Page 660 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 27

Fluid Therapy with Macromolecular

Plasma Volume Expanders

Dez Hughes and Amanda Boag

“Those who fill our professional ranks are habitually conservative. This salutary mental attitude expresses itself
peculiarly in our communal relations; namely, when a new idea appears which is more or less subversive to old
notions and practices, he who originates the idea must strike sledge hammer blows in order to secure even a
momentary attention. This must then be followed by a long, patient, propaganda and advertising until in the
grand finale, the public, indifferent at first, is aroused, proceeds to discuss, and finally accepts the iconoclastic
proposal as a long-accepted fact of its own invention and asks wonderingly, ‘Why such a bother? What after all
is new about this? We knew it long ago!”
Howard A. Kelly, MD. Electrosurgery in gynaecology, Ann Surg 93:323, 1931.

In the late nineteenth century, Ernest Starling proposed for a more complete discussion of solute and solvent
the concept that the balance between hydrostatic and exchange among the microvasculature, interstitium,
osmotic pressure gradients between the intravascular and lymphatics. 5,127,154 The main aim of this chapter
and interstitial fluid compartments governed is to objectively address the complexities and
transvascular fluid exchange. 151 He postulated that a controversies of colloid therapy while avoiding the ten-
hydrostatic pressure gradient in excess of the osmotic gra- dency toward bias apparent in some articles dealing with
dient at the arterial end of the capillary bed results in a net the crystalloid-colloid controversy. A deeper appreciation
transudation of fluid into the interstitium. At the venous of the relevant issues should ensure a more rational
end of the capillary bed, plasma proteins (which do not approach when deciding whether colloid therapy is
normally pass out of the blood vessels) exert an osmotic appropriate. The present chapter is exhaustive in its deal-
force in excess of the hydrostatic gradient, resulting in a ing with some issues but not all-inclusive, and the reader
net fluid flux into vessels. More than a century of research also is referred to several reviews of colloid fluid therapy
has confirmed that Starling’s hypothesis provides the available in the veterinary 31,54,98,135 and human medical
foundation for microvascular fluid exchange but also literature. 55,56,103,129,177
has revealed that the anatomy and physiology of the
microvasculature, interstitium, and lymphatic system THE MICROVASCULAR
are much more complex. Consequently, a much deeper BARRIER
understanding of transvascular fluid dynamics is neces-
sary for a logical and rational approach to intravenous In simple terms, the healthy microvascular barrier is a cap-
therapy with fluids containing macromolecules. This illary wall that is relatively impermeable to protein.
chapter assumes the reader is familiar with the informa- In addition to the endothelial cell and the capillary base-
tion given in Chapter 1 explaining the fluid ment membrane, a luminal surface layer (the glycocalyx)
compartments of the body and the mechanisms of water and the interstitial matrix also contribute to the selective
and solute flow among compartments. Although this permeability of the microvascular barrier. 5,127,180 The
chapter discusses the anatomy, physiology, and biophysics glycocalyx coats the luminal aspect of the endothelial cell
of transvascular fluid dynamics in some depth, compre- and is composed of proteins, glycoproteins, and
hensive reviews and texts are available on the subject glycolipids that modify the permeability of the

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