Fluid shift volume from legs to head and neck,inside diameter of upper airway, water content of neck soft tissue, percutaneous PaCO2, severity of sleep apnea, minimum SP02, lung-to-finger calculation time and loop gain were measured at 22:00 and repeated at 6:00 next morning in patients with OSA.Correlations between changes of fluid shift volume and changes in diameter of upper airway, water content of neck soft tissue, PaCO2, minimum SP02, lung-to-finger calculation time and loop gain were analyzed.