A comparative study of accuracy in major adaptive filters for motion artifact removal in sleep apnea tests

Sleep apnea is probably the most common respiratory disorder; respiration and blood oxygen saturation (SpO₂) are major concerns in sleep apnea and are also the two main parameters checked by polysomnography (PSG, the gold standard for diagnosing sleep apnea). In this study, we used a simple, non-invasive monitoring system based on photoplethysmography (PPG) to continuously monitor SpO₂ and heart rate (HR) for individuals at home. Various breathing experiments were conducted to investigate the relationship between SpO₂, HR, and apnea under different conditions, where two techniques (empirical formula and customized formula) for calculating SpO₂ and two methods (resting HR and instantaneous HR) for assessing HR were compared. Various adaptive filters were implemented to compare the effectiveness in removing motion artifacts (MAs) during the tests. This study fills the gap in the literature by comparing the performance of different adaptive filters on estimating SpO₂ and HR during apnea. The results showed that up-down finger motion introduced more MA than left-right motion, and the errors in SpO₂ estimation were increased as the frequency of movement was increased; due to the low sampling frequency features of these tests, the insertion of adaptive filter increased the noise in the data instead of eliminating the MA for SpO₂ estimation; the normal least mean squares (NLMS) filter is more effective in removing MA in HR estimation than other filters.