Upper Airway During Sleep

Abnormal upper airway anatomy predisposes to airway collapse during sleep. The onset of sleep is associated with a number of changes in the factors affecting pha-ryngeal patency including neuromuscular activity, ventilation, chemical, and load responses. Central drive to the respiratory muscle apparatus and upper airway dilators is reduced at the transition from wakefulness to NREM sleep. This leads to reduced upper airway cross-sectional area and increased airway resistance, which together compromise the airway's ability to remain patent. Sites of upper airway narrowing in wakefulness do not necessarily correlate with the site of obstruction during sleep (2,25). Several studies employing nasopharyngoscopy, CT, and MRI have shown that the narrowing/closure site in the airway during sleep is most commonly in the retropalatal area (14,25,41,48,138). Studies have also demonstrated that reductions in airway dimensions occur in the lateral and/or anteroposterior axes in apneics and normals (14,25,41,71,72,138) (Fig. 6) The reason for the former is thought to be secondary to thickening of the lateral pharyngeal walls and the latter is thought to be secondary to posterior displacement of the soft palate (138). Videoendoscopy during sleep has shown that pharyngeal occlusion appears to be due to a combination of inspiratory and expiratory narrowing in the breaths that precede an apnea. Expiratory narrowing renders the lumen vulnerable to complete collapse during the

Sleep

Wake

Sleep

Wake

FIGURE 6 State-dependent magnetic resonance imaging in the retro-palatal region of a normal subject (apnea-hypopnea index = 0 events/ hour). Airway area is smaller during sleep in this normal subject. The state-dependent change in airway caliber is secondary to reductions in the lateral and anterior-posterior airway dimensions. Source: From Ref. 138.

FIGURE 6 State-dependent magnetic resonance imaging in the retro-palatal region of a normal subject (apnea-hypopnea index = 0 events/ hour). Airway area is smaller during sleep in this normal subject. The state-dependent change in airway caliber is secondary to reductions in the lateral and anterior-posterior airway dimensions. Source: From Ref. 138.

subsequent inspiratory effort (72). Contrary to prior beliefs, the airway is not a homogeneous tube that collapses uniformly (Fig. 7). It behaves more like a Starling resistor where pharyngeal occlusion occurs once the intraluminal pressure decreases below the surrounding pressure. Airflow limitation occurs despite continued increases in driving pressure as a result of the upstream segment (e.g., nose) and the transmural pressure surrounding the collapsible segment. Studying the upper airway anatomical differences between apneics and normals during sleep is necessary to eliminate the confounding effects of muscle activity. One such study used endoscopic techniques to assess pharyngeal size in OSA patients and controls under general anesthesia with full paralysis (69). Results obtained showed a smaller airway caliber, especially in the retropalatal area and increased collapsibility in apnea patients (69). An MRI study confirmed this differential narrowing by quantifying volume reduction in the retropalatal (19% volume reduction) compared with the retroglossal (4% volume reduction) regions during sleep in normal patients (138) (Fig. 7). The specific mechanisms explaining the predominance of narrowing in the retropalatal region during sleep remain to be unraveled. Nonetheless, such data indicate that during sleep, normals, in addition to apneics, narrow their upper airway primarily in the retropalatal region.

Sleep Apnea

Sleep Apnea

Have You Been Told Over And Over Again That You Snore A Lot, But You Choose To Ignore It? Have you been experiencing lack of sleep at night and find yourself waking up in the wee hours of the morning to find yourself gasping for air?

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