A Conversion Formula for Comparing Pulse Oximeter Desaturation Rates Obtained with Different Averaging Times

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.