Diffusion-limited tissue equilibration and arteriovenous diffusion shunt describe skeletal muscle nitrous oxide...

This study evaluated the relative importance of perfusion and diffusion mechanisms in compartmental models of blood : tissue inert gas exchange in skeletal muscle. Nitrous oxide kinetics in a hind limb skeletal muscle bed were determined during and after 20 min of nitrous oxide inhalation, at separate low and high steady states of hind limb blood flow in five sheep under halothane anaesthesia. Nitrous oxide concentrations in arterial and femoral vein blood were determined using gas chromatographic analysis and femoral vein blood flow was monitored continuously. Parameters and model selection criteria of various perfusion- or diffusion-limited structural models of skeletal muscle were estimated by simultaneous fitting of the models to the mean observed femoral vein nitrous oxide concentration for both blood flow states. Purely perfusion-limited models fit the data poorly. Models that allowed a diffusion-limited exchange of nitrous oxide between a perfusion-limited tissue compartment and an unperfused deep compartment provided better overall fit of the data and credible parameter estimates. The data was best described by allowing, in addition to diffusion-limited tissue equilibration, counter current diffusion of nitrous oxide between arterial and venous blood. The level of tissue blood flow modifies the magnitudes of both these diffusion effects. These results suggest a dual role of diffusion in blood : tissue inert gas equilibration in skeletal muscle.