Upper airway lengthening caused by weight increase in obstructive sleep apnea patients.

BACKGROUND: The longer upper airway is more collapsible during sleep. This study aims to reveal relationships among upper airway length, weight, and obstructive sleep apnea (OSA), particularly to answer why the upper airway of OSA patients is longer than that of healthy people and why some obese people suffer from OSA while others do not. METHODS: We perform head and neck MRI on male patients and controls, and measure > 20 morphological parameters, including several never before investigated, to quantify the effect of weight change on upper airway length. RESULTS: The upper airway length is longer in patients and correlates strongly to body weight. Weight increase leads to significant fat infiltration in the tongue, causing the hyoid to move downward and lengthen the airway in patients. The apnea-hypopnea index (AHI) strongly correlates to airway length and tongue size. Surprisingly, a distance parameter h and angle ß near the occipital bone both show significant differences between healthy males and patients due to their different head backward tilt angle, and strongly correlates with AHI. The contributions of downward hyoid movement and head tilt on airway lengthening are 67.4-80.5% and19.5-32.6%, respectively, in patients. The parapharyngeal fat pad also correlates strongly with AHI. CONCLUSIONS: The findings in this study reveal that the amount of body weight and distribution of deposited fat both affect airway length, and therefore OSA. Fat distribution plays a larger impact than the amount of weight, and is a better predictor of who among obese people are more prone to OSA.

A pilot study in men to show the effects of postural fluid shifts on the severity of obstructive sleep apnea.

PURPOSE: Studies reveal that rostral fluid shifts due to body posture changes from standing to lying down may narrow the upper airway. However, without credible and direct experimental evidence, it remains unclear what the role of natural fluid redistribution in the neck is in affecting obstructive sleep apnea (OSA) severity. Our aim is using direct experimental evidence to determine whether or not postural fluid shifts affect OSA severity. METHODS: We performed overnight polysomnography on two consecutive nights for 22 men. The bed was set horizontally on the control night, while its tail part was lowered by 30° on the experimental night to reduce the amount of fluid shifted into the neck. We measured sleep and anthropometric parameters on each night. RESULTS: The mean (95% CI) apnea-hypopnea index (AHI) in the supine head and trunk position decreased from 66.6 events per hour (57.6-75.6) to 61.2 (52.0-70.4) (t = 4.507, p <0.001), and the oxygen desaturation index from 69.5 events per hour (56.4-82.6) to 61.6 (50.5-72.6) (t = 3.293, p = 0.004), from the control to the experimental night with a decrease in the change of leg fluid volume from 17.7% (15.7-19.8) to 4.7% (1.9-7.5) (t = 11.659, p < 0.001). CONCLUSIONS: Our findings provide direct experimental evidence to show that natural fluid shift caused by the day-to-night posture change does contribute to OSA pathogenesis and severity. It is likely that the neck fluid increase from an actual day-to-night position change, with 90° change in posture, would produce a much larger AHI increase than the 11.2% found in this study, which contains only a 30° change in posture. These findings suggest that reducing the amount of fluid in the neck region may relieve airway obstructions for patients with moderate and severe OSA.

Threshold of the upper airway cross-section for hypopnea onset during sleep and its identification under waking condition.

BACKGROUND: There is currently no method that can predict whether or under what condition hypopnea, even obstructive sleep apnea (OSA), will occur during sleep for individuals based on credible parameters measured under waking condition. We propose a threshold concept based on the narrowest cross-sectional area of the upper airway (CSA-UA) and aim to prove our hypothesis on the threshold of the area for hypopnea onset (TAHO), which can be used as an indicator of hypopnea onset during sleep and measured while awake. METHODS: We performed magnetic resonance imaging for 20 OSA patients to observe CSA-UA changes during fluid accumulation in the neck caused by elevating their legs, and identified TAHO by capturing the sudden enlargement in CSA-UA. Correlation analyses between TAHO and the body mass index (BMI), and between the reduction in CSA-UA and the increase in the neck circumference (NC) with fluid accumulation were performed. Logistic regression analysis was performed for identifying OSA patients based on the behaviors of their CSA-UA changes during leg raising. Shape changes of airway cross-section were also investigated. RESULTS: Four CSA-UA change patterns after fluid redistribution were identified. Six patients had similar CSA-UA variation behaviors observed in healthy subjects. From the other three change patterns involving 14 patients, a threshold value of CSA-UA 0.63 ± 0.21 cm2 was identified for normal breathing. Data showed a positive correlation between TAHO and BMI (r = 0.681, p = 0.0007), and a negative correlation between the reduction in CSA-UA and the increase in NC (r = - 0.513, p = 0.051) with fluid accumulation. A sigmoid function for the probability of being a OSA patient p = 1/[1 + exp. (4.836 + 3.850 t-8.4 h)] was obtained to effectively separate OSA patients from normal subjects. The upper airway narrowing occurred in anteroposterior, lateral, or both directions, suggesting different tendencies of upper airway collapse in patients. Three types of shape changes in the cross-section of the upper airway, which had different effects on airway resistance, were measured. CONCLUSIONS: Our findings prove TAHO hypothesis. The threshold measured while awake for normal breathing can be used clinically as the indicator of hypopnea onset during sleep, and therefore to identify OSA patients under waking condition and design effective personalized treatments for OSA patients. Both shape and size changes in the cross-section of the upper airway affect airway resistance significantly. Shape change in the cross-section of the upper airway can provide key clinical information on the collapse patterns of the upper airway for individuals.