Rate of disappearance of labeled carbon dioxide from the lungs of humans during breath holding: a method for studying the dynamics of pulmonary CO<sub>2</sub> exchange

Richard W. Hyde; Ricardo J. M. Puy; William F. Raub; Robert E. Forster

doi:10.1172/JCI105846

Rate of disappearance of labeled carbon dioxide from the lungs of humans during breath holding: a method for studying the dynamics of pulmonary CO₂ exchange

Richard W. Hyde, Ricardo J. M. Puy, William F. Raub, and Robert E. Forster

Published July 1, 1968 - More info

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Abstract

The dynamics of CO₂ exchange in the lungs of man was studied by observing the rate of disappearance of a stable isotope of CO₂ (¹³CO₂) from the alveolar gas during breath holding. Over 50% of the inspired isotope disappeared within the first 3 sec followed by a moderately rapid logarithmic decline in which one-half of the remaining ¹³CO₂ disappeared every 10 sec.

The large initial disappearance of ¹³CO₂ indicated that alveolar ¹³CO₂ equilibrated in less than 3 sec with the CO₂ stored in the pulmonary tissues and capillary blood. The volume of CO₂ in the pulmonary tissues calculated from this initial disappearance was 200 ml or 0.33 ml of CO₂ per milliliter of pulmonary tissue volume.

The alveolar to end-capillary gradient for ¹³CO₂ was calculated by comparing the simultaneous disappearance rates of ¹³CO₂ and acetylene. At rest and during exercise this gradient for ¹³CO₂ was either very small or not discernible, and diffusing capacity for CO₂ (D_{L_CO₂}) exceeded 200 ml/(min × mm Hg).

After the administration of a carbonic anhydrase inhibitor the rate of disappearance of ¹³CO₂ decreased markedly. D_{L_CO₂} fell to 42 ml/(min × mm Hg) and at least 70% of the exchange of ¹³CO₂ with the CO₂ stores in the pulmonary tissues and blood was blocked by the inhibitor. These changes were attributed to impairment of exchange of ¹³CO₂ with the bicarbonate in the pulmonary tissues and blood.

The pH of the pulmonary tissues (V_tis) was determined by a method based on the premise that the CO₂ space in the pulmonary tissues blocked by the inhibitor represented total bicarbonate content. At an alveolar P_CO₂ of 40 mm Hg pH of V_tis equalled 6.97 ± 0.09.