Countercurrent compartmental models describe hind limb skeletal muscle helium kinetics at resting and low blood flows in sheep

Abstract

This study evaluated the relative importance of perfusion and diffusion mechanisms in compartmental models of blood : tissue helium exchange in a predominantly skeletal muscle tissue bed in the sheep hind limb. Helium has different physiochemical properties from previously studied gases and is a common diluent gas in underwater diving where decompression schedules are based on theoretical models of inert gas kinetics. Helium kinetics across skeletal muscle were determined during and after 20 min of helium inhalation, at separate resting and low steady-states of femoral vein blood flow in six sheep under isoflurane anaesthesia. Helium 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-limited or perfusion-diffusion compartmental models of skeletal muscle were estimated by simultaneous fitting of the models to the femoral vein helium concentrations for both blood flow states. A model comprising two parallel perfusion-limited compartment models fitted the data well but required a 51-fold difference in relative compartment perfusion that did not seem physiologically plausible. Models that allowed a countercurrent diffusion exchange of helium between arterial and venous vessels outside of the tissue compartments provided better overall fit of the data and credible parameter estimates. These results suggest a role of arterial-venous diffusion in blood : tissue helium equilibration in skeletal muscle.