pulmonary disorders
Treatment of Acute COPD Exacerbation
Oxygen in Acute COPD Exacerbation
Many patients require oxygen supplementation during a COPD exacerbation, even those who do not need it chronically. Hypercapnia may worsen in patients given oxygen. This worsening has traditionally been thought to result from an attenuation of hypoxic respiratory drive. However, increased ventilation/perfusion (V/Q) mismatch probably is a more important factor. Before oxygen administration, pulmonary vasoconstriction minimizes V/Q mismatch by decreasing perfusion of the most poorly ventilated areas of the lungs. Increased V/Q mismatch occurs because oxygen administration attenuates this hypoxic pulmonary vasoconstriction. The Haldane effect may also contribute to worsening hypercapnia, although this theory is controversial. The Haldane effect is a decrease in hemoglobin's affinity for carbon dioxide, which results in increased amounts of carbon dioxide dissolved in plasma. Oxygen administration, even though it may worsen hypercapnia, is recommended; many patients with COPD have chronic as well as acute hypercapnia and thus severe central nervous system depression is unlikely unless PaCO2 is>85 mm Hg. The target level for PaO2 is about 60 mm Hg; higher levels offer little advantage and increase the risk of hypercapnia. In patients who are prone to hypercarbia (ie, an elevated serum bicarbonate may indicate the presence of a compensated respiratory acidosis), oxygen is given via nasal prongs or Venturi mask so it can be closely regulated, and the patient is closely monitored. Patients whose condition deteriorates with oxygen therapy (eg, those with severe acidemia or central nervous system depression) require ventilatory assistance. Many patients who require oxygen at home for the first time when they are discharged from the hospital after an exacerbation improve within 30 days and no longer require oxygen. Thus, the need for home oxygen should be reassessed 60 to 90 days after discharge. 