Mechanism

The primary cause of obstructive sleep apnea remains uncertain (1)

The sequence of events is as follows:

• the principal pathophysiological characteristic of the disorder is collapse of the upper airway during sleep (1)
  • this causes a fall in tone of the upper airway during sleep
• contributing factors include
  • abnormalities of airway structure- enlarged tonsils and uvula
  • abnormal airway function - primarily a reduced activity of the dilator muscles of the pharynx (1)
  • weight of the neck
  • airflow pressure - including chronic nasal obstruction
  • tone - reduced by alcohol
• progressive upper-airway closure during the transition to sleep may be accompanied by either of the following:
  • complete obstruction of airflow (apnea)
  • partial obstruction (hypopnea).
• the resulting hypoventilation causes
  • asphyxia - PaO2 as low as 2kPa ( hypoxemia)
  • hypercapnia
  • acidosis
  • generation of negative intrathoracic pressure (1)
• these changes stimulate arousal centers in the central nervous system
• this leads to increased respiratory and pharyngeal-muscle activity.
  • patient becomes completely or partially aroused
  • these changes in neurologic function ultimately overcome the obstruction, and ventilation resumes.
  • there is increased drive to the pharyngeal muscles and these are pulled apart causing a loud snort
• the patient then returns to sleep, the pharyngeal musculature relaxes, and the cycle repeats itself.
• the cycle repeats after 1/2 - 2 minutes
• there is associated increased right and left ventricular afterload, decreased left ventricular compliance, increased pulmonary-artery pressures, and increased myocardial oxygen demand (1)

Reference:

• (1) Basner RC. Continuous Positive Airway Pressure for Obstructive Sleep Apnea.N Engl J Med 2007; 356:1751