Journal of Applied Physiology, Vol 55, Issue 1 32-36, Copyright © 1983 by American Physiological Society
Graphical analysis of multiple inert gas elimination data
W. E. Stewart and S. M. Mastenbrook Jr
A plot of measured retention-excretion ratios [(Ri/Ei)obs] vs. reciprocal
solubility (1/lambda i) for selected inert gases allows quick detection of
shunt and ventilation-perfusion (V/Q) inhomogeneity in the lung. We derive
simple rules for constructing a smooth R/E function from the data, using a
multicompartmental model of the lung. If mixed venous inert gas
measurements are available, the values [lambda i(1-Ri)/Ei]obs for the
infused gases can be used to estimate the overall VT/QT ratio and provide
an additional test of the consistency of the data. For any set of
equilibrium compartments ventilated and perfused in parallel, we show that
d(R/E)/d(1/lambda) cannot be negative, nor can d2(R/E)/d(1/lambda)2 be
greater than zero. A rectilinear R/E function implies a narrow distribution
of V/Q among the gas exchange compartments, whereas a downward-concave
curve implies a broader distribution. The shunt perfusion and dead-space
ventilation can be estimated from the asymptotes of the R/E function. The
range of V/Q for the gas exchange compartments can also be bracketed if a
well-defined region of curvature is present in the graph. Finally, from the
R/E vs. 1/lambda graph and (if mixed venous data are available) from the
lambda(1-R)/E values, we can determine quickly whether the data deserve the
detailed numerical analysis outlined in our companion paper.
