Calibration-Free Pulse Oximetry Based on Two Wavelengths in the Infrared — A Preliminary Study

Abstract

The assessment of oxygen saturation in arterial blood by pulse oximetry (SpO2) is based on the different light absorption spectra for oxygenated and deoxygenated hemoglobin and the analysis of photoplethysmographic (PPG) signals acquired at two wavelengths. Commercial pulse oximeters use two wavelengths in the red and infrared regions which have different pathlengths and the relationship between the PPG-derived parameters and oxygen saturation in arterial blood is determined by means of an empirical calibration. This calibration results in an inherent error, and pulse oximetry thus has an error of about 4%, which is too high for some clinical problems. We present calibration-free pulse oximetry for measurement of SpO2, based on PPG pulses of two nearby wavelengths in the infrared. By neglecting the difference between the path-lengths of the two nearby wavelengths, SpO2 can be derived from the PPG parameters with no need for calibration. In the current study we used three laser diodes of wavelengths 780, 785 and 808 nm, with narrow spectral line-width. SaO2 was calculated by using each pair of PPG signals selected from the three wavelengths. In measurements on healthy subjects, SpO2 values, obtained by the 780–808 nm wavelength pair were found to be in the normal range. The measurement of SpO2 by two nearby wavelengths in the infrared with narrow line-width enables the assessment of SpO2 without calibration.

Pulse Oximetry with Two Infrared Wavelengths without Calibration in Extracted Arterial Blood

“…As described in our article in 2014 [ 16 ], the peak intensity of the PPG pulse, , exhibits fluctuations of low and high frequencies, and the value of the PPG signal at the time of the pulse peak (end-diastole) might be different than what its value would have been at the time of the pulse minimum. The value of , used for a given pulse, was taken as the value of the point of the line connecting the two maxima of the pulse at the time of the pulse minimum (see Figure 7 in [ 16 ]).…”

“…By using two nearby wavelengths in the infrared region, the difference between the corresponding scattering constants and path-lengths is reduced [ 9 , 16 ]. In this case, the natural variation in the path-lengths ratio between persons is expected to lead to errors that are small relative to those experienced when using pulse oximetry that uses two wavelengths, one of which is in the red and the other in the infrared region.…”

“…In this case, the natural variation in the path-lengths ratio between persons is expected to lead to errors that are small relative to those experienced when using pulse oximetry that uses two wavelengths, one of which is in the red and the other in the infrared region. In our study in 2014 [ 16 ], measurements on healthy persons by a pulse oximeter which was based on a pair of laser diodes with wavelengths of 780 nm and 808 nm exhibited SpO 2 values between 95.3% and 100.5%. These results were similar to those of a commercial pulse oximeter: the maximal difference between them for each examinee was 2.5%.…”

The Various Oximetric Techniques Used for the Evaluation of Blood Oxygenation

“…In the last few years, pulse oximetry that makes use of two close wavelengths in the infrared has been developed [ 39 , 41 , 59 ]. The choice of light with two close wavelengths reduces the differences between their scattering constants and path-lengths, so that the error due to the inter-subject variation in l 2 / l 1 is expected to be small relative to that experienced for red and infrared pulse oximetry.…”

Signal Quality Analysis in Pulse Oximetry: Modelling and Detection of Motion Artifact

“…can be affected without the location of the cardiac pulses being obscured. Nitzan et al further refined the use of the calibration-free method [14]. Laser diodes were used to improve the precision and accuracy of the emission wavelength in the probe, and SpO 2 calculations were performed using multiple extinction coefficient data sets [15].…”