Oxygen and Carbon Dioxide Retention in COPD

Revised and reviewed 6 March 2014

OVERVIEW

• Excessive oxygen administration can lead to hypercapnic respiratory failure in some COPD patients
• COPD patients with more severe hypoxemia are at higher risk of CO2 retention from uncontrolled O2 administration
• The same phenomenon has also been described in severe asthma, community-acquired pneumonia and obesity hypoventilation syndrome and any patient with chronic respiratory failure may be at risk

MECHANISM

The traditional explanation is wrong

• the traditional theory is that oxygen administration to CO2 retainers causes loss of hypoxic drive, resulting in hypoventilation and type 2 respiratory failure. This is a myth.
• Patients suffering from COPD exacerbation, regardless of whether they have CO2 retention, generally have supra-normal respiratory drive (unless there is impending hypercapnic coma)

The real explanation involves:

• increased V/Q mismatch (most important)
• the Haldane effect

V/Q mismatch

• In COPD, patients optimise their gas exchange by hypoxic vasoconstriction leading to altered alveolar ventilation-perfusion (Va/Q) ratios
• Excessive oxygen administration overcomes this, leading to increased blood flow to poorly ventilated alveoli, and thus increased Va/Q mismatch and increased physiological deadspace
• this increase in Va/Q mismatch occurs in both CO2 retainers and non-retainers, the difference is presumably one of degree

The Haldane effect

• deoxygenated hemoglobin (Hb) binds CO2 with greater affinity than oxygenated hemoglobin (HbO2)
• hence oxygen induces a rightward shift of the CO2 dissociation curve, which is called the Haldane effect
• in patients with severe COPD who cannot increase minute ventilation, the Haldane effect accounts for about 25% of the total PaCO2 increase due to O2 administration

MANAGEMENT

• Target SaO2 88-92% in these patients
• the targeted approach is associated with decreased mortality in COPD patients and less respiratory acidosis
• The oxygen flow rate administered is not important, the (alveolar) PAO2 (and, indirectly, the SaO2) achieved is
• Never withhold oxygen from a seriously ill hypoxic patient due to fear of cause hypercapnic respiratory failure

References and Links

Journal articles

• Abdo WF, Heunks LM. Oxygen-induced hypercapnia in COPD: myths and facts. Crit Care. 2012 Oct 29;16(5):323. PMC3682248

FOAM and web resources

• DundeeChest — Why do COPD patients retain CO2 when given too much oxygen? (features brief animations showing loss of hypoxic vasoconstriction leading to V/Q mismatch in COPD following oxygen administration)