Expansion
Ventilation falls, carbon dioxide rises slightly and upper airway tone is reduced
Expansion
Normal changes in sleep:
- Minute ventilation falls by 10 to 15 per cent
- Arterial carbon dioxide rises by about 0.5 kPa (3 to 4 mmHg)
- Chemoreceptor sensitivity is reduced
- Upper airway dilator muscle tone falls, so airway resistance rises
- Loss of the wakefulness drive to breathe
In REM sleep these are exaggerated: breathing becomes irregular, and skeletal muscles including the intercostals and accessory muscles are atonic, leaving the diaphragm as the only effective respiratory muscle.
Consequences:
- Obstructive sleep apnoea: reduced dilator tone allows pharyngeal collapse; worsened by obesity, alcohol, sedatives and supine posture
- Patients relying on accessory muscles (neuromuscular disease, kyphoscoliosis, severe chronic obstructive pulmonary disease) hypoventilate most in REM sleep, so nocturnal hypoventilation and morning headache precede daytime respiratory failure
- Diaphragmatic weakness causes profound REM-related desaturation