Upper Airway Compliance

Upper airway compliance, or collapsibility, is determined by many factors such as tonic and phasic upper airway muscle activation, biomechanical properties (e.g., connective tissue composition, surface tension), and response of the upper airway to positional stresses. Perhaps more important than the differences in static upper airway properties between men and women, differences in upper airway compliance predicts a divergence in the propensity for upper airway collapse by gender.

Upper airway imaging techniques have demonstrated an increased upper airway compliance (and subsequent increase in upper airway collapsibility) in proportion to the neck circumference (28), which tends to be higher in men than in women. Neck circumference alone, however, may only be a surrogate for the underlying soft tissue and muscle composition in the neck.

Local upper airway dilator muscle activation in response to upper airway obstruction can protect the patency of the upper airway (see section: Neuromuscular Reflexes) Little is known about the biomechanical properties of the upper airway in patients with OSA, particularly with regards to gender differences.

Body Habitus

Obesity is the strongest risk factor for OSA (7,29), with even small changes in weight resulting in clinically significant changes in upper airway collapsibility and severity of upper airway obstruction (29). The effect of obesity on obstructive apneic and hypopneic events is proposed to be mass loading of the upper airway by adiposity surrounding the pharynx, causing upper airway collapse. Therefore, the distribution of total body fat and not just overall BMI, is relevant to the risk of OSA. Women are more likely to be obese (BMI > 30 kg/m2) than men (30), but differences in fat distribution may be one reason why women have lower rates of OSA. When comparing men and women with comparable BMI and waist circumferences, men exhibited greater upper body obesity, as measured by smaller hip circumferences and greater subscapular skin fold thickness (31). The finding that women with a comparable severity of OSA as men have a greater BMI (7), may be explained by differences in distribution of body weight by gender.

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