31-05-2012, 01:30 PM
How can SpO2 readings differ from manufacturer to manufacturer
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Principle
Pulse oximetry combines the principles of optical plethysmography and spectrophotometry to determine arterial oxygen saturation values. Optical plethysmography uses 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. Theses 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 amplitudes of the red and infrared signals are detected and measured to produce a Ratio value.
SpO2 Computation
Each manufacturer uses 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 within accuracy specifications.
Accuracy Specifications
The industry standard for pulse oximetry accuracy specifications is +/- 2 digits. This 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.
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 manufacturer to manufacturer and still be accurate.
[attachment=23683]
Principle
Pulse oximetry combines the principles of optical plethysmography and spectrophotometry to determine arterial oxygen saturation values. Optical plethysmography uses 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. Theses 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 amplitudes of the red and infrared signals are detected and measured to produce a Ratio value.
SpO2 Computation
Each manufacturer uses 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 within accuracy specifications.
Accuracy Specifications
The industry standard for pulse oximetry accuracy specifications is +/- 2 digits. This 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.
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 manufacturer to manufacturer and still be accurate.