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Smiths BCI Report on how SpO2 Readings can Differ among Pulse Oximeters
Principle
Pulse oximetry combines the principles of optical plethysmography and spectrophotmetry to determine arterial oxygen saturation values. Optical plethysmography used light absorbance technology to reproduce waveforms produced by pulsating blood. Spectrophotometry uses various wavelengths of light to perform quantitative measurements about light absorption through given substances.
Technology
Two wavelengths of light are passed through body tissue via light emitting diodes (LED) to a photodetector. The two LEDs are red light and infrared light. These two LEDs are chosen because light absorption varies with the oxygen concentration of hemoglobin in both the red (660nm) and infrared (890-940nm) light. The pulse amplitude of the red and infrared signals are detected and measures to produce a ratio value.
The red light amplitude (AC red) is divided by the infrared light pulse amplitude (AC ir) to form the intermediate value called Ratio.
Ratio=AC red / DC red divided by AC ir / DC ir
Ratio is used as the input to a “lookup table” function in the pulse oximeter. The SpO2 value is the result of the “lookup” function.
SpO2 Computation
Each manufacturer used a unique calibration curve. A calibration curve is an algorithm that is empirically derived as a result of data obtained from desaturation studies. It relates light transmittance to oxygen saturation mathematically. Each pulse oximetry manufacturer develops it’s own proprietary calibration curve, no two are alike.
Desaturation studies consist of recording data from many human test subjects at different levels of saturation or desaturation. Measured arterial blood saturations and SpO2 readings are obtained and recorded at each level of desaturation. Desaturation is induced by having the subjects breath a hypoxic gas mixture. This process is repeated over and over with many different test subjects. The data is then plotted on a graph. A proprietary calibration curve is developed by making the best fit of the SpO2 data against the measured data with accuracy specifications.
Accuracy Specifications
The industry standard for pulse oximetry accuracy specifications is +/- 2 digits. The specification is usually equal to +/- 1 standard deviation of 68% of the test population; 1 standard deviation above and below the line of identity on a graph relating true to measured values. Therefore any two pulse oximeters, regardless of manufacturer, can display different SpO2 readings and still be accurate. Some manufacturer’s SpO2 readings are higher than an actual measured SaO2, some are lower and some read the same, but they are all within +/- 2 digit specifications.
Below is an example of three different manufacturer’s SpO2 readings compared to the actual arterial blood saturation measured with a laboratory co-oximeter. All three manufacturer’s readings are considered accurate because they are within said specifications. Remember, a higher reading does not necessarily equal a more accurate reading, as demonstrated with the 100& SpO2 below. It is not possible to be 100% oxygen saturated due to the normal anatomical ventilation to perfusion mismatches of the human body, yet pulse oximeters read 100% and are considered accurate.
Manufacturer 1...SpO2 ...96%....-2%
Manufacturer 2...SpO2 ...98%.... 0
Manufacturer 3...SpO2 ..100%....+2%
Co-Oximeter.......SpO2 ...98%.....Measured (Actual)
Summary
Each pulse oximetry manufacturer uses various wavelength LEDs which will affect the Ratio obtained. The calibration curve is empirically derived from dynamic data that contains many variables. Manufacturers adjust their proprietary calibration curve in order to meet a specification range. Based on this information, it is understandable how an SpO2 reading can vary from one manufacturer to manufacturer and still be accurate.
Information provided by Smiths Medical BCI at
http://www.smiths-medical.com/Upload/products/product_relateddocs/How%20can%20SpO2%20readings%20differ.pdf |
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