The Discussion of Spo2 Sensor Accuracy

Research and development of blood oxygen sensor

1. The basic principle of Spo2 measurement

The Spo2 measurement process contains two parts. The first part uses a blood oxygen sensor to collect light and convert them into electrical signals on the receiving tube. The second part uses the spo2 monitor to convert the electrical signal into the corresponding blood oxygen value according to the preset curve and display it on the screen. Therefore, all the factors that affect the optical signal intensity and total amount will influence the accuracy of the measurement. Also, whether the R curve is accurate and matches the sensor directly affects the blood oxygen reading. Therefore, sensors and monitors both have an impact on the spo2 accuracy.


Factors that affect the precision of the blood oxygen sensor


1, Luminous tube parameters: the absorption curves by arterial blood of red light and infrared light are different. The amount of light that can penetrate the cortex, blood vessels, and muscle tissue of the sensor positioning to reach the receiving tube under the same conditions is different because of the light waves and powers differences.

2, Receiving tube parameters: the different sizes and sensitivity of the receiving tube cause different light-receiving abilities. Generally speaking, the large area of ​​the receiving tube will receive more light and less of a small space.

3, Sensor design: Different sensor designs have different resistance to interference light.

4, The materials that make up the sensor: Different sensor materials have different light absorption and interference.


Therefore, the parameters and designs of different sensors have different effects on light, and each sensor has unique optical characteristics!


Production and adjustment of R curve

According to Beer-Lambert's law, the functional relationship between the ratio of red light and infrared light (R/IR) and arterial blood oxygen saturation (SaO2) should be linear. However, due to the complexity of the optical properties of biological tissues, the correlation curve (referred to as the R curve, the same below) can only be determined by experimental methods.

In other words, during the production, we need to adjust R-curve for each sensor. In other words, each sensor has its R curve, such as, there are 10-15 models of Nellcor Oximax technology sensors, each with a chip, which stores more than a dozen blood oxygen curves. When plugged into the instrument, the instrument will automatically recognize and use the corresponding R curve in the measurement.


The process of making and adjusting the R curve varies because of different companies, but all include the following three steps:

1) Establish the R curve of the sensor with a blood oxygen simulator. However, the simulator cannot simulate the light absorption mode of blood under hypoxic conditions. 

2) The R curve is adjusted by a self-built low oxygen environment and compared with standard instruments.

3) Finally, perform blood gas comparison verification in the standard blood oxygen laboratory according to ISO 80601-2-61 (YY-0784).


This process may need to be repeated several times until the displayed values of the sensor under certain circumstances and conditions in the allowable error with blood gas analysis result.


It is how the OEM sensor and R curve are made and verified. If there is an error, engineers generally reduce the error by adjusting the R curve. So, OEM sensors are relatively easy to make. The technical requirements for sensor production are not high. Oximeter manufacturers can make up for the lack of accuracy by adjusting the R curve. Of course, once the adjustment of the R curve is completed, the sensor design and parameters used for the adjustment are the original sensors and must be controlled and sealed and cannot be changed at will. If there is any change, we must re-check the curve! Therefore, OEM manufacturers generally do not modify the sensor design and parameters at will.


How is the research and development of compatible spo2 sensors going on? 

As there are no own instruments and cannot change the R curve, they can only adjust the sensor design and parameter to make the lights received by the tube is the same as the original one or at least close to it under the same situation. Only in this way can make it be compatible with the R curve set in the instrument. However, due to too many factors influencing the light of sensor receiving, it is not easy to accurately adjust the amount of light received by the receiving tube. In particular, the instrument manufacturer does not disclose the R curve and sensor parameters, nor does it notify the compatibility sensor manufacturer of the instrument design and R curve changes in real-time. The compatibility sensor manufacturer can only pay attention to the manufacturer's updates and conduct compatibility testing regularly. It requires labor force, material resources, and technology and is difficult than making OEM sensors.


The standard of blood oxygen sensor


The well-designed sensor and well-adjusted R curve are only a part of the blood oxygen sensor makes, and it has to meet relevant standards. Only passed all the tests, then it can be considered a medical product!


The accuracy of blood oxygen products is expressed by the root mean square RMS of the difference between the instrument and the standard blood gas readings. International standards and Chinese standards stipulate that the RMS of the sensor in the 70%-100% interval must be less than 4; the US FDA standard is that the RMS in the 70%-100% interval must be less than 3.


A sensor that performs well on one brand does not mean that it can perform well on an instrument of another brand. Therefore, one sensor cannot be compatible with all or multiple oximeters with different R curves!




The compatibility of the sensor and the curve is the top influence on the accuracy of the blood oxygen sensor. And from what we discussed above; we can get:

The blood oxygen sensor has a specific R curve;

Changing the design parameters of the sensor is equivalent to changing the R curve;

The product needs to be verified by clinical trials;

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