A conversion formula for comparing pulse oximeter desaturation rates obtained with different averaging times

Abstract

Objective: The number of desaturations determined in recordings of pulse oximeter saturation (SpO2) primarily depends on the time over which values are averaged. As the averaging time in pulse oximeters is not standardized, it varies considerably between centers. To make SpO2 data comparable, it is thus desirable to have a formula that allows conversion between desaturation rates obtained using different averaging times for various desaturation levels and minimal durations.

Methods: Oxygen saturation was measured for 170 hours in 12 preterm infants with a mean number of 65 desaturations <90% per hour of arbitrary duration by using a pulse oximeter in a 2-4 s averaging mode. Using 7 different averaging times between 3 and 16 seconds, the raw red-to-infrared data were reprocessed to determine the number of desaturations (D). The whole procedure was carried out for 7 different minimal desaturation durations (≥ 1, ≥ 5, ≥ 10, ≥ 15, ≥ 20, ≥ 25, ≥ 30 s) below SpO2 threshold values of 80%, 85% or 90% to finally reach a conversion formula. The formula was validated by splitting the infants into two groups of six children each and using one group each as a training set and the other one as a test set.

Results: Based on the linear relationship found between the logarithm of the desaturation rate and the logarithm of the averaging time, the conversion formula is: D2 = D1 (T2/T1)(c), where D2 is the desaturation rate for the desired averaging time T2, and D1 is the desaturation rate for the original averaging time T1, with the exponent c depending on the desaturation threshold and the minimal desaturation duration. The median error when applying this formula was 2.6%.

Conclusion: This formula enables the conversion of desaturation rates between different averaging times for various desaturation thresholds and minimal desaturation durations.

Figure 1

a. Bivariate fit of observed and predicted desaturations per hour with Group 1 as training set. Group 1 has been used as training set to generate the different exponents c for the formula. For group 2 (test set) predicted desaturations have been calculated (based on the exponents of group 1). The predicted desaturations (Npred/h) for Group 2 are plotted against the observed desaturations (Nobs/h) of Group 2. The concordance correlation coefficient was 0.997 b. Bivariate fit of observed and predicted desaturations per hour with Group 2 as training set. Group 2 has been used as training and Group 1 as test set. The predicted desaturations (Npred/h) for Group 1 are plotted against the observed desaturations (Nobs/h) of Group 1. The concordance correlation coefficient was 0.998

Figure 2. Influence of the averaging time and the desaturation duration on the number of desaturations.

SpO₂ recordings for averaging times of 3 seconds (green), 10 seconds (red) and 16 seconds (blue). For an alarm threshold at 80% SpO₂ (straight line) and an arbitrary desaturation duration, an averaging time of 3 s results in 6 desaturations, while an averaging time of 10 or 16 s results in 3 and 1 desaturation(s), respectively. For a desaturation duration ≥15 s, an averaging time of 3 s or 10 s results in no desaturation, while an averaging time of 16 s results in 1 desaturation.