Oxygen saturation resolution influences regularity measurements

The measurement of regularity in the oxygen saturation (SpO₂) signal has been suggested for use in identifying subjects with sleep disordered breathing (SDB). Previous work has shown that children with SDB have lower SpO₂ regularity than subjects without SDB (NonSDB). Regularity was measured using non-linear methods like approximate entropy (ApEn), sample entropy (SamEn) and Lempel-Ziv (LZ) complexity. Different manufacturer´s pulse oximeters provide SpO₂ at various resolutions and the effect of this resolution difference on SpO₂ regularity, has not been studied. To investigate this effect, we used the SpO₂ signal of children with and without SDB, recorded from the Phone Oximeter (0.1% resolution) and the same SpO₂ signal rounded to the nearest integer (artificial 1% resolution). To further validate the effect of rounding, we also used the SpO₂ signal (1% resolution) recorded simultaneously from polysomnography (PSG), as a control signal. We estimated SpO₂ regularity by computing the ApEn, SamEn and LZ complexity, using a 5-min sliding window and showed that different resolutions provided significantly different results. The regularity calculated using 0.1% SpO₂ resolution provided no significant differences between SDB and NonSDB. However, the artificial 1% resolution SpO₂ provided significant differences between SDB and NonSDB, showing a more random SpO₂ pattern (lower SpO₂ regularity) in SDB children, as suggested in the past. Similar results were obtained with the SpO₂ recorded from PSG (1% resolution), which further validated that this SpO₂ regularity change was due to the rounding effect. Therefore, the SpO₂ resolution has a great influence in regularity measurements like ApEn, SamEn and LZ complexity that should be considered when studying the SpO₂ pattern in children with SDB.