Page 665 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

652 SPECIAL THERAPY

tissues, and the potential for self-regulation of Artificial colloids are polydisperse; that is, they contain
transvascular fluid fluxes often is underestimated. When molecules of different molecular weight. In contrast, in a
considering fluid therapy with macromolecular volume monodisperse colloid such as albumin, molecules are all
expanders, a great deal of emphasis has been placed on the same size. The artificial colloids have extremely com-
the manipulation of individual Starling forces (such as plex pharmacokinetics in part because of this large range
intravascular COP) in isolation rather than addressing of molecular sizes. 84 The smaller molecules pass rapidly
the system in its entirety. Maintenance of intravascular into the urine and interstitium, whereas the larger
volume depends on an intricate and dynamic interaction molecules remain in circulation and gradually are
between the intravascular and interstitial Starling forces hydrolyzed by amylase or removed by the monocyte
and the structure and function of the microvascular bar- phagocytic system. 161 This initial rapid excretion of small,
rier, interstitium, and lymphatic system. Infusion of intra- osmotically active molecules followed by gradual elimina-
venous fluids can change all of the Starling forces, modify tion of large molecules results in an exponential decline in
the permeability of the microvascular barrier, change the intravascular expansion. Manufacturer data sheets can be
volume and composition of the interstitium, and increase misleading because they may imply that a major propor-
lymphatic flow. Furthermore, the magnitude and relative tion of the volume expansion lasts for 24 to 36 hours.
significance of these changes vary among and within Estimates of the degree of initial plasma volume expan-
tissues. Consequently, it is a gross and potentially danger- sion for hetastarch and dextran 70 vary from 70% to
ous oversimplification to view the body as the homoge- 170% of the infused volume. 67,77,87,91,124 This decreases
nous sum of its individual parts when contemplating to approximately 50% of the infused volume after 6 hours.
intravenous fluid therapy. From a clinical standpoint, Volume expansion with hydroxyethyl starch then declines
the differences between the lungs and the systemic circu- gradually from 60% to 40% of the infused volume during
lation are of the utmost importance. For example, in a the next 12 to 18 hours, whereas with dextran 70 it
dog with systemic inflammatory response syndrome decreases gradually from 40% to 20% of the infused vol-
and aspiration pneumonia causing pulmonary edema by ume. 161 In experimental dogs, blood volume was
means of increased microvascular permeability, colloid increased by approximately 25% both immediately and
therapy may be effective in limiting subcutaneous edema 4 hours following infusion of 20mL/kg of both dextran
at the expense of worsening pulmonary fluid 70 and hetastarch. 147 In dogs with hypoalbuminemia of
extravasation. various causes receiving hydroxyethyl starch, COP was
Despite this great heterogeneity, the concept that net not significantly different from baseline 12 hours after
fluid extravasation depends on the balance between intra- infusion. 106 In the authors’ experience, the duration of
vascular COP and capillary hydrostatic pressure forms the volume expansion with artificial colloids can be even
basis for intravenous colloid therapy. 64,73,90,174 By virtue shorter, especially with capillary leak syndromes. This rel-
of their larger molecular size, and in the absence of an atively short duration of action and the high cost of arti-
increase in microvascular permeability, colloid molecules ficial colloids have led some authors to question the cost-
are retained within the vasculature to a greater degree effectiveness of colloid infusions in veterinary patients. 173
than are crystalloids. Consequently, smaller volumes of The duration of action of colloids may be expressed in
colloid result in greater plasma volume expansion com- terms of plasma colloid concentrations, plasma COP
pared with crystalloid, 51,144,145 and crystalloid is measurements, or degree of volume expansion. The initial
expected to leak into the interstitium to a greater degree volume of intravascular expansion is the result of the COP
than colloid and cause more interstitial expansion or of the infused colloid, which is determined by the number
edema. 27 This may be beneficial if the animal has an inter- of molecules, not their size. This concept is extremely
stitial fluid deficit or deleterious if there is interstitial important because the distribution of molecular weights
57,58
edema. One hour after infusion of a crystalloid solution, is narrowed after intravenous infusion. The smaller
as little as 10% of the infused volume may remain in the molecules that are responsible for a large part of the
intravascular space. 145 Some evidence indicates that tissue COP and intravascular volume expansion are extravasated
perfusion is better after volume expansion with colloids orexcretedwithinhours.Theintravascularcolloidconcen-
than with crystalloids, even when resuscitation is titrated tration (i.e., mass per unit volume) is still high due to the
to physiologic endpoints. 63 Unfortunately larger colloids large molecules, but the COP is relatively low. COP and
may reduce tissue perfusion by increasing plasma viscos- degree of volume expansion tend to decrease faster than
ity. 22 Many factors influence the volume and duration of does the plasma concentration of colloid. Data from an
intravascular expansion associated with artificial colloids, experimental study of euvolemic human volunteers given
including the species of animal, dose, specific colloid for- twice the usual dose of a high molecular weight form of
mulation, preinfusion intravascular volume status, and hydroxyethyl starch may therefore have little bearing on
the microvascular permeability. These factors likely the effects of commercially available hydroxyethylstarch
explain the great variability in intravascular persistence in a dog with systemic inflammatory response syndrome
and volume expansion in published studies. in hypodynamic, septic shock.