Effects of fluid shift on upper airway patency and neck circumference in normal-weight subjects.
BACKGROUND
Fluid shift from the lower body into the neck could narrow the upper airway (UA) and increase the propensity of its obstruction in people with obstructive sleep apnea. Although studies have demonstrated an increase in the neck circumference (NC) due to fluid shift, it remains unclear as to whether a large increase in NC means a large reduction in the cross-sectional area of the UA (CSA-UA). This study tested a hypothesis that a significant UA narrowing due to fluid redistribution is not necessarily linked to an apparent increase in NC, and vice versa.


METHODS
Magnetic resonance imaging (MRI) studies were performed on 30 male and 20 female normal-weight subjects. Fluid shift was achieved by raising their legs by >50°. The coordinates of the neck and UA boundaries were extracted from the MRI to calculate the NC and CSA-UA.


RESULTS
After elevating the legs for 8 min, the CSA-UA was reduced by 27.6% while the NC increased by 1.5% in 50 subjects (p < 0.001). In 10% of the males and 10% of the females, NC was almost unchanged, while the reduction in CSA-UA was large. In 7% of the males and 20% of the females, the change in NC was large, while that in CSA-UA was small. The correlation coefficient between the changes in NC and CSA-UA was -0.211, which was consistent with the hypothesis.


CONCLUSIONS
The amount of NC increase should not be used to judge the degree of reduction in CSA-UA. Clinically, those people who have little NC increase after body position changes have more risk of CSA-UA reduction at bedtime.
School of Biomedical Engineering, Capital Medical University, Beijing, China; Beijing Key Laboratory of Fundamental Research on Biomechanics in Clinical Application, Capital Medical University, Beijing, China.
Yunqiang
Y
An
School of Biomedical Engineering, Capital Medical University, Beijing, China; Beijing Key Laboratory of Fundamental Research on Biomechanics in Clinical Application, Capital Medical University, Beijing, China.
Yong
Y
Li
Department of Radiology, Beijing Tongren Hospital, Capital Medical University, Beijing, China.
Zhaohui
Z
Liu
School of Biomedical Engineering, Capital Medical University, Beijing, China; Beijing Key Laboratory of Fundamental Research on Biomechanics in Clinical Application, Capital Medical University, Beijing, China.
Jianxia
J
Wang
Department of Radiology, Beijing Tongren Hospital, Capital Medical University, Beijing, China.
Ting
T
Li
School of Biomedical Engineering, Capital Medical University, Beijing, China; Beijing Key Laboratory of Fundamental Research on Biomechanics in Clinical Application, Capital Medical University, Beijing, China.
Huahui
H
Xiong
Department of Radiology, Beijing Tongren Hospital, Capital Medical University, Beijing, China.
Hongxia
H
Yin
School of Biomedical Engineering, Capital Medical University, Beijing, China; Beijing Key Laboratory of Fundamental Research on Biomechanics in Clinical Application, Capital Medical University, Beijing, China.
Xinyue
X
Zhang
Department of Radiology, Beijing Tongren Hospital, Capital Medical University, Beijing, China. Electronic address: cjr.xianjunfang@vip.163.com.
Junfang
J
Xian
School of Biomedical Engineering, Capital Medical University, Beijing, China; Beijing Key Laboratory of Fundamental Research on Biomechanics in Clinical Application, Capital Medical University, Beijing, China. Electronic address: yqhuang@ccmu.edu.cn.
Yaqi
Y
Huang
D001835:Body Weight / Q000502:physiology; D005260:Female; D018495:Fluid Shifts / Q000502:physiology*; D006801:Humans; D008297:Male; D009333:Neck / Q000503:physiopathology*; D010614:Pharynx / Q000503:physiopathology; D017286:Polysomnography; D020181:Sleep Apnea, Obstructive / Q000503:physiopathology*
Head and neck imaging; Neck circumference; Neck skin elastic modulus; Obstructive sleep apnea; Rostral fluid redistribution; Upper airway narrowing