A Conversion Formula to Correct for Pulse Oximeter Desaturations Obtained with Different Averaging Times

Abstract

Study objectives: Pulse oximetry has gained widespread use in neonatology and intensive care medicine to measure arterial oxygen saturation (SpO2) continuously. The SpO2 values are usually obtained by averaging over preceding measurements. As the averaging time, usually between 2 to 16 seconds, affects the number of desaturations, it is necessary to have a conversion formula to compare results obtained using different averaging times.Methods: Oxygen saturation was recorded for 168 hours in fifteen infants with a mean number 60.9 desaturations < 90 % per hour for using a pulse oximeter (Radical, Masimo). The raw data were reprocessed using 7 different averaging times between 2 and 16 seconds to determine the number of desaturations (D) below the threshold values of 80%, 85% or 90% for 7 different minimal desaturation durations (>0, >5, >10, > 15, > 20, >25, >30 seconds).Measurements and Results: We found a linear relationship between the logarithm of the desaturation rate and the logarithm of the averaging time. Based on this linear relationships 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, where the exponent c depends on the desaturation threshold and the minimal desaturation duration. The median percentage error was found to be 2.5%.Conclusion: A formula is presented to convert between the numbers of desaturations measured with different averaging times for different desaturation thresholds and minimal durations.