Complications

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Oxygen-induced hypercapnia

• Overview
  • PaCO2 > 45 mm Hg or a rise of > 5 mm Hg in a chronically hypercapnic patient
  • One of the most common and serious complications of oxygen therapy
  • Can be fatal if left untreated.
• Mechanism
  • Thought to predominantly occur via a combination of two mechanisms when supplemental oxygen is administered:
    • ↓ Hypoxic pulmonary vasoconstriction: ↑ FiO2 → ↑ alveolar O2 tension → ↓ hypoxic pulmonary vasoconstriction → ↑ V/Q mismatch and hypercapnia
    • Haldane effect: ↑ FiO2 → ↑ oxygenated Hb, which has a reduced affinity to bind CO2 (right shift in the CO2 dissociation curve) → CO2 being released from Hb and RBCs → ↑ PaCO2
• Risk factors
  • Any patient with risk factors for hypercapnic respiratory failure who is receiving supplemental oxygen
  • PaO2 is > 75 mm Hg
  • Acute illness or new oxygen therapy in patients with chronic hypercapnic respiratory failure
• Management
  • Gradually titrate oxygen back to 88–92%.
  • Noninvasive ventilation in patients with decompensated hypercapnic respiratory failure who are within target saturations.
• Prevention
  • Close monitoring for symptoms of hypercapnia.
  • Patients at risk of hypercapnic respiratory failure: ABGs should be performed if drowsiness or other symptoms of hypercapnia develop, if saturations deteriorate, or if acute breathlessness occurs.