Journal of Applied Physiology, Vol 67, Issue 4 1489-1494, Copyright © 1989 by American Physiological Society
Alveolar pressure inhomogeneity and gas exchange during constant-flow ventilation in dogs
J. I. Sznajder, A. Nahum, G. Crawford, E. R. Pollak, P. T. Schumacker and L. D. Wood
Section of Pulmonary and Critical Care Medicine, Michael Reese Hospital and Medical Center, Chicago, Illinois.
Analysis of momentum transfer between inflow jets and resident gas during
constant-flow ventilation (CFV) predicts inhomogeneity of alveolar
pressures (PA) and volume, which might account for specific
ventilation-variance in the lung. Using alveolar needles to measure
pressures (PA) during CFV in eight anesthetized dogs with wide thoracotomy,
we observed random dispersion of PA among lobes of up to 12.5 cmH2O. Within
each lobe, the PA dispersion was up to 10 cmH2O at CFV of 90 l/min; when
flow decreased, PA at all sites decreased, as did the intralobar
dispersion. These pressure differences were not observed during
conventional mechanical ventilation (CMV). During CFV with room air, dogs
were hypoxemic [arterial PO2 (Pao2) 54 +/- 15 Torr] and the venous
admixture (Qva/QT) was 50 +/- 15%. When inspiratory O2 fraction was
increased to 0.4, Pao2 increased to 172 +/- 35 Torr and Qva/QT dropped to
13.5 +/- 8.4%, confirming considerable ventilation-perfusion (VA/Q)
variance not observed during CMV. We conclude that momentum transfer
between the inflow stream and resident gas caused inhomogeneities of
alveolar pressures, volumes, and ventilation responsible for VA/Q variance
and hypoxemia during CFV. Conceivably, the abnormal ventilation
distribution is minimized by collateral ventilation and forces of
interdependence between regions of high and low alveolar pressures.
Momentum transfer also predicted the mucosal damage observed on
histological evaluation of the bronchial walls near the site of inflow jet
impact.
