Colloids are more effective than crystalloids for correcting intravascular volume deficits and for improving systemic and
microcirculatory blood flow, even if the crystalloid is rapidly infused.14-17 Colloid solutions contain molecules, such as albumin, that produce a colloid osmotic (oncotic) pressure (COP) and, thus,
help retain fluid within the blood vessel. Colloidal solutions can be either natural (e.g. albumin) or synthetic (e.g. dextran, hydroxyethyl starch) and are frequently categorized based on their molecular weight.
The benefits of COP
Compared with crystalloids, colloids are excellent replacement fluids because of their ability to produce COP.14,18,19 The capacity of a colloid to expand the blood volume, although variable, is directly related to its ability to increase
COP (Table 2).19,20 The COP is related to the number of active particles available in solution.
Table 2: Hydroxyethyl starch solutions
Increases in plasma COP help retain fluid in the vascular compartment and, under the right circumstances, promote fluid movement
from the interstitial fluid compartment into the vascular fluid compartment (autotransfusion).
Colloids produce a greater and more sustained increase in vascular volume than crystalloids do. Notably, 70% to 80% of a crystalloid
solution (COP = 0 mm Hg) is distributed into the interstitial fluid compartment within 15 to 30 minutes of its administration,
while colloidal solutions produce effects that are variably sustained for hours after their administration. The colloid effects
depend on their colligative and chemical properties and the integrity of the vessel wall.8,21
A look at synthetic colloids
The synthetic colloids hetastarch (e.g. Hespan—Abbott Animal Health; Hextend—Hospira, Inc.) and tetrastarch (e.g. VetStarch—Abbott Animal Health; Voluven—Hospira, Inc.) are hydroxyethyl starch solutions that vary in molecular weight, molar
substitution (number of hydroxyethyl residues per glucose subunits), and the pattern of substitution of hydroxyethyl residues
on glucose subunits (C2/C6 ratio).20 These properties determine the magnitude and duration of their volume-expanding effects (Table 2). Higher molecular weight, molar substitution, and C2/C6 ratios prolong plasma retention and the duration of volume-expanding
Hydroxyethyl starch solutions with lower molecular weight and molar substitution have shorter half-lives and produce fewer
side effects, yet retain clinically important volume-expanding efficacy because they contain more molecules per milliliter
(produce a greater COP) than a product with a high molecular weight.
Tetrastarch (4 hydroxyethyl residues per 10 glucose subunits; 4/10 = 0.4) in 0.9% sodium chloride has a mean molecular weight
of 130 kDa and molar substitution of 0.4 (130/0.4). It produces immediate and sustained increases in hemodynamics with comparatively
lower risk of fluid overload, coagulation derangements (factor VIII, platelet function), renal impairment, or tissue accumulation
than hetastrach solutions, provided that dosing guidelines are followed.20,22-28 Tissue oxygenation is improved29 by improving microcirculatory blood flow.14,17
Note that in contrast to other commercially available hydroxyethyl starch solutions, tetrastarch (VetStarch) is approved for
both the treatment and prevention of hypovolemia. Total daily doses of VetStarch should not exceed 50 ml/kg/day for most animals
and 15 ml/kg/day for animals with prolonged bleeding times (prothrombin time >13 sec; partial thromboplastin time > 24 sec),
reduced platelet numbers (< 50,000/ml), or elevated creatinine concentrations (> 1.8 mg/dl [157 mmol/L]).
Clinical experience after administration of tetrastarch (VetStarch) doses, ranging from 20 to 50 ml/kg/day, in critically
ill dogs suggests good to excellent volume replacement without discernable adverse effects (Table 3). Focused clinical trials determining efficacy and effects on coagulation and renal function in critically ill animals are
Table 3: Results of administering tetrastarch to 100 critically ill dogs*
Caution is always advised when administering a colloid in dehydrated animals or animals with evidence of coagulopathy, head
trauma, or impaired renal function (elevated BUN or creatinine concentrations).30-32 Adverse effects and toxicities caused by colloidal solutions are dose-dependent, and guidelines for dosing should be strictly
followed. Notably, human hospital trials suggest that tetrastarch (HES 130/0.4)-associated renal impairment is low and, compared
with other hydroxyethyl starches, is most likely to occur in animals who have failed to receive sufficient water or who remain
hypotensive or when recommended maximum daily doses are exceeded.31-33
Provided that arterial blood pressure remains within normal limits, isotonic balanced crystalloids solutions are adequate
therapy for perioperative treatment caused by insensible fluid loss (e.g. evaporation from open body cavities) and anesthesia-associated systemic vasodilatation (relative hypovolemia). Colloids produce
more immediate and sustained beneficial hemodynamic (macrocirculatory and microcirculatory) effects and are effective replacement
therapy for perioperative hypotension and hypovolemia (relative or absolute).
The packed cell volume should be monitored during fluid therapy and maintained at values greater than 20%. Current knowledge
and veterinary clinical guidelines for perioperative fluid management are primarily based on opinion, nonclinical animal research,
and human clinical trials. Human clinical trials recommend not to administer colloidal solutions in any subject with evidence
of head trauma or increased risk of acute kidney injury.
The author thanks Dava Cazzolli, DVM, DACVECC; Jaime Chandler, DVM, DACVECC; Michelle Albino, LVT, CVT (Anesth); and Yukie
Ueyama, DVM, for their assistance in abstracting data.
William W. Muir, DVM, MS, PhD, DACVA, DACVECC
338 W. 7th Ave.
Columbus, OH 43201
For this article's reference citations, go to http://dvm360.com/FluidTherapy2Refs.