Distribution of blood flow and ventilation-perfusion ratio in the lung, measured with radioactive CO₂

TLDR

Inhaled radioactive CO₂ is rapidly taken up by pulmonary blood. By externally counting radioactive CO₂ over the chest during breath holding, the clearance rate from the counting field, proportional to regional perfusion, is recorded, and by relating the end‑inspiration counting rate to lung volume in the field, ventilation differences between upper and lower zones are measured and found to be small. In normal subjects, the clearance rate ranged from ~20 %/s at the lung base to near zero at the apex, decreasing linearly with height; this perfusion gradient was attenuated by moderate exercise and abolished supine, and the resulting ventilation‑perfusion ratio variation produced an alveolar‑arterial O₂ gradient of ~4 mm Hg, indicating that blood‑flow differences between upper and lower lung regions explain the V/Q inequality. Submitted on November 23, 1959.

Abstract

Inhaled radioactive CO 2 is rapidly taken up by pulmonary blood. By external counting over the chest during breath holding, the clearance rate of radioactive CO 2 xs from the counting field can be recorded, and is proportional to the regional perfusion. In normal subjects, the clearance rate varied from about 20%/ sec. at the base of the lung to virtually nil at the apex, and the change was approximately linear with distance up the chest. The difference between upper and lower zones was reduced on moderate exercise and eliminated when the subject lay on his back. By relating the counting rate at the end of inspiration to the volume of lung in the counting field, the difference in ventilation between upper and lower zones was measured and found to be small. Variation in ventilation-perfusion ratio was thus determined. Alveolar-arterial O 2 gradient expected from this ventilation-perfusion ratio inequality was calculated to be about 4 mm Hg. This suggests that the variation in blood flow between upper and lower parts of the lung in erect man accounts for the whole of the ventilation-perfusion ratio inequality in the normal lung. Submitted on November 23, 1959