Ka. Overholser et al., DEDUCTION OF PULMONARY MICROVASCULAR HEMATOCRIT FROM INDICATOR DILUTION CURVES, Bulletin of mathematical biology, 56(2), 1994, pp. 225-247
We have developed a new model describing the relationship between plas
ma and red cell tracers flowing through the lung. The model is the res
ult of an analysis of the transport of radiolabeled plasma albumin bet
ween two flowing phases and shows that differences between red cell an
d plasma tracer curves are related to microvascular hematocrit. The mo
del was tested in an isolated, blood-perfused dog lung preparation in
which we injected Cr-51-labeled red cells and I-125-labeled plasma alb
umin into the pulmonary artery. From the tracer concentration time cur
ves at the venous outflow, we calculated h(r), the ratio of microvascu
lar hematocrit to large-vessel hematocrit. In 18 baseline experiments,
h(r) = 0.92 +/- 0.01 (mn +/- sem) at a blood flow rate of 10.7 +/- 0.
3 ml s-1. We determined the effects of (a) glass bead embolization, (b
) alloxan, and (c) lobe ligation on h(r). Embolization attenuated the
separation between plasma and red cells (increased h(r)), probably as
a consequence of passive vasodilation. Alloxan enhanced separation of
plasma and red cells (decreased h(r)), possibly as a result of arterio
lar vasoconstriction. Ligation of a fraction of the perfused tissue at
constant flow did not cause significant change in hr in the remaining
perfused tissue. The model assumes that large-vessel transit times ar
e uniform and that all dispersion occurs in the microvasculature. A th
eoretical analysis apportioning dispersion between large and small ves
sels disclosed that the error associated with these assumptions is lik
ely to be less than 15% of the measured h(r). We conclude from this st
udy that the microvascular hematocrit model describes experimental pla
sma and red cell curves. The results imply that h(r) can be readily de
duced from tagged red cells and plasma and can be accounted for in cal
culating permeability-surface area in diffusing tracer experiments.