Medical near-infrared spectroscopy
1.1. Medical near-infrared spectroscopy technology
In medicine, near-infrared spectroscopy is mainly used to monitor regional tissue oxygen saturation(rSO2). It is a technology that reflects the systemic hemodynamics and the perfusion state of tissues and organs and has the advantages of non-invasive and real-time and monitor the oxygen saturation of different parts of the brain, kidneys, intestines, and muscles.
NIRS in medicine is mainly in the range of 700nm-1000nm, which waveband has good penetration to human tissues such as bones, skin, fat, muscles, etc. The main light-absorbing groups in the blood are HbO2 and deoxyhemoglobin (HbR). The Hb oxidation state determines the light absorption range. HbR mainly absorbs red light, while HbO2 mainly absorbs infrared light.
1.2. Blood oxygen saturation measurement
Blood oxygen saturation (SaO2) is a parameter that reflects the content of oxygenated hemoglobin in the blood. It is the percentage of oxygenated hemoglobin (HbO2) to the total binding capacity of hemoglobin (Hb). Clinically, it is measured based on the principle that arterial blood absorbs light with the change of arterial pulsation. This measurement result is called pulse oximetry (SpO2).
The picture shows the light-emitting spectrum of light-emitting diodes and the absorption rate curve of blood for different blood oxygen concentrations.
The measurement principle of blood oxygen saturation is Lambert-Bear's law, also known as the fundamental law of light absorption. Its physical meaning is when a beam of parallel monochromatic light passes through a uniform non-scattering light-absorbing material vertically, its absorbance is proportional to the concentration c of the light-absorbing material and the thickness of the absorbing layer b and is inversely related to the transmittance T.
The light transmittance T = I0/I, I0 is the intensity of incident light, and I is the intensity of transmitted light. A is the absorbance, which is the degree of light absorption. B is the thickness of the liquid layer in cm. c is the concentration of the solution in mol/L. θ is the molar absorption coefficient, in L/mol.cm, the absorption material at a particular wavelength and the characteristic constants under solvent conditions.
According to Lambert Beer's law, the absorption of light by blood varies with the ratio of oxygenated hemoglobin. But the absorption of other tissues such as skin, muscle, bone, and venous blood is constant. Oxyhemoglobin has a greater absorbance of light with a wavelength of 660 nm, and non-oxygenated hemoglobin has a greater absorbance of light with a wavelength of 660 nm. but, Where R is the ratio of the light absorption rate of the two wavelengths, and a and b are constants.
The fingertip sensor: the upper wall of the sensor fixes two LEDs placed side by side, and the opposite wall is a photosensitive absorption device, which converts the red light and infrared light on the finger into electrical signals. The two LEDs emit light at a time interval and with a lower duty cycle. The whole blood absorption rate is calculated according to the ratio of the luminous intensity of the LEDs and the intensity of the transmitted light received by the photoelectric tube.
1.3. Cerebral oximeter
The cerebral oximeter is a device that focuses on the blood oxygen saturation of brain tissue, and its light can penetrate the skull for monitoring. It consists of a light source and two signal probes. A cerebral oximeter uses curved reflection to measure the spo2, so its measuring depth is about half the distance between the light source and the receiver.
The figure shows the measurement principle diagram of the cerebral oximeter.
The light from the light source directly enters the skull, diffuses through the cell to the entire area, and part of it reaches the receiving end through repeated reflections. The light curve in the legend is a schematic diagram.
Cerebral blood oxygen values lower than 58 indicate cerebral blood oxygen or ischemia, and higher than 81.5 indicate the risk of cerebral hyperperfusion. A decrease of more than 20% is sufficient to accompany the loss of consciousness. But between 58-81.5, there is a risk of cerebral hypoperfusion or over perfusion.
1.4. Measurement technology
The blood in the human body is consists of arterial blood and venous blood. Regardless of the composition of the blood oxygen value, there is no meaning.
The spo2 level of the fingertip is only arterial blood oxygen. It uses the ascending branch and descending branch of the pulse wave to calculate the subtraction between the highest point and the lowest point to obtain the two-state values after blood reflux and perfusion to calculate the oxygen saturation of arterial blood.
The blood oxygen monitoring in all parts of the arm, brain, or others is tissue blood oxygen. And It can only detect only the blood oxygen saturation of arteries + veins. Some measurement techniques fit the measured value and arterial blood oxygen to a mathematical model to calculate arterial blood oxygen. It is not accurate.
Normal arterial blood oxygen is more than 95%, and the blood oxygen values commonly used clinically are arterial blood oxygen. Venous blood oxygen is about 50%-60%. The measurement result of tissue blood oxygen is the weighted combination of arteries and veins at that location.
0 Comments