Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

for the evaluation of acute fluid and electrolyte alterations.
If one assumes that there has been no gain or loss of erythrocytes
or protein from the vascular compartment, changes
in PCV and TPP reflect changes in plasma volume ( Boyd,
1981 ). For the estimation of alterations in plasma volume
in human subjects, the change in PCV or hemoglobin concentration
is preferred over change in protein concentration
(van Beaumont et al ., 1973 ). This preference exists because
fluxes of protein both into and out of the plasma volume
may occur in certain rapidly changing clinical circumstances
(Landis and Pappenheimer, 1963 ; Senay, 1970 ). There is a
substantial extravascular protein pool, and the volume distribution
of both albumin and globulins is approximately
two to three times that of the plasma volume ( Landis and
Pappenheimer, 1963 ; Matteeuws et al ., 1966 ). This situation
is one of the reasons that plasma transfusions have limited
effects on the plasma protein concentration of hypoproteinemic
patients. In most animal species, there is a much wider
range of normal for PCV than for TPP. In some of these species,
most notably the horse, mobilization of erythrocytes
from splenic reserves in response to excitement, pain, or catecholamine
release can result in marked variability in PCV,
erythrocyte count, or hemoglobin concentration ( Carlson,
1987 ; Persson, 1967 ; Persson et al ., 1973 ). For these reasons,
it has proven most useful to utilize the change in both
PCV and TPP as a crude index of change in plasma volume
in domestic animals. For these estimations to be valid, initial
values for PCV and TPP must be known, and there must
have been no loss of blood or protein. The most common
causes for decreases in plasma volume are the sequestration
or loss of sodium-containing fluid as in obstructive bowel
disease, diarrhea, vomiting, renal disease, or in heavily
sweating horses. In these circumstances, both PCV and TPP
increase in relation to the change in plasma volume, but not
necessarily by the same proportion ( Boyd, 1981 ).
If it is assumed that the quantity of plasma proteins
within the plasma volume remains constant (this assumption
is not always correct), it is possible to calculate the
percentage change in plasma volume based on protein concentration
( Boyd, 1981 ):
%PV [(PP 1/PP 2) 1]
100 (17-13)