Pathophysiology of obstructive sleep apnea in adults
- M Safwan Badr, MD
M Safwan Badr, MD
- Section Editor — Sleep Related Breathing Disorders
- Professor and Chief, Pulmonary Critical Care and Sleep Medicine
- Wayne State University School of Medicine
Obstructive sleep apnea (OSA) is a fairly common disorder with significant adverse health consequences [1-4]. OSA is characterized by recurrent obstruction of the pharyngeal airway during sleep, with resultant hypoxia and sleep fragmentation. The pathogenesis of OSA is due to the interaction between unfavorable anatomic upper airway (UA) susceptibility and sleep-related changes in UA function . However, the mechanisms linking sleep-related physiologic changes to UA obstruction in some individuals are not fully understood. In addition, the majority of studies investigating UA obstruction during sleep have been conducted during nonrapid eye movement (NREM) sleep, given the difficulty in achieving rapid eye movement (REM) during invasive studies in the laboratory environment.
This topic will review the effects of sleep on respiratory mechanics, the determinants of UA patency, and the pathophysiology of UA obstruction during sleep. The pathophysiology of OSA in children and the clinical features, diagnosis, and treatment of OSA in children and adults are reviewed separately. (See "Mechanisms and predisposing factors for sleep-related breathing disorders in children" and "Overview of obstructive sleep apnea in adults" and "Clinical presentation and diagnosis of obstructive sleep apnea in adults" and "Management of obstructive sleep apnea in adults" and "Management of obstructive sleep apnea in children".)
EFFECT OF SLEEP ON RESPIRATORY MECHANICS
Sleep is accompanied by multiple physiologic changes relevant to ventilation and respiration (algorithm 1). Sleep is viewed as a quiet resting period, judging by the limited movement, decreased responsiveness, and the passive appearance of a sleeping individual. Sleep is associated with a decreased metabolic rate, loss of the wakefulness drive to breathe , and a subsequent decrease in ventilatory motor output to respiratory muscles, including upper airway (UA) muscle. Furthermore, the loss of the wakefulness drive to breathe renders respiration during sleep critically dependent on the level of chemoreceptor and mechanoreceptor stimuli , and hence susceptible to central apnea and to upper airway obstruction. The pathogenesis of central apnea is discussed separately. (See "Central sleep apnea: Pathogenesis".)
Upper airway mechanics
Decreased muscle activity — Reduced UA muscle activity during sleep is a physiologic phenomenon of little consequence in healthy individuals, but it may promote UA narrowing in susceptible individuals.
Decreased ventilatory motor output is associated with decreased UA muscle activity, particularly in muscles that display tonic activity (independent of the phase of respiration). For example, the tensor palatini demonstrates immediate decrease in activity with sleep onset, with associated reduction in inspiratory flow [7,8]. Studies investigating respiratory muscle activity at sleep onset demonstrate that the activity of respiratory pump muscles and UA-dilating muscles changes less when the dominant electroencephalogram (EEG) waveform is theta (light sleep) versus alpha (wakefulness) .
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- EFFECT OF SLEEP ON RESPIRATORY MECHANICS
- Upper airway mechanics
- - Decreased muscle activity
- - Changes in caliber and compliance
- - Loss of load compensation
- Thoracic cage dynamics
- Increased arterial carbon dioxide
- DETERMINANTS OF UPPER AIRWAY PATENCY
- Structural factors
- - Craniofacial structure
- - Soft tissue structures
- Vascular factors
- - Rostral fluid displacement
- Neuromuscular factors
- - Upper airway muscle activity
- Decreased activity
- Decreased responsiveness
- Mechanical corollary
- Changes during rapid eye movement sleep
- - Effect of lung volume
- PHARYNGEAL OBSTRUCTION DURING SLEEP
- Upper airway mechanics
- Ventilatory motor output
- Surrounding tissue
- Role of expiratory narrowing