Plasma Volume Expanders

Plasma Volume Expanders

Major blood loss entails the danger of
life-threatening circulatory failure, i.e.,
hypovolemic shock. The immediate
threat results not so much from the loss
of erythrocytes, i.e., oxygen carriers, as
from the reduction in volume of circulating
blood.
To eliminate the threat of shock, replenishment
of the circulation is essential.
With moderate loss of blood, administration
of a plasma volume expander
may be sufficient. Blood plasma
consists basically of water, electrolytes,
and plasma proteins. However, a plasma
substitute need not contain plasma
proteins. These can be suitably replaced
with macromolecules (“colloids”)
that, like plasma proteins, (1) do
not readily leave the circulation and are
poorly filtrable in the renal glomerulus;
and (2) bind water along with its solutes
due to their colloid osmotic properties. In
this manner, they will maintain circulatory
filling pressure for many hours. On
the other hand, volume substitution is
only transiently needed and therefore
complete elimination of these colloids
from the body is clearly desirable.
Compared with whole blood or
plasma, plasma substitutes offer several
advantages: they can be produced more
easily and at lower cost, have a longer
shelf life, and are free of pathogens such
as hepatitis B or C or AIDS viruses.
Three colloids are currently employed
as plasma volume expanders—
the two polysaccharides, dextran and
hydroxyethyl starch, as well as the polypeptide,
gelatin.
Dextran is a glucose polymer
formed by bacteria and linked by a 1!6
instead of the typical 1!4 bond. Commercial
solutions contain dextran of a
mean molecular weight of 70 kDa (dextran
70) or 40 kDa (lower-molecularweight
dextran, dextran 40). The chain
length of single molecules, however,
varies widely. Smaller dextran molecules
can be filtered at the glomerulus
and slowly excreted in urine; the larger
ones are eventually taken up and degraded
by cells of the reticuloendothelial
system. Apart from restoring blood
volume, dextran solutions are used for
hemodilution in the management of
blood flow disorders.
As for microcirculatory improvement,
it is occasionally emphasized that
low-molecular-weight dextran, unlike
dextran 70, may directly reduce the aggregability
of erythrocytes by altering
their surface properties. With prolonged
use, larger molecules will accumulate
due to the more rapid renal excretion
of the smaller ones. Consequently,
the molecular weight of dextran circulating
in blood will tend towards a
higher mean molecular weight with the
passage of time.
The most important adverse effect
results from the antigenicity of dextrans,
which may lead to an anaphylactic
reaction.
Hydroxyethyl starch (hetastarch) is
produced from starch. By virtue of its
hydroxyethyl groups, it is metabolized
more slowly and retained significantly
longer in blood than would be the case
with infused starch. Hydroxyethyl
starch resembles dextrans in terms of
its pharmacological properties and
therapeutic applications.
Gelatin colloids consist of crosslinked
peptide chains obtained from
collagen. They are employed for blood
replacement, but not for hemodilution,
in circulatory disturbances.