Kinetics of pulmonary CO2 transfer studied by using labeled carbon dioxide C16O18O

The kinetics of alveolar-capillary CO2 transfer was investigated, using 18O-labeled carbon dioxide (C16O18O), in 21 experiments on 2 resting human subjects. Single-breath experiments were performed. After the subjects expired to residual volume, they inspired a gas mixture containing 20% O2 and 2.8% CO2 - a quarter of CO2 was labeled with 18O. The breath-hold time ranged between 0.5 and 20 sec. After expiration the gas mixture was analysed by mass spectrometry. The C16O18O concentration of the end-expiratory gas decreased with the increasing breath-hold time. The relationship of both showed a biexponential characteristic with the two time constants 0.4 and 16 sec. The lower time constant characterizes the CO2 transfer from the alveolar space to the capillary blood, as well as the isotopic equilibration of the 18O within the blood. The higher time constant could result from small admixtures of the label from the dead spaces or the lung tissues. The diffusing capacity of C16O18O was calculated by using the first part of the indicator clearance curve. The derived mean value of 862 ml X mm Hg-1 X min-1 is considered to be a lower limit of the 'membrane diffusing capacity' for CO2 (DMCO2).