COLLOIDS

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

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 effect.

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]).

Table 3: Results of administering tetrastarch to 100 critically ill dogs*

Precautions

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

CONCLUSION

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.

ACKNOWLEDGMENTS

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
VCPCS
338 W. 7th Ave.
Columbus, OH 43201

For this article's reference citations, go to http://dvm360.com/FluidTherapy2Refs.