1. Sixteen patients with chronic bronchitis and airways obstruction were given radioactive nitrogen (13N) by intravenous injection and by inhalation, while breathing air and after 10–20 min breathing 30% oxygen. The clearance of 13N from four zones of each patient's whole lung field was monitored.

2. The 13N clearance of each region in these patients with chronic bronchitis was much slower than in normal subjects. Oxygen breathing produced a significant delay in the clearance of intravenously administered 13N in 23 zones in 10 patients but no systematic change in clearance after inhaled 13N.

3. With inhalation of 30% oxygen there was no significant change in the mean minute ventilation, tidal volume or arterial Pco2.

4. The results suggest that local hypoxic vasoconstriction is present in some patients on breathing air and that this is relieved by 30% oxygen, resulting in a diversion of local blood flow from well-ventilated to more poorly ventilated areas. The fall in V̇A/Q̇ on 30% oxygen is insufficient to increase arterial Pco2.

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