Using mathematical function formulas, the effect on the AHI of various treatment modalities can be calculated. These formulas described in the paper titled: " Reliable Calculation of the Efficacy of Non-Surgical and Surgical Treatment of Obstructive Sleep Apnea Revisited, M.J.L. Ravesloot, N. de Vries. Sleep 2011;34(1):105-110."3 These formulas were constructed to compare non optimal use of optimal therapy (CPAP) with the continuous effect (100%) of often non optimal therapy (surgery).
Total sleep time (TST)
Baseline AHI: AHI on diagnostic polysomnogram.
AHI whilst using CPAP: Whilst using CPAP, ideally the AHI is reduced to 0,1,2,3,4 or 5. Whilst not using CPAP (hours without CPAP), we assume the AHI reverts to baseline.
Hours of CPAP use: hours spent using CPAP during sleep.
Some clinicians prefer other PSG variables as an outcome measure; e.g., the apnea index or desaturation index (DI) as a measure of intermittent hypoxia. The latter is also considered to be less susceptible to nightly variability. Instead of the AHI one can use the the apnea index (AI) or desaturation index (DI) instead. The mathematical principle remains the same.