Transpleural CO2 diffusion in excised dog lungs

Previous studies of transpleural gas diffusion have reported both linear and nonlinear relationships between total gas transfer and the transpleural driving gradient estimated as the difference between pleural and end-tidal gas tensions. The present studies were undertaken to explain these conflicting results by examining the role of convective mixing as well as the validity of using measurements of end-tidal gas to estimate subpleural gas tensions and calculate the transpleural partial pressure gradient. Nine excised nonperfused dog lungs were ventilated with room air under varying conditions of frequency (f), tidal volume (VT) and pleural CO2 concentration (FplCO2). The relationship between CO2 elimination (VCO2) and the transpleural CO2 concentration gradient estimated by (FplCO2-FETCO2) was found to be linear for every level of ventilation (constant convective mixing) studied. The slope of this relationship, equal to the pleural CO2 diffusing capacity (DplCO2) was found to decrease as ventilation was decreased whether achieved by reductions in f or VT. Because changes in membrane geometry were insufficient to explain the differences in DplCO2, seven additional excised dog lungs were studied ultrastructurally following 4 h of ventilation with room air at a pleural CO2 concentration of 6%. Previous studies under similar conditions have shown that changes in type II pneumocyte lamellar body volume density Vv(LB/cyto) are related to the alveolar concentration of CO2. The results indicated that the concentration of CO2 in alveoli located within 1 mm of the pleural surface was substantially greater than the value estimated by FETCO2. Consequently, (FplCO2-FETCO2) over-estimates the transpleural concentration gradient and produces erroneously low estimates of the diffusing capacity of the pleural membrane.