Influence of increased alveolar CO₂ tension on pulmonary diffusing capacity for CO in man

¹ Department of Physiology and Pharmacology, Graduate School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania

Since the pulmonary diffusing capacity for CO theoretically varies with changes in the size of the pulmonary capillary bed, it should be a useful tool for the investigation of the effects of CO₂ on the pulmonary capillaries. Dl was measured during hypercapnia in nine normal resting subjects by the CO-helium breath-holding technique. a) When 10% CO₂ was simply added to the inspired mixture used in the measurement of Dl, CO disappeared more rapidly from the lung. Dl was increased on the average only 5% after 10 seconds of breath holding. Pulmonary capillary blood volume (Vc) calculated by the method of Roughton and Forster was measured in two subjects and was increased approximately 46.5%. b) After the subject breathed a gas mixture containing 7.5% CO₂ for 10 minutes, Dl was increased an average of 24% in all subjects and Vc was increased 112% in one subject studied. In five subjects repeated measurements of Dl, together with measurements of ventilation, systemic blood pressure, pulse rate and pulmonary blood flow in three of them, were made before, during and after a period of hypercapnia. After starting to breath the CO₂-enriched gas mixture, Dl rose abruptly, reaching near maximal values in 2–5 minutes, while the other measurements of respiratory and circulatory phenomena continued to rise for 6–10 minutes, suggesting that the presumed increase in the pulmonary capillary bed was not dependent on the systemic respiratory and circulatory changes.