Journal of Applied Physiology, Vol 68, Issue 1 88-93, Copyright © 1990 by American Physiological Society
Mechanisms of gas exchange with different gases during constant-flow ventilation
F. J. Chen, A. S. Menon, S. V. Lichtenstein, N. Zamel and A. S. Slutsky
Department of Medicine and Surgery, University of Toronto, Mount Sinai Hospital, Ontario, Canada.
To investigate the mechanisms responsible for the difference in gas
exchange during constant-flow ventilation (CFV) when using gases with
different physical properties, we used mixtures of 70% N2-30% O2 (N2-O2)
and 70% He-30% O2 (He-O2) as the insufflating gases in 12 dogs. All dogs
but one had higher arterial PCO2 (PaCO2) with He-O2 compared with N2-O2. At
a flow of 0.37 +/- 0.12 l/s, the mean PaCO2's with N2-O2 and He-O2 were
41.3 +/- 13.9 and 53.7 +/- 20.3 Torr, respectively (P less than 0.01); at a
flow rate of 0.84 +/- 0.17 l/s, the mean PaCO2's were 29.1 +/- 11.3 and
35.3 +/- 13.6 Torr, respectively (P less than 0.01). The chest was then
opened to alter the apposition between heart and the lungs, thereby
reducing the extent of cardiogenic oscillations by 58.4 +/- 18.4%. This
intervention did not significantly alter the difference in PaCO2 between
N2-O2 and He-O2 from that observed in the intact animals, although the
individual PaCO2 values for each gas mixture did increase. When the PaCO2
was plotted against stagnation pressure (rho V2), the difference in PaCO2
between N2-O2 and He-O2 was nearly abolished in both the closed- and
open-chest animals. These findings suggest that the different PaCO2's
obtained by insufflating gases with different physical properties at a
fixed flow rate, catheter position, and lung volume result mainly from a
difference in the properties of the jet.
