely, the constant change in blood pressure inside the aortic root vessels is transmitted to the downstream vessels connected to it and even the entire arterial canal system, resulting in the continuous expansion and contraction of the entire arterial vessel wall. In other words, the heart's beating generates pulse waves in the aorta, which propagate in waves along the vessel walls and out through the entire arterial system. When the heart expands and contracts, pressure changes in the arterial system occur, resulting in a periodic pulse wave. This is referred to as pulse wave generation. Pulse wave characteristics can reflect a wide range of physiological information, including heart rate, blood pressure, and blood flow, and can provide critical information for non-invasive detection of specific body signs. In medicine, the pulse wave is usually classified into two types: pressure pulse wave and volume pulse wave. The pressure pulse wave primarily represented blood pressure transmission, whereas the volumetric pulse wave represented periodic changes in blood flow. The volume pulse wave contains more important information about the body's blood vessels, blood flow, and other cardiovascular factors than the pressure pulse wave.

Photoelectric volumetric pulse tomography can detect the volumetric pulse wave non-invasively. After reflection or transmission, the light beam reaches the photoelectric sensor. The effective characteristic information of the volumetric pulse wave will be carried by the received light beam. Because blood volume changes with the expansion and contraction of the heart, when the heart is diastolic, the blood volume is the smallest, and the light intensity detected by the sensor is the greatest; when the heart contracts, the volume is maximum, and the light intensity detected by the sensor is minimum.

Tip of the finger

In the non-invasive detection with blood flow volume pulse wave as the direct measurement data, the selection of spectral measurement sites should follow the following principles:

(1) The vascular veins should be relatively rich, and the proportion of effective information such as hemoglobin and ICG in the total material information in the spectrum should be increased;

(2) It has obvious characteristics of blood flow volume change to effectively collect volumetric pulse wave signals;

(3) It is not susceptible to external interference, and the tissue characteristics are less affected by individual differences, to obtain the human spectrum with good repeatability and stability;

(4) It is convenient for the implementation of spectral detection and easy to be accepted by the subjects, to avoid causing interference factors such as rapid heart rate and measurement position movement under pressure.

Because the pulse wave cannot be detected from the arm position, it is unsuitable for detecting blood flow volume pulse waves. The wrist is near the radial artery, the pressure pulse wave signal is strong, and the skin is easily mechanically vibrated, which may lead to signal detection. It is difficult to accurately characterize the characteristics of blood flow volume change and is not suitable for the measurement position because the volumetric pulse wave also carries the pulse information of the skin surface reflection. Although the palm is a common clinical location for blood sampling, its bone is thicker than that of the finger, and the volumetric pulse wave signal amplitude collected by diffuse reflection is lower. The distribution of blood vessels in the palm is depicted in Figure 2-5. The figure shows that the anterior segment of the finger has a dense capillary network that can effectively reflect the hemoglobin content in the human body. Furthermore, this location has obvious blood flow volume change characteristics and is an ideal location for measuring volumetric pulse waves. Because the muscle and bone tissues of the fingers are thin, the influence of background interference information is minimal. Furthermore, the finger front is simple to measure, and the subject has no psychological burden, making it possible to obtain stable spectral signals with high SNR.

Model of arterial blood density in human peripheral tissue

Bone, nail, skin, tissue, venous blood, and arterial blood are all components of the human finger. The optical path of other components does not change during the entire process of light interaction because of the heart contraction to the artery congestion, resulting in changes in the measurement of the optical path; however, the optical path of other components does not change during the entire process of light interaction, which is a constant.

When a specific wavelength of light is applied to the skin of a human fingertip, the finger can be thought of as a mixture of two parts: the static material (the optical path is constant) and the dynamic material (the optical path changes with the material volume). When the light is absorbed by the fingertip tissue, the photodetector receives the transmitted light. The light absorption ability of the tissue components of human fingers reduces the intensity of transmitted light collected by the sensor. The equivalent model of human finger light absorption is established based on this characteristic.


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