Abstract
When breathing air, the average arterial oxygen tension in eight patients with acute pulmonary oedema was significantly higher than in eight other patients suffering from an acute exacerbation of chronic bronchitis, but the mixed venous oxygen tension was very similar in both groups. This largely arose from the smaller arteriovenous difference of oxygen content in the bronchitic cases, presumably due to their higher cardiac output, associated with raised arterial CO(2) tensions. Oxygen therapy (60-90% for pulmonary oedema, 30% for the bronchitics) raised the mixed venous oxygen tensions to a similar level in both groups. We suggest that the major need for oxygen therapy lies in patients who maintain their oxygen consumption but show a reduction in mixed venous tension when breathing air. Although partial correction of arterial hypoxaemia is adequate in chronic bronchitis-in which the cardiac output is maintained-high concentrations of oxygen are necessary in pulmonary oedema, in which the cardiac output is low.