Physiological Changes in Respiration During Sleep
During sleep, respiration undergoes predictable physiological changes including increased upper airway resistance, reduced minute ventilation, elevated PaCO2 by 2-7 mm Hg, decreased tidal volume, and altered ventilatory control with loss of behavioral respiratory drive.
Changes in Ventilation and Gas Exchange
Normal sleep causes a physiological increase in PaCO2 of 2-7 mm Hg from wakefulness to sleep 1. This occurs because breathing during sleep becomes primarily dependent on chemical control rather than behavioral control, making ventilation more vulnerable to instability 2.
- Minute ventilation decreases significantly during all stages of sleep compared to wakefulness (from 7.66 L/min awake to 7.18 L/min in non-REM and 6.46 L/min in REM sleep) 3
- REM sleep produces the greatest reduction in ventilation, with tidal volume reduced to 73% of waking levels 3
- Mean inspiratory flow rate (VT/Ti), an index of inspiratory drive, is significantly lower in REM sleep than during wakefulness or non-REM sleep 3
Upper Airway Changes
Airflow resistance increases substantially during sleep, rising from 3.93 cmH2O·L⁻¹·s during wakefulness to 7.96 in stage 2 sleep and 8.66 in REM sleep 4. This increase occurs almost entirely in the upper airway above the larynx 4.
- Pharyngeal dilator muscle tone decreases during sleep, resulting in increased upper airway resistance and collapsibility 5
- Changes in pharyngeal dilator muscle tone contribute to hypoventilation during sleep 5
Breathing Pattern Alterations
The breathing pattern during all stages of sleep becomes significantly more rapid and shallow than during wakefulness 3.
- Tidal volume decreases during sleep without statistically significant changes in dynamic compliance of the lungs 4
- Relative hypotonia of respiratory muscles, body posture changes, and altered ventilatory control result in additional physiologic changes contributing to hypoventilation 5
Central Respiratory Control Changes
Sleep states exert direct effects at multiple respiratory regulatory sites, including the central respiratory pattern generator, respiratory premotor pathways, and motoneurons that innervate the respiratory pump and upper airway muscles 2.
- Ventilatory responses to hypoxia and hypercapnia are altered during sleep 6
- Intercostal muscle tone decreases during sleep, particularly during REM sleep 6
Clinical Significance and Pathological Thresholds
Pathological hypoventilation during sleep is defined as either PaCO2 >55 mm Hg for ≥10 minutes, OR ≥10 mm Hg increase in PaCO2 during sleep (compared to awake supine value) reaching >50 mm Hg for ≥10 minutes 7, 1.
Common Pitfalls
- Do not assume that normal daytime PaCO2 excludes significant nocturnal hypoventilation, particularly in patients with neuromuscular disease or obesity hypoventilation syndrome 1
- Elevated serum bicarbonate (≥27 mmol/L) in obese patients should prompt investigation for sleep-related hypoventilation even if daytime blood gases appear acceptable 1
- These physiological changes are further magnified in disease states such as chronic obstructive pulmonary disease, restrictive respiratory disorders, neuromuscular conditions, and cardiac diseases 6