Journal of Applied Physiology, Vol 65, Issue 5 2132-2137, Copyright © 1988 by American Physiological Society
Lobar contribution to VA/Q inequality during constant-flow ventilation
P. T. Schumacker, J. Solway, L. D. Wood and J. I. Sznajder
Section of Pulmonary and Critical Care Medicine, University of Chicago, Illinois 60637.
Previous studies have shown that normal arterial PCO2 can be maintained
during apnea in anesthetized dogs by delivering a continuous stream of
inspired ventilation through cannulas aimed down the main stem bronchi,
although this constant-flow ventilation (CFV) was also associated with a
significant increase in ventilation-perfusion (VA/Q) inequality, compared
with conventional mechanical ventilation (IPPV). Conceivably, this VA/Q
inequality might result from differences in VA/Q ratios among lobes caused
by nonuniform distribution of ventilation, even though individual lobes are
relatively homogeneous. Alternatively, the VA/Q inequality may occur at a
lobar level if those factors causing the VA/Q mismatch also existed within
lobes. We compared the efficiency of gas exchange simultaneously in whole
lung and left lower lobe by use of the multiple inert gas elimination
technique in nine anesthetized open-chest dogs. Measurements of whole lung
and left lower lobe gas exchange allowed comparison of the degree of VA/Q
inequality within vs. among lobes. During IPPV with positive end-expiratory
pressure, arterial PO2 and PCO2 (183 +/- 41 and 34.3 +/- 3.1 Torr,
respectively) were similar to lobar venous PO2 and PCO2 (172 +/- 64 and
35.7 +/- 4.1 Torr, respectively; inspired O2 fraction = 0.44 +/- 0.02).
Switching to CFV (3 l.kg-1.min-1) decreased arterial PO2 (112 +/- 26 Torr,
P less than 0.001) and lobar venous PO2 (120 +/- 27 Torr, P less than 0.01)
but did not change the shunt measured with inert gases (P greater than
0.5).(ABSTRACT TRUNCATED AT 250 WORDS)
