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

Open Access
Publisher Website
Google Scholar
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 (DLCO2 ) exceeded 200 ml/(min × mm Hg). After the administration of a carbonic anhydrase inhibitor the rate of disappearance of 13CO2 decreased markedly. DLCO2 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 (Vtis) 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 PCO2 of 40 mm Hg pH of Vtis equalled 6.97 ± 0.09.