Oxygen desaturation index (ODIx)

The ODI_x corresponds to the average number of desaturation events per hour. A desaturation is defined as a SpO₂ drop of x% below the baseline. The ODI_x is a widely used measure in the field of sleep medicine, where desaturations are characteristic of apnea and hypopnea events⁴⁹. Indeed, obstruction of the airway leads to reduced entrance of oxygenated air to the lungs, which leads to a drop in oxygenated hemoglobin until airway patency is restored. These manifest as transient hypoxemic events or desaturations. Traditionally x = 3 or 4%. There exist many implementations of the ODI with variable definitions. In the present work, the implementation of the ODI_x detection algorithm was developed and validated by Behar et al.⁵⁰, building on the parent model and desaturation definition of Jung et al.⁵¹. Specifically, the model of Jung et al.⁵¹ defines three fiducial points A, B, and C, to determine the occurrence of a desaturation. Fiducial point A is defined as the point where the SpO₂ value decreases by ≥1 and ≤3%, fiducial point B as the value that reaches a minimum of at least 3% below A, and fiducial point C as the point where the SpO₂ value returns to a level either 1% below A or 3% above B. Some additional constraints are imposed, including that fluctuations in consecutive SpO₂ values should be <1% between A and B and >−1% between B and C. Finally, the time interval between A and C must be ≥10 and ≤60 s⁵¹. In the original paper, 90 s was used as the time limit, but to ensure capture of resaturation, 60 s is used here. An additional fiducial point D was defined as a point posterior to C at which the SpO₂ time series reaches a level of at least 1% below A and where the time interval between A and D is ≤60 s. From the detected desaturation events, several oximetry biomarkers can be computed (Fig. ​(Fig.7).7). Within our context of single-channel SpO₂ analysis, i.e., when no reference EEG channel is available to quantify sleep time, the ODI_x is defined as:

where N_desat is the number of desaturations in the signal and TRT is the total recording time in hours.

A, B, C, and D are the fiducials points delimiting a desaturation event.