The impact of study type and sleep measurement on oxygen desaturation index calculation

Abstract

Study objectives: The oxygen desaturation index (ODI) is an important measure of sleep-disordered breathing during polysomnography (PSG); however, the AASM Manual (V3) does not specify whether to include oxygen desaturations occurring during wake epochs. Additionally, an ODI obtained from PSG can differ from an ODI using home sleep apnea tests (HSATs) that do not measure sleep, hampering diagnostic and treatment decision reliability. This study aimed to (1) compare an ODI that included all desaturations with an ODI that excluded desaturations occurring during wake epochs in PSG and (2) compare ODIs obtained from PSG with HSAT.

Methods: 100 consecutive PSGs for investigation of obstructive sleep apnea were compared. ODIs were calculated including all desaturations (ODI_all) and by excluding desaturations entirely during wake epochs (ODI_sleep). Additionally, we compared ODI_all with an ODI calculated using monitoring time as the denominator (ODI_HSAT).

Results: The median (interquartile range) 3% ODI for ODI_all was 22.8 (13.1, 44.1) events/h and ODI_sleep was 17.6 (11.5, 35.2) events/h (median difference: -3.9 events/h [-8.2, -0.9]; 21.0% [8.7%, 33.2%]). This discrepancy was larger with increasing ODI and decreasing sleep efficiency. The ODI_HSAT was 17.4 (11.3, 35.2) events/h and the median reduction in ODI_HSAT vs ODI_all was -4.5 (-10.9, -2.0) events/h (21.6%; 11.1%, 33.8).

Conclusions: ODI was significantly reduced when desaturations in wake epochs were excluded, and when ODI was based on monitoring time rather than sleep time, with the potential for underestimation of disease severity. Results suggest that ODI can differ substantially depending on the calculation and study type used, and that there is a need for standardization to ensure consistent diagnosis and treatment outcomes.

Citation: Whenn CB, Wilson DL, Ruehland WR, Churchward TJ, Worsnop C, Tolson J. The impact of study type and sleep measurement on oxygen desaturation index calculation. J Clin Sleep Med. 2024;20(5):709-717.

Keywords: calculation; desaturation; epoch; oxygen; sleep.

Figure 1. Example PSG fragment demonstrating a respiratory event beginning in a sleep epoch, with a corresponding oxygen desaturation occurring in a wake epoch.

Epochs are labeled as W = wake, 1 = stage 1, or 2 = stage 2 sleep. Desat = desaturation, Abdo = abdominal RIP signal, Flow = nasal flow signal, Pleth = oximeter plethysmography, PSG = polysomnography, SpO₂% = % oxygen saturation, Therm = thermistor signal, Thor = thoracic respiratory inductance plethysmography (RIP) signal.

Figure 2. Example PSG epochs demonstrating probable wakefulness used for calculation of monitoring time (A) and when sleep would be continued (B).

Abdo = abdominal RIP signal, Flow = nasal flow signal, Pleth = oximeter plethysmography, Pos = body position, PSG = polysomnography, Snore = snore signal, SpO₂% = % oxygen saturation, Therm = thermistor signal, Thor = thoracic RIP signal.

Figure 3. Bland-Altman plot illustrating agreement between ODIs (3%) including (ODI_all) vs excluding oxygen desaturations entirely in wake epochs (ODI_sleep), showing median, 5th, and 95th percentiles.

The data demonstrate that, as the average of ODI_all and ODI_sleep increases, the discrepancy between these oxygen desaturation indices becomes larger with a wider spread. ODI = oxygen desaturation index, ODI_all = oxygen desaturation index calculated including all oxygen desaturations with total sleep time as the denominator, ODI_sleep = oxygen desaturation index calculated with desaturations entirely in wake epochs excluded, with total sleep time as the denominator.

Figure 4. Comparison between ODI indices generated during polysomnography.

ODI_all includes all oxygen desaturations, ODI_sleep includes oxygen desaturations spanning sleep epochs, and ODI_start includes only oxygen desaturations that begin in sleep epochs. Total sleep time was the denominator for all indices. All pairwise comparisons were significant (P < .001). ODI = oxygen desaturation index.

Figure 5. The median (IQR) decrease in ODI across sleep efficiency quartiles when desaturations entirely in wake are included (ODI_all) compared with when they are excluded (ODI_sleep) from calculations.

All pairwise comparisons were significant (P < .025). IQR = interquartile range, ODI = oxygen desaturation index, ODI_all = oxygen desaturation index calculated including all oxygen desaturations with total sleep time as the denominator, ODI_sleep = oxygen desaturation index calculated with desaturations entirely in wake epochs excluded, with total sleep time as the denominator.

Figure 6. Bland-Altman plot illustrating agreement between ODIs based on PSG vs HSAT, showing median, 5th, and 95th percentiles.

The data demonstrate that, as the average of ODI_all and ODI_HSAT increases, the discrepancy between these oxygen desaturation indices becomes larger with a wider spread. HSAT = home sleep apnea test, ODI = oxygen desaturation index, ODI_all = oxygen desaturation index calculated with total sleep time as the denominator, ODI_HSAT = oxygen desaturation index calculated with monitoring time as the denominator, PSG = polysomnography.