Gas concentration profiles along airways of dog lungs during high-frequency ventilation

Following equilibration with inert gases (He and SF6), dog lungs were partially washed out, either by high-frequency ventilation (HFV) or by conventional mechanical ventilation (CMV), to varied inert gas clearance levels, at which expirograms were recorded by mass spectrometry. Relative alveolar slopes were distinctly positive for HFV and tended to increase with lung clearance; they were, however, smaller than those of CMV and were smaller for He than for SF6 during both ventilatory modes. Fowler dead space was smaller for HFV than for CMV, with significant differences between test gases only during CMV. Plots of concentration against linear distance between measuring site and alveolar region showed that most of the total concentration drop during HFV occurred, with nearly linear slope, along the endotracheal tube and the upper airways, with no difference between He and SF6. In the alveolar region, on the other hand, relative concentration gradients were similar for HFV and CMV, both showing separation of He and SF6. The data suggest that gas transport in the upper airways during HFV is not diffusion limited. Gas mixing in alveolar regions, although more complete for HFV than for CMV, is limited by diffusion; however, this incomplete gas mixing does not appreciably limit overall gas transport during HFV.