1. Pulmonary blood volume was measured in 39 patients with severe chronic bronchitis, of whom 29 had recently recovered from an acute exacerbation. Pa,o2 was below 10·3 kPa in all but five patients, Pa,co2 was above 5·6 kPa in 26 cases, and pulmonary artery mean pressure was above 20 mmHg in 29 cases. Pulmonary blood volume was measured with the double-injection—single-sampling technique using Indocyanine Green and pulmonary wedge injection.

2. Pulmonary blood volume in the 39 patients was significantly lower (343 ± 95 ml) than in a group of 16 normal subjects (469 ± 117 ml) studied previously by the same method. There was no correlation between pulmonary blood volume and driving pressure across the pulmonary circulation, or between pulmonary blood volume and Pa,o2.

3. Breathing 26–29% oxygen in 19 patients caused no significant changes in either pulmonary blood volume or pulmonary vascular resistance. Therefore the reduced pulmonary blood volume could not be attributed to hypoxic pulmonary vasoconstriction.

4. The finding of a diminished pulmonary blood volume in chronic bronchitis is best explained by a loss of pulmonary vessels. Pulmonary blood volume was not correlated with total lung capacity nor with the residual volume/total lung capacity ratio, nor was it significantly higher in the 16 patients with normal total lung capacity (352 ± 73 ml) than in the 23 patients with a total lung capacity greater than 110% of normal (331 ± 110 ml). There was therefore no correlation between the amount of emphysema and the reduction in pulmonary blood volume.

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