Journal of Applied Physiology, Vol 69, Issue 3 937-944, Copyright © 1990 by American Physiological Society
Alveolar slope and dead space of He and SF6 in dogs: comparison of airway and venous loading
M. Meyer, K. D. Schuster, H. Schulz, M. Mohr and J. Piiper
Department of Physiology, Max Planck Institute for Experimental Medicine, Gottingen, Federal Republic of Germany.
Series (Fowler) dead space (VD) and slope of the alveolar plateau of two
inert gases (He and SF6) with similar blood-gas partition coefficients
(approximately 0.01) but different diffusivities were analyzed in 10
anesthetized paralyzed mechanically ventilated dogs (mean body wt 20 kg).
Single-breath constant-flow expirograms were simultaneously recorded in two
conditions: 1) after equilibration of lung gas with the inert gases at
tracer concentrations [airway loading (AL)] and 2) during steady-state
elimination of the inert gases continuously introduced into venous blood by
a membrane oxygenator and partial arteriovenous bypass [venous loading
(VL)]. VD was consistently larger for SF6 than for He, but there was no
difference between AL and VL. The relative alveolar slope, defined as
increment of partial pressure per increment of expired volume and
normalized to mixed expired-inspired partial pressure difference, was
larger by a factor of two in VL than in AL for both He and SF6. The
He-to-SF6 ratio of relative alveolar slope was generally smaller than unity
in both VL and AL. Whereas unequal ventilation-volume distribution combined
with sequential emptying of parallel lung regions appears to be responsible
for the sloping alveolar plateau during AL, the steeper slope during VL is
attributed to the combined effects of continuing gas exchange and
ventilation-perfusion inequality coupled with sequential emptying. The
differences between He and SF6 point at the contributing role of
diffusion-dependent mechanisms in intrapulmonary gas mixing.
