## 2 Materials and Methods  
Between March 2015 and May 2020, we enrolled consecutive patients with very severe OSA who met these criteria to the study:  
* Age \(\geq\) 20 years
* Unsuccessful or refusal of CPAP
* AHI \(\geq\) 60 events/h
* Received a one-stage multi-level sleep surgery with the modified ZPP performed with one-layer closure and open partial tongue-base glossectomy
* Available preoperative and postoperative PSGs for required recordings, including desaturation index and SpO\({}_{2}\).  
Patients were not specially selected for surgery by any other criteria unlisted above. There were no specific exclusion criteria. Every PSG was conducted overnight in the level 1 sleep laboratory of the authors' tertiary referral hospital. In a PSG, an apnea is defined as the complete cessation of airflow for at least 10 s. A hypopnea is defined as a decrease in airflow \(\geq\) 30% for at least 10 s that is accompanied by either EEG signs of arousal or by a 4% or greater decrease in oxygen saturation. We performed preand post-operative endoscopies to evaluate the anatomy. Patients received general anesthesia by oral intubation to allow routine septomeatoplasty, and breathed through the mouth for 2 to 3 days before we removed the nasal packing. Modified Z-palatoplasty (ZPP) with one-layer closure was performed as illustrated in our previous report [37]. Partial open tongue-base resection was completed with transoral (CO\({}_{2}\)) laser microsurgery or transoral robotic surgery (TORS) for hypopharyngeal obstruction according to the preoperative endoscopic assessment. After the surgery, we cared for all patients in general ward areas with oximeter monitor. Intravenous Dynastat was prescribed twice daily on patients' request. No intravenous or oral narcotics were given to prevent respiratory depression.  
A PSG followed about 6 months (193 \(\pm\) 67 days) after the surgery, and sleep parameters were considered stable. We measured the mean change in AHI compared to the mean AHI before surgery, AHI reduction, as suggested by [PERSON] et al. [38]. In the components of AHI, we then calculated the percentage of hypopnea in AHI and compared the desaturation index, the mean SpO\({}_{2}\) desaturation, and the mean SpO\({}_{2}\) before and after the surgery. We performed a paired \(t\)-test to examine the changes in AHI reduction, percentage of hypopnea in AHI, desaturation index, mean SpO\({}_{2}\) desaturation, and mean SpO\({}_{2}\), against no change after the surgery. We plotted scatter graph to exclude outliers and justify the application of correlation coefficient. We calculated the correlation coefficient to show the relationship between proportion change of hypopnea/AHI (defined here as (postoperative-preoperative)/preoperative fraction of hypopnea in AHI) and change of mean SpO\({}_{2}\) (defined here as (postoperative-preoperative)/preoperative mean SpO\({}_{2}\)). We then tested the change of mean SpO\({}_{2}\) in \"non-surgical success\" patients to see if clinical and statistical improvements can be seen if a classical surgical success could not be qualified. The statistical significance was tested as \(\alpha\) = 0.05.  
All statistical examinations were performed in MATLAB 9.4.0.813654 (MathWorks, Natick, MA, USA).