Colloid solutions used to expand vascular volume

Q: Could you explain what colloid solutions are and how they should be used?

A: In today's practice, both crystalloid and colloid solutionsshould be used in routine fluid therapy.

At the 18th Annual Veterinary Medical Forum of the American Collegeof Veterinary Internal Medicine, Dr. Karol A. Mathews of Guelph, Ontario,Canada presented a pertinent review on colloids and their use in small animalpractice. The following is a modified summary of the well-presented review.

Whole blood, plasma, human serum albumin, dextrans, hydroxyethyl starches,pentastarch, oxypolygelatin and hemoglobin-based oxygen carriers are examplesof colloid solutions. Some synthetic colloid solutions, such as dextrans,hydroxyethyl starches, pentastarch and oxypolygelatin, contain large particlesthat do not readily leave the vascular space.

These particles act to hold, and in some instances draw, water into thevascular space, and, thereby, expands the vascular volume. They exert acolloidal osmotic pressure in a similar manner as does plasma proteins.

Colloid solutions are usually selected for those dogs and cats requiringintravascular volume expansion to increase perfusion pressure and oxygendelivery on an emergent basis or to maintain adequate oncotic and perfusionpressure where crystalloid solutions alone are inadequate. Examples wherecolloid solutions may be of benefit and superior to crystalloid solutionsalone are in those dogs and cats that are:

· Hypovolemic and hypoproteinemic.

· Traumatized and hypovolemic

· Hypovolemic with head trauma

· Hypotensive and in shock

· Severe sepsis

· Ascites and peripheral edema

· Hypotension during anesthesia

Combinations

Colloid solutions may be used in combination with hypertonic saline solutionto increase the duration of effect of hypertonic saline solution and reducethe volume of crystalloid solution required to attain and maintain adequatesystemic arterial blood pressure and tissue perfusion in a hypovolemic crisis.

Where active hemorrhage is occurring, bleeding should be controlled beforeoptimizing systemic blood pressure. The use of colloid solutions alone,or in combination with crystalloid solutions, can be tailored to the individualanimal to optimize outcome.

The infusion of the various colloid products will depend on the animal'shistory (acute or chronic illness, trauma, inflammation) physical examinationfindings (hydration and mental status, third space losses, blood loss, underlyingdisease), serum chemistry profile results (serum albumin and total proteinlevels, renal and liver function, electrolytes) and PCV (anemia).

Albumin

Albumin accounts for 80 percent of plasma colloidal osmotic pressure­ 40 percent of extracellular fluid albumin is found in the intravascularspace and interstitial sites contain the remaining 60 percent.

Albumin synthesis in the liver is increased subsequent to losses, providednutrient supply to the liver is sufficient. The permeability of endotheliumis increased during hypoalbuminemic states. When serum albumin levels normalize,the size of the microvascular pores between the endothelial cells returnto normal reducing endothelial permeability and edema.

Fresh or fresh frozen plasma contains many substances in addition toalbumin, including fibronectin, alpha-macroglobulins, antithrombin III,anti-trypsin and coagulation factors; all important when treating animalswith sepsis of varying causes, DIC, or congenital or acquired coagulationdisorders.

Hypoproteinemic states

Stored, frozen plasma may be of benefit in hypoproteinemic states duringvolume resuscitation in shock, for rodenticide-induced coagulopathy, aftertrauma or in a perioperative period to increase intravascular volume withoutdecreasing oncotic pressure further and to provide transport proteins foradministered medications.

Frozen plasma is ineffective in treatment of chronic hypoproteinemicstates as intravenously administered plasma redistributes to the interstitialspace. The intravascular half-life of transfused plasma (for its albumincomponent) is about 16 hours.

The recommended volume of plasma to be administered is approximately20-30 ml/kg/day, which can be administered continuously over 24 hours, asa two- to four-hour infusion (or faster if needed), or in 6-10 ml/kg aliquotsand given as one-hour infusions every eight hours.

Plasma transfusions

Plasma transfusions are recommended when serum albumin levels drop below2.0 g/dl and the animal is deteriorating. If the animal is improving, valueslower than 2.0 g/dl are well tolerated and increase with time. Approximately22.5 ml/kg plasma is required to raise the recipient's albumin level to5.0 g/dl. Potential side effects of plasma infusions are hypocalcemia withlarge volume transfusions, reactions to plasma proteins and infection fromcontaminated products.

The time taken to thaw frozen plasma makes its use as a first line colloidsolution inappropriate in the shocky animal. Calcium-containing fluids,such as lactated Ringer's solution, should not be given through the sameintravenous line as plasma as the calcium will precipitate the citrate inthe anticoagulant.

Human serum albumin

Human serum albumin solution (Plasbumin-25, Bayer Corp.) is an alternativeto species-specific plasma. Human serum albumin has been rarely used inveterinary medicine but is now being used with increased frequency.

Because of its high oncotic load, this product will result in an increasein the intravascular volume of almost five times the volume administeredwithin 30-60 minutes; therefore, continuous monitoring is required to preventpotential volume overload and pulmonary edema.

It can be administered at 2 ml/kg over four hours, or faster in emergenthypotensive states refractory to other colloid solutions and continued asa constant rate infusion at 0.1 ml/kg/h in severely hypoproteinemic states.Human serum albumin is beneficial in septic animals with an exposed abdomen.Repeat transfusion may result in anaphylaxis and is contraindicated in thosedogs with circulatory overload (congestive heart failure, renal insufficiencyor stabilized chronic anemia).

