Objective When body posture changes from standing or sitting to a recumbent 

posture at bedtime,fluid displaces from the lower body into the neck due to gravity,which

may increase the volume of the soft tissues surrounding the upper airway and tissue 

pressure. As a result,it may narrow the upper airway and increase the propensity of airway 

obstruction in patients with obstructive sleep apnea (OSA). The aim of this study is to 

analyze the effects of fluid shift on the shape of the airway cross section and flow 

resistance. Method Using three-dimensional spoiled gradient echo sequence,we perform 

magnetic resonance imaging (MRI) for OSA subjects under two conditions:with subjects lying supine (the control),and elevating both legs for 8min,which causes fluid redistribution. We analyze the changes in the cross-sectional area and shape of the upper airway using selected axial images,and evaluate the effects of different shape changes on the airway resistance based on hydraulic radius theory. Results Compared to the control stage,there are three types of shape changes in the cross-section of the upper airway after leg raising for 8min:the shape basically remains unchanged;the ratio of the major to minor axis 

increases,which means that the shape becomes more elliptical;and the ratio of the major to 

minor axis decreases,which means that the shape becomes more circular. The changes in the 

cross-sectional shape have significant effects on the flow resistance. Conclusions Fluid 

shift induced by posture change can change not only the cross-sectional area,but also the 

shape of the upper airway. The change in shape has significant effects on flow resistance. 

This study provides an important basis for further investigations on OSA mechanism.