Page 662 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Fluid Therapy with Macromolecular Plasma Volume Expanders 649

Further research showed that fluid flow from vessels solute concentration gradient across the membrane (DC).
differed among tissues depending on the surface area of Therefore the expression representing macromolecular
the capillary beds in the organ and the hydraulic conduc- flux becomes:
tance (i.e., the ease of fluid flow) through the microvascu-
lar barrier. To account for this variability, the fluid flux J s ¼ J v ð1 sÞ C þ PSDC
equationismodifiedbythefiltrationcoefficient(K fc ).This
term simply implies that fluid flow is equal to a fraction of Solute flow ¼ convective flow þ Diffusion
the effective hydrostatic and osmotic pressure gradients.
At normal lymph flow rates, convection has been
J v ¼ K fc ½ðP c P i Þ sðp p p i ފ estimated to account for approximately 30% of the total
flux of albumin into lymph. 123 An important point that
Each different constituent of plasma may differ in its rate of warrants further emphasis is that the rate of solute efflux
efflux from a vessel depending on such factors as its molec- is dependent on the rate of solvent efflux. Any condition
ular radius, shape, and charge, and the permeability of the that increases the rate of fluid flow across a membrane can
microvascular barrier to the constituent in question. The increase the extravasation of macromolecules. Hence,
twomajorgroupsofmoleculeswithrespecttotransvascular intravenous fluid therapy with crystalloid or colloid can
124
fluid flux are termed the solvent phase and the solute phase, increase albumin loss into the interstitium.
andexpressionsweredevelopedtopredicttheegressofboth These mathematical expressions give the impression of
major groups of molecules from the microvascula- a constant hydrostatic pressure gradient acting across a
ture. 83,92,110,115 Thesolventphaseincludeswaterandthose single membrane of static and uniform conductivity
moleculesthatarenotsignificantlyimpededintheirpassage and permeability (homoporous), with filtration opposed
through the microvascular barrier, whereas the solute flux by an osmotic pressure resulting from a single
equationdescribesthepassageofmoleculesthatdonotflow impermeant solute, the plasma “protein.” In fact, the
freely from the vasculature. hydrostatic pressure and osmotic pressure gradients vary
The solvent flow equation remains the same as the pre- among different tissues and at different levels of the cap-
vious expression of fluid flow except that the filtration illary bed within the same tissue. 121,156,159 In disease
coefficient is subdivided into the hydraulic conductance states, the differences among organs may be significant
(L p ) and the membrane surface area (S), and the hydro- and the clinician must consider the possibility of individ-
static and osmotic gradients are expressed as DP and ual organ edema (e.g., pulmonary, myocardial, or intesti-
Dp, respectively: nal edema) even if there are no overt signs of a systemic
edematous state. The total osmotic gradient is a summa-
J v ¼ L p SðDP sDpÞ tion of all the impermeant solutes present within plasma,
which all have unique reflection coefficients and efflux
The two major mechanisms of solute flow through the rates. 156 Furthermore, the surface area of the capillary
microvascular barrier are convection (i.e., carriage in a bed may change depending on precapillary sphincter
bulk flow of fluid) and diffusion (i.e., random motion activity and the permeability of the microvascular barrier
resulting in net movement of molecules from an area of and interstitium may also vary physiologically and in
high concentration to an area of lower concentration). 127 disease states. 8,71,113,180,181
An analogy to illustrate the two mechanisms would be a NORMAL STARLING FORCES
wave breaking on a beach. Some of the sodium molecules
in the wave will be moving away from the beach by diffu- AND THE TISSUE SAFETY
sion; however, the forward convective flow of the wave FACTORS
carries them in the opposite direction.
The solute flow equation is the most relevant expres- PLASMA COLLOID OSMOTIC
sion with respect to intravenous therapy with fluids PRESSURE
containing macromolecules. It states that the rate of sol-
ute flux (J s ) is equal to the sum of the convective flow and Although in popular usage colloid is interpreted most
the diffusional movement. often as referring to a macromolecule that cannot pass
through a membrane, the strict definition refers to the
Solute flowðJ s Þ¼ convective flow þ diffusion dispersion in a gas, liquid, or solid medium of atoms or
molecules that resist sedimentation, diffusion, and filtra-
Convective flow is equal to the product of fluid flow (J v ), tion. This definition is in contradistinction to crystalloids,
the fractional permeability of the membrane (1 s ), and which are freely diffusible. Oncotic pressure is defined as
the mean intramembrane solute concentration,C. Diffu- the osmotic pressure exerted by colloids in solution
sion is equal to the product of the solute permeability (P), (hence it is redundant to use the phrase colloid oncotic
the surface area of the microvascular barrier (S), and the pressure). Proteins in plasma are truly in solution, but