Morphometric estimates of diffusing capacity in lungs fixed under zone II and zone III conditions

Comparative morphometric estimates of the diffusing capacity (DL) were made in rabbit lungs fixed by vascular perfusion under lower zone II and zone III conditions and in lungs fixed by instillation of fixatives into the airways. Owing to a reduction of both capillary volume and membrane diffusing capacity DL of zone II lungs (0.074±0.007 (SD) ml · sec^-1 · mbar^-1) was found to be lower by some 25% than DL of instillation-fixed lungs (0.102±0.012 (SD) ml · sec^-1 · mbar^-1). The average value of DL of air-filled zone III lungs, on the other hand, almost matched the DL of instillation-fixed lungs. However, DL is not equal in all regions but increases along the vertical axis of zone III lungs. Hence, the previous conclusion that morphometric estimates of DL in instillation-fixed lungs reflects a structural limit for O₂ diffusion, which cannot be reached under physiologic conditions, must be revised.