62° Congresso Internazionale Multisala SCIVAC
The good and bad characteristics of intravenous colloid fluids Steve Haskins DVM, MS, Dipl ACVA, Dipl ACVECC, California, USA
Colloid solutions contain large molecules (many thousand Daltons) that do not readily cross the normal vascular membrane. These solutions are largely retained within the vascular fluid compartment and therefore these fluids are more efficient blood volume expanders, and are less edemagenic, compared to crystalloids. Dextrans are mixtures of straight chain polysaccharides produced by the bacteria, Leuconostoc mesenteroides or lactobacilli grown on sucrose media. Different molecular weights are produced by acid hydrolysis of macromolecules. Dextrans not filtered by the glomerulus are metabolized by dextrinases in various tissues. Dextran-70 (many commercial sources) is the most commonly used example. Hetastarch and Pentastarch (Abbott®) are modified branched-chain glucose polymers produced by hydrolysis of the highly branched starch amylopectin. Starches are metabolized by plasma and interstitial alpha-amylases, and plasma amylase may be elevated following their administration. Colloid solutions contain many different solute sizes. Molecular weights below about 50,000 Daltons are rapidly excreted in the urine. This causes a transient osmotic diuresis and can increase urine specific gravity (without a proportionate increase in urine osmolality). Larger molecules are slowly metabolized and tend to accumulate in the body with daily administration of the colloid. Commercial colloidal solutions are iso-osmotic (they are suspended in saline or another ECFlike crystalloid) and are hyperoncotic in the bottle. Artificial colloids are commonly administered to increase circulating blood volume and to support colloid osmotic pressure in hypoproteinemic patients. Loading dosages of colloid generally range from 5 to 30 ml/kg in 5 ml/kg aliquots (with appropriate monitoring of cardiovascular endpoints). For cats, the range is from 2.5 to 15 ml/kg in 2.5 ml/kg aliquots. Continuous rate infusions of colloids of 1 to 2 ml/kg/hr would be approximately equivalent to a crystalloid infusion of 5-10 ml/kg/hr, and could be used for intraoperative support of circulating blood volume. The same dosage rate has been used for support of colloid osmotic pressure in hypoproteinemic patients. In addition to worsening of congestive heart failure, oliguric/anuric edema, rebleeding, and hemodilution, the artificial colloids produce a dose-related defect of primary hemostasis, which is somewhat greater than that due to hemodilution alone (Concannon KT, 1992; Cope JT, 1997; Gollub S, 1967; Wierenga J, 2007). Prolongation of aPTT is attributed to a reduction of VIII:C activity (Gollu S, 1967; Aberg M, 1979). Prolonged bleeding time and decreased platelet adhesiveness is attributed to inhibition of the vWf:ag (Aberg, 1979). This effect on coagulation
is more pronounced when colloids are administered in larger dosages or for longer periods of time and appear to be proportional to the molecular weight of the circulating macromolecules (Trieb J, 1997). It is not expected that even large doses would induce bleeding in normal patients. However, some coagulopathic patients and some postsurgical patients may develop hemorrhagic problems (Cope JT, 1997). The coagulation effects of artificial colloids may be beneficial in the hypercoagulable phase of disseminated intravascular coagulation where an objective of therapy is to slow activated coagulation and platelet cascades. Molecular sizes below 50,000 are rapidly excreted in the urine and this will cause a transient osmotic diuresis. The filtered colloidal molecules can be concentrated resulting in an increase in urine viscosity and specific gravity. Dextran 40 has a larger initial impact on plasma volume but a shorster duration of action compared to Dextran-70. Due to its smaller average molecular size is more rapidly filtered by the glomerulus and excreted. In states of active tubular reabsorption (dehydration), the dextran molecules concentrate in the tubular lumen, increase filtrate viscosity and predispose to acute renal failure (Ferraboli R, 1997). Urine specific gravity is often used as a surrogate marker of urine concentration and when increased, it is usually taken as a sign of dehydration. Urine specific gravity must be interpreted with caution following or during artificial colloid administration; urine osmolality should be used instead as the marker of urinary concentration. Colloids may interfere with cross-matching procedures by causing red cell clumping (Daniels MJ, 1982). Allergic reactions are extremely rare, but are possible.
References Aberg M, Hedner U, Bergentz SE, Effect of dextran on factor VIII (antihemophilic factor) and platelet function, Ann Surg 1979; 189:243-247. Concannon KT, Haskins SC, Feldman BF, Hemostatic defects associated with two infusion rates of dextran 70 in dogs, Am J Vet Res 1992; 53:1369-1375. Cope JT, Banks D, Mauney MC, et al, Intraoperative hetastarch infusion impairs hemostasis after cardiac operations, Ann Thor Surg 1997; 63:78-83. Daniels MJ, Strausss RG, Smith-Floss AM, Effects of hydroxyethyl starch on erythrocyte typing and blood cross matching, Transfusion 1982; 22:226-228. Ferraboli R, Malheiro PS, Abdulkader RC. et al, Anuric acute renal failure caused by dextran 40 administration, Ren Fail 1997; 19:303-306. Gollub S, Schaefer C, Squitieri A, The bleeding tendency associated with plasma expanders, Surg Gynec Obst 1967; 124:1203-1211. Treib J, Hass A, Pindur G, Coagulation disorders caused by hydroxyethyl starch, Throm Haemost 1997; 78:974-983. Wierenga J, Jandrey K, Haskins SC, Tablin, F, The effect of Hetastarch and Hextend on platelet function in vitro, Am J Vet Res in press, 2007.
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