September 2021

In this VETgirl online veterinary continuing education blog, Amy Newfield, CVT, VTS (ECC) reviews fluid therapy in veterinary medicine. Does it matter what type of IV fluids you reach for? Should you reach for an isotonic fluid? What about a colloid? Does IV fluid therapy make a difference in patients regardless of what type of IV fluid bag you reach for? Read it below or tune into our VETgirl podcast HERE to listen to it!

**NOTE: In this VETgirl podcast, Amy mentioned that packed red blood cells (pRBC) are colloids. Please note this is an error, as pRBCs carry very little plasma proteins, and while considered a life-saving transfusion, are not considered in the “colloid” group of fluid therapy.

FLUID THERAPY: IT’S 2021. What’s available in veterinary medicine?

By Amy Newfield, MS, CVT, VTS (ECC)

Only 30 years ago, there were only about 3 types of fluids which most veterinarians had access to for all patient issues and ailments. Fluid types are more numerous and readily available for all practitioners. Let’s talk about what types of fluids are available in this high level fluid therapy 101 blog.

Crystalloids: Still #1
There are three categories of crystalloid fluids:
• Isotonic
• Hypotonic
• Hypertonic

Isotonic crystalloids (Lactated Ringers, Normosol-R, 0.9% NaCl, Plasmalyte-A) are similar to extracellular fluid because they contain similar electrolyte concentrations (sodium, chloride, potassium, magnesium, calcium and bicarbonate-like anions). Within 30 minutes 75-98% of the fluids shift into the extravascular space, so therefore the infusion must be continuous and you need large volumes in order to make a difference. Isotonic fluids are used to restore fluid deficits and to provide maintenance fluid requirements. They continue to be the most common type of fluid used.

Fluid Pump - Bag

Hypotonic fluids (5% Dextrose in Water, 0.45% NaCl) have a lower osmotic pressure compared to isotonic. They should not be used to treat shock because they contain too much water and will redistribute too quickly. Hypotonic fluids should be considered when a patient has a free water deficit, hypernatremia, or for a patient in congestive heart failure or liver failure. Congestive heart failure and hepatic failure are associated with increased sodium retention which is why an infusion of a hypotonic fluid is ideal.

Hypertonic fluids (7%-7.5% NaCl) have a higher osmotic pressure. Hypertonic fluids cause fluids to shift from the interstitial space into the intravascular space in order to improve venous return and cardiac output. It is a fluid of choice when