Starch-based colloid solutions

The commercially available starch-based colloid solutions are mixturesof glucose polymers of various molecular weights - average molecular weightof hetastarch is 450,000 daltons and pentastarch is 260,000 daltons. Ofimportance with starch-based colloid solutions is the extent and durationof increase in oncotic pressure and volume expansion. Until recently, 6%hetastarch (Hespan, B. Braun/McGaw), 10% pentastarch (Pentaspan, B. Braun/McGaw),and 6% dextran-70 (Macrodex, Medisan Pharmaceuticals) in 0.9% saline and5% dextrose (dextran 70 only) solution with an osmolarity ranging from 310-326mOsm/L were the most common synthetic colloids used in veterinary practice.

A commercially available bovine bone gelatin-based colloid solution,oxypolygelatin (RapidVet Plasm-ex, DMS Laboratories) in a balanced electrolytesolution at neutral pH, has recently been approved for use in companionanimals. It has average molecular weight of 30,000 daltons, and is excretedthrough the kidneys with less than 10 percent metabolized and has no tissueaccumulation.

These colloid solutions can be administered as a slow push in emergentsituations, over one to several hours to treat edematous animals, or asa 24-hour constant rate infusion along with a low infusion rate of a crystalloidfluid for maintenance in animals that are hypoalbuminemic.

Post-op, critically ill

As a general rule, when using these colloid solutions for managementof the postoperative or critically ill, hypoproteinemic animal, one-thirdof the calculated fluid volume requirement should be administered as colloidsolution and the remaining two-thirds as crystalloid solution.

The extent of volume expansion and increase in oncotic pressure willvary with each colloid product.

In general, all colloid products are very effective volume expanderswith more expansion observed with a higher rate of administration.

Because rapid volume expansion can be detrimental, caution should beused when administering starch-based and gelatin-based colloid productsto animals with oliguric or anuric renal failure or congestive heart failure.All colloid solutions may reduce von-Willebrand factor activity and theactivity of serum coagulation factors, especially factor VIII, to some degree,beyond that of dilutional effects alone. Most of the time this will notresult in clinical bleeding problems.

The recommended dose for hetastarch, pentastarch, or dextrans is 20 ml/kg/dayfor dogs and 10 ml/kg/day for cats. A relative contraindication to largevolumes of hydroxy-ethyl starch products might be an animal with a moderatecoagulopathy where hemorrhage may be a concern after surgery or splenicand/or liver trauma. An absolute contraindication would be animals witha severe coagulopathy.

Anaphylaxis has been seen with the use of dextrans but not with hetastarch,pentastarch or oxypolygelatin product. While generally contraindicated inanimals with congestive heart failure, pentastarch has been used in theseanimals with renal insufficiency. After congestion is resolved, infusionsup to 5 ml/kg/day in cats and up to 10 ml/kg/day in dogs delivered over24 hours have been administered to enhance renal perfusion and urine production.The concomitant reduction in the volume of crystalloid fluids decreasedthe formation of interstitial edema while improving renal function.

Dextran excretion

Excretion of dextrans is significantly reduced in renal compromised animals.Dextran-40 and, to a lesser extent, dextran-70 lowers blood viscosity whichmay improve microcirculatory blood flow and tissue oxygenation, possiblybecause of a hemodilution-related decrease in blood viscosity or to primaryeffects on the RBC and endothelial cell adhesiveness. However, dextran-70may cause increased RBC sludging in the microcirculation in dehydrated animals.Dextran-40 may also cause renal failure by precipitating in the renal tubulesin animals with severe dehydration or impaired renal perfusion.

Crystalloid solutions should be administered concurrently to reduce thiseffect. Dextrans reduce clotting factor levels by hemodilution, coatingblood vessel walls and cellular elements and impair the elasticity and tensilestrength of the fibrin clots.

Hemostatic problems may occur more readily in thrombcytopenic animalsand animals with renal failure because of uremic platelet dysfunction. Therecommended dose for dextrans in dogs is 10-20 ml/kg/day and 5-10 ml/kg/dayin cats.

Oxypolygelatin is recommended for maintenance and circulatory equilibriumin the presence of actual, or incipient, circulatory disturbances. Becauseoxypolygelatin does not interfere with platelet function, this product maybe preferred over hetastarch or pentastarch in animals with hemorrhage,or at increased risk for hemorrhage following trauma or surgery.

This product can be used at 3-5 ml/kg over 15 minutes in emergent situations,in combination with crystalloid solutions, with continued administrationat a slower rate as needed with a total volume of 10-20 ml/kg/dose givenin a 24-hour period.

The volume effect, when given as a bolus, lasts approximately four hoursmaking it an ideal solution for short-term hypovolemic, hypotensive shock.This product may be used for several days without noticeable side effects.Anuria in dogs due to shock may be reversed with normal urine productionachieved within three hours after infusion with oxypolygelatin.

However, oxypolygelatin administration to animals in renal failure dueto other causes should be administered with caution. The urine specificgravity and protein concentration will be increased due to renal excretionof the gelatin protein. This product is available in 250 ml glass bottlesand has a four-year shelf life.

What's your question? Send your pediatric/geriatric relatedquestions to: Pediatric/Geriatric Protocol, DVM Newsmagazine, 7500Old Oak Blvd., Cleveland, OH 44130. Your questions will be answered by Dr.Hoskins in upcoming columns.

Dr. Hoskins is owner of DocuTech Services in Baton Rouge, La. He isa diplomate of the American College of Veterinary Internal Medicine withspecialities in small animal pediatrics. Formerly a professor in the Schoolof Veterinary Medicine at Louisiana State University, Hoskins is also theauthor of clinical textbooks on pediatrics and geriatrics. He founded anInternet service called "Vet-Web.com", where pet owners can e-mailanimal health related questions and he responds via e-mail. The Internetaddress is www.vet-web.com. He can be reached at (225) 751-9272.