Stratification does not limit O₂ uptake in rabbit lungs

Abstract

This study was performed to assess the role of stratification, i.e. axial gas mixing deficit within alveolar space, in limiting alveolar gas exchange for oxygen. The single-breath method for varying breath-holding time with oxygen-labelled carbon dioxide, C18O2, was applied to 10 anaesthetized, paralysed and artificially ventilated rabbits. Alveolar partial pressure of C18O2 was analysed using respiratory mass spectrometry. Starting from residual volume, the lungs were rapidly inflated using 40 mL of indicator gas mixture (1% C18O2 in nitrogen). After executing breath-holding, the lungs were rapidly deflated. Pulmonary diffusing capacity of carbon monoxide was determined in the same way. On the basis of a serial compartment model, the lower limit of the stratificational conductance of oxygen was estimated, using the rate constant of C18O2 removal from alveolar space (4 s-1) and Graham's law. We found that the stratificational conductance in rabbits amounts to at least 13.5 mL mmHg-1 min-1. The pulmonary diffusing capacity of oxygen was calculated by multiplying the carbon monoxide diffusing capacity of rabbit lungs by a factor of 1.2, yielding a value of 0.77 mL mmHg-1 min-1. These results show that stratificational conductance is at least 17.5 times higher than pulmonary oxygen diffusing capacity, indicating that stratification does not limit oxygen uptake in rabbit lungs.

Density-dependent reduction of nitric oxide diffusing capacity after pneumonectomy

“…Model simulations by Paiva and Engel (21,22) suggest that inhomogeneity of gas concentrations within a single acinus, although present at rest normally, is functionally small and contributes little to the alveolar-arterial O 2 tension difference (A-aDO 2 ). The normal acinus functions as a single gas exchange unit, consistent with reports by Heller and Schuster (10) in rabbits, by Worth et al (41) in adult dogs, and in our present results.…”

“…Altered Taylor's dispersion has been invoked to explain the density-dependent increase of steady-state CO uptake observed in human subjects (5,16). However, model simulations and experimental data (10,20,37) suggest that normal resting A-aDO 2 is predominantly caused by ventilation-perfusion heterogeneity among acini rather than by diffusion limitation within an acinus. Hence, this explanation does not fully account for the reported effect of SF 6 on resting A-aDO 2 .…”

Lung Structure and the Intrinsic Challenges of Gas Exchange

“…The effects of diffusion limitation within alveolar airspace, termed gas-phase resistance or stratified inhomogeneity, on gas transfer have been simulated (139, 272, 313) and summarized recently (356). Because axial or gas phase conductance is at least 17.5 times higher than conductance across alveolar tissue-blood barrier, the relatively small magnitude of stratified inhomogeneity does not normally limit alveolar O 2 transfer (133, 270, 316). Stratified inhomogeneity becomes detectable when distal airways and acinar air spaces become elongated and dilated such as that in the expanded remaining lung following pneumonectomy (158, 167).…”