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 solutions should be used in routine fluid therapy.
At the 18th Annual Veterinary Medical Forum of the American College of Veterinary Internal Medicine, Dr. Karol A. Mathews of Guelph, Ontario, Canada presented a pertinent review on colloids and their use in small animal practice. 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 examples of colloid solutions. Some synthetic colloid solutions, such as dextrans, hydroxyethyl starches, pentastarch and oxypolygelatin, contain large particles that do not readily leave the vascular space.
These particles act to hold, and in some instances draw, water into the vascular space, and, thereby, expands the vascular volume. They exert a colloidal osmotic pressure in a similar manner as does plasma proteins.
Colloid solutions are usually selected for those dogs and cats requiring intravascular volume expansion to increase perfusion pressure and oxygen delivery on an emergent basis or to maintain adequate oncotic and perfusion pressure where crystalloid solutions alone are inadequate. Examples where colloid solutions may be of benefit and superior to crystalloid solutions alone 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
Colloid solutions may be used in combination with hypertonic saline solution to increase the duration of effect of hypertonic saline solution and reduce the volume of crystalloid solution required to attain and maintain adequate systemic arterial blood pressure and tissue perfusion in a hypovolemic crisis.
Where active hemorrhage is occurring, bleeding should be controlled before optimizing systemic blood pressure. The use of colloid solutions alone, or in combination with crystalloid solutions, can be tailored to the individual animal to optimize outcome.
The infusion of the various colloid products will depend on the animal's history (acute or chronic illness, trauma, inflammation) physical examination findings (hydration and mental status, third space losses, blood loss, underlying disease), serum chemistry profile results (serum albumin and total protein levels, renal and liver function, electrolytes) and PCV (anemia).
Albumin accounts for 80 percent of plasma colloidal osmotic pressure 40 percent of extracellular fluid albumin is found in the intravascular space and interstitial sites contain the remaining 60 percent.
Albumin synthesis in the liver is increased subsequent to losses, provided nutrient supply to the liver is sufficient. The permeability of endothelium is increased during hypoalbuminemic states. When serum albumin levels normalize, the size of the microvascular pores between the endothelial cells return to normal reducing endothelial permeability and edema.
Fresh or fresh frozen plasma contains many substances in addition to albumin, including fibronectin, alpha-macroglobulins, antithrombin III, anti-trypsin and coagulation factors; all important when treating animals with sepsis of varying causes, DIC, or congenital or acquired coagulation disorders.
Stored, frozen plasma may be of benefit in hypoproteinemic states during volume resuscitation in shock, for rodenticide-induced coagulopathy, after trauma or in a perioperative period to increase intravascular volume without decreasing oncotic pressure further and to provide transport proteins for administered medications.
Frozen plasma is ineffective in treatment of chronic hypoproteinemic states as intravenously administered plasma redistributes to the interstitial space. The intravascular half-life of transfused plasma (for its albumin component) is about 16 hours.
The recommended volume of plasma to be administered is approximately 20-30 ml/kg/day, which can be administered continuously over 24 hours, as a two- to four-hour infusion (or faster if needed), or in 6-10 ml/kg aliquots and given as one-hour infusions every eight hours.
Plasma transfusions are recommended when serum albumin levels drop below 2.0 g/dl and the animal is deteriorating. If the animal is improving, values lower than 2.0 g/dl are well tolerated and increase with time. Approximately 22.5 ml/kg plasma is required to raise the recipient's albumin level to 5.0 g/dl. Potential side effects of plasma infusions are hypocalcemia with large volume transfusions, reactions to plasma proteins and infection from contaminated products.
The time taken to thaw frozen plasma makes its use as a first line colloid solution inappropriate in the shocky animal. Calcium-containing fluids, such as lactated Ringer's solution, should not be given through the same intravenous line as plasma as the calcium will precipitate the citrate in the anticoagulant.
Human serum albumin
Human serum albumin solution (Plasbumin-25, Bayer Corp.) is an alternative to species-specific plasma. Human serum albumin has been rarely used in veterinary medicine but is now being used with increased frequency.
Because of its high oncotic load, this product will result in an increase in the intravascular volume of almost five times the volume administered within 30-60 minutes; therefore, continuous monitoring is required to prevent potential volume overload and pulmonary edema.
It can be administered at 2 ml/kg over four hours, or faster in emergent hypotensive states refractory to other colloid solutions and continued as a 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 those dogs with circulatory overload (congestive heart failure, renal insufficiency or stabilized chronic anemia).
Starch-based colloid solutions
The commercially available starch-based colloid solutions are mixtures of glucose polymers of various molecular weights - average molecular weight of hetastarch is 450,000 daltons and pentastarch is 260,000 daltons. Of importance with starch-based colloid solutions is the extent and duration of 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 and 5% dextrose (dextran 70 only) solution with an osmolarity ranging from 310-326 mOsm/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 electrolyte solution at neutral pH, has recently been approved for use in companion animals. It has average molecular weight of 30,000 daltons, and is excreted through the kidneys with less than 10 percent metabolized and has no tissue accumulation.
These colloid solutions can be administered as a slow push in emergent situations, over one to several hours to treat edematous animals, or as a 24-hour constant rate infusion along with a low infusion rate of a crystalloid fluid for maintenance in animals that are hypoalbuminemic.
Post-op, critically ill
As a general rule, when using these colloid solutions for management of the postoperative or critically ill, hypoproteinemic animal, one-third of the calculated fluid volume requirement should be administered as colloid solution and the remaining two-thirds as crystalloid solution.
The extent of volume expansion and increase in oncotic pressure will vary with each colloid product.
In general, all colloid products are very effective volume expanders with more expansion observed with a higher rate of administration.
Because rapid volume expansion can be detrimental, caution should be used when administering starch-based and gelatin-based colloid products to animals with oliguric or anuric renal failure or congestive heart failure. All colloid solutions may reduce von-Willebrand factor activity and the activity of serum coagulation factors, especially factor VIII, to some degree, beyond that of dilutional effects alone. Most of the time this will not result in clinical bleeding problems.
The recommended dose for hetastarch, pentastarch, or dextrans is 20 ml/kg/day for dogs and 10 ml/kg/day for cats. A relative contraindication to large volumes of hydroxy-ethyl starch products might be an animal with a moderate coagulopathy where hemorrhage may be a concern after surgery or splenic and/or liver trauma. An absolute contraindication would be animals with a severe coagulopathy.
Anaphylaxis has been seen with the use of dextrans but not with hetastarch, pentastarch or oxypolygelatin product. While generally contraindicated in animals with congestive heart failure, pentastarch has been used in these animals with renal insufficiency. After congestion is resolved, infusions up to 5 ml/kg/day in cats and up to 10 ml/kg/day in dogs delivered over 24 hours have been administered to enhance renal perfusion and urine production. The concomitant reduction in the volume of crystalloid fluids decreased the formation of interstitial edema while improving renal function.
Excretion of dextrans is significantly reduced in renal compromised animals. Dextran-40 and, to a lesser extent, dextran-70 lowers blood viscosity which may improve microcirculatory blood flow and tissue oxygenation, possibly because of a hemodilution-related decrease in blood viscosity or to primary effects on the RBC and endothelial cell adhesiveness. However, dextran-70 may cause increased RBC sludging in the microcirculation in dehydrated animals. Dextran-40 may also cause renal failure by precipitating in the renal tubules in animals with severe dehydration or impaired renal perfusion.
Crystalloid solutions should be administered concurrently to reduce this effect. Dextrans reduce clotting factor levels by hemodilution, coating blood vessel walls and cellular elements and impair the elasticity and tensile strength of the fibrin clots.
Hemostatic problems may occur more readily in thrombcytopenic animals and animals with renal failure because of uremic platelet dysfunction. The recommended dose for dextrans in dogs is 10-20 ml/kg/day and 5-10 ml/kg/day in cats.
Oxypolygelatin is recommended for maintenance and circulatory equilibrium in the presence of actual, or incipient, circulatory disturbances. Because oxypolygelatin does not interfere with platelet function, this product may be 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 administration at a slower rate as needed with a total volume of 10-20 ml/kg/dose given in a 24-hour period.
The volume effect, when given as a bolus, lasts approximately four hours making 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 production achieved within three hours after infusion with oxypolygelatin.
However, oxypolygelatin administration to animals in renal failure due to other causes should be administered with caution. The urine specific gravity and protein concentration will be increased due to renal excretion of the gelatin protein. This product is available in 250 ml glass bottles and has a four-year shelf life.
What's your question? Send your pediatric/geriatric related questions to: Pediatric/Geriatric Protocol, DVM Newsmagazine, 7500 Old 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 is a diplomate of the American College of Veterinary Internal Medicine with specialities in small animal pediatrics. Formerly a professor in the School of Veterinary Medicine at Louisiana State University, Hoskins is also the author of clinical textbooks on pediatrics and geriatrics. He founded an Internet service called "Vet-Web.com", where pet owners can e-mail animal health related questions and he responds via e-mail. The Internet address is www.vet-web.com. He can be reached at (225) 751-9272.