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  • PaCO2 >45
  • Exclusively caused by alveolar hypoventilation
    • Results from decrease in respiratory rate, tidal volume, or increase in dead space

Clinical Features

  • Headache, confusion, lethargy, seizure, coma
  • Extreme hypercapnia (acute elevation >100) can result in cardiovascular collapse

Differential Diagnosis

  • Depressed central respiratory drive
    • Structural CNS disease: brainstem lesions
    • Drug depression of respiratory center: opioids, sedatives, anesthetics
    • Endogenous toxins: tetanus
  • Thoracic cage disorders
    • Kyphoscoliosis
    • Morbid obesity
  • Neuromuscular impairment
    • Neuromuscular disease: myasthenia gravis, Guillain-Barré
    • Neuromuscular toxin: organophosphate poisoning, botulism
  • Intrinsic lung disease associated with increased dead space
  • Upper airway obstruction


  • ABG
    • HCO3 increases 1 mEq/L for each 10mmHg increase in PaCO2 (acute)
    • HCO3 increasess 3.5 mEq/L for each 10mmHg increase in PaCO2 (chronic)


  • Increase minute ventilation (rate and/or tidal volume)

See Also