Updated: 2/13/2018

Supplemental Oxygen-Induced Hypercapnia

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Introduction
  • Definition
    • supplemental oxygen given to a patient with chronic hypercapnia worsens CO2  retention 
      • chronic hypercapnia seen in longstanding COPD as a result of V/Q mismatching (increased dead space)
        • higher minute ventilation required to maintain normal PaCO2
  • Pathophysiology
    • blood pH, determined by changes in PaCO2, is main driver of ventilation 
      • peripheral and central chemoreceptors
      • elevated PaCO2 normally triggers hyperventilation
        • chronic elevations alter sensitivity of respiratory center to CO2 and new threshold is set
        • this makes O2 receptors in the carotid bodies, and hypoxemia an important driver for respiratory activity in patients with COPD
  • Changes with oxygen supplementation that result in increased PaCO2
    • Decreased minute ventilation
      • hypoxic drive for ventilation may be reduced, but this is likely not major factor in worsening hypercapnia
      • decrese in minute ventilation is insufficient to fully explain rise in CO2
    • Increased dead space to tidal volume ratio
      • worsens V/Q matching because of loss of hypoxic vasoconstriction
      • most important contributing factor to worsening hypercapnia
    • Haldane effect
      • oxyhemoglobin binds CO2 less avidly than deoxyhemoglobin
      • increased dissolved CO2 in blood = increased PaCO2
  • Presentation
    • altered mental status after supplemental oxygen as a result of acute on chronic respiratory acidosis and elevated PaCO2

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