nocturnal hypoxemia has various pathophysiology

hypoxemia can occur in anyone on the steep curve of oxyhemoglobin dissociation curve¹
overnight oximetry cannot discriminate between different etiologies of hypoxemia
use PSG to determine presence of apneas and capnography to determine presence of hypoventilation
patients with OSA/CSA have hypoxemia from apnea (using Aa gradient to determine reason for hypoxemia, sleep disordered breathing is characterized by apnea, hyponea, hypoventilation, RERA)
in patients with OHS, hypoxia has variety of reasons:
- hypoventilation
  - presence of OSA or CSA
  - change in O2 sensor from leptin and resultant central hypoventilation
- V/Q mismatch and increased deadspace ventilation²
  - decreased chest wall and abdominal wall compliance causing restriction
  - increased airway resistance from bronchospasm
  - decreased muscle strength
  - low ERV leads to VQ mismatch that's worse in supine position. Patients adapt by taking lower tidal volume with higher respiratory rate.
in patients with COPD, there's V/Q mismatch and deadspace from air trapping and emphysema
in patients with restrictive lung disease, there's decreased lung compliance. Hypoxia can also happen with NMS and diaphragmatic paralysis

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