A MRI Study On The Effects Of Fluid Shift On Sleep Respiration

Obstructive sleep apnea (OSA) is a common disorder of respiratory systems. Theperiodic hypoxia resulting from the repetitive collapse and obstruction of the upperairway during sleep could lead to serious consequences, including severecardiovascular diseases.In the recent years, foreign investigators have found a narrowing of the upperairway due to fluid shift from the lower body into the neck when changing the bodyposition from standing or sitting to a recumbent posture. This might increase thetendency of the upper airway obstruction during sleep. Currently we have not foundrelated publications from other Chinese investigators, except one study from HongKong scholars. There are also some shortcomings in the currently used experimentalmethods in other countries, such as forcing fluid shift into the neck using antishocktrousers or measuring the cross-sectional area of the upper airway (CSA-UA) usingacoustic pharyngometry. The objective of our study is to explore the effects of fluidshift caused by body posture changes on the upper airway patency and respiratoryfunctions. To reach this, we developed a three dimensional (3D) magnetic resonanceimaging (MRI) procedure for the fluid shift study. In our experimental design, thefluid was shifted from the legs into the neck naturally.Investigators have observed an increase in the neck circumference (NC) if fluidshifts into the neck after body posture changes, but an important question that has notbeen previously realized and should be answered is, for different individuals, whetheran increase in NC is always accompanied by a significant reduction in CSA-UA, orvice versa. This is critical because whether or not the amount of NC increase resultedfrom body posture changes could be used in clinics to grade the effect of fluid shift onthe upper airway patency and respiratory functions. Regarding this question, we advanced a hypothesis that there is no strong correlation between the change in NCand the CSA-UA reduction.To test our hypothesis, we proposed a MRI procedure for3D-SPGR volumescanning of axial planes from the top of nasal cavity to laryngeal prominence of fiftyhealthy subjects (30males and20females). The scanning was conducted under fourconditions: the control test with subjects lying supine, the states after fluidredistribution resulting from elevating both legs for1min and8min, and the state ofprostration after putting down the legs. A boundary segmentation procedure wasconducted on the images for chosen tissues. The coordinates of the neck and theupper airway boundaries were extracted under these four conditions, and the values ofNC and CSA-UA were calculated. Finally, a statistical analysis of these data wasperformed to study the changes of CSA-UA and NC size resulting from fluidredistribution.The experimental results confirmed our hypothesis. It showed that, perhapscounterintuitively, it was not necessary to have a significant reduction in CSA-UAwhile NC obviously increased due to fluid shift, or vice versa. In addition to theimportant results obtained from our study, the method we proposed of using a3D-SPGR volume MRI to study the effect of fluid shift on the cross section of theupper airway also opens a new door for exploring the effects of body posture changeson the respiratory functions during sleep.