Convection, diffusion and cardiogenic mixing of inspired gas in the lung; an experimental approach

In 8 open-chested dogs, we measured the FN2 within 26 airways, 2.5-8.6 mm in diameter, during constant flow inflations with 0.5 liter of O2. At low flows the FN2 did not fall to zero but reached a plateau, at a value that was inversely related to inspiratory flow. When inspiratory flow is constant, the measured FN2 represents one point on a stationary front separating inspired and alveolar gas. At all points on the front the convective and diffusive transport of N2 is equal and opposite in direction. We quantitated cardiogenic gas mixing by comparing in vivo and post mortem the flow which resulted in a given stable FN2 value within the same airway. In vivo, this flow and therefore the calculated effective diffusion coefficient (D') was more than 5 times greater than that post mortem. Our results confiem some of the predictions made from model analyses of gas transport in the lung. However, calculations based on molecular diffusion as the sole mixing mechanism necessarily overestimate diffusion times and the magnitude of stratification.