Unstable respiratory control (high loop gain) is an important non-anatomical risk factor for obstructive sleep apnea. Studies showed high loop gain might also be acquired from long-term hypoxemia/hypercapnia due to OSA, and could be decreased by CPAP therapy in some of the individuals. Whether another treatment, i.e. upper airway surgery, could achieve a similar improvement in is not known. We hypothesize that 1) high LG could be reversible with improved hypoxemia and reduced apnea hypopnea index (AHI) by surgical treatment; 2) high loop gain at baseline may be associated with poor treatment outcomes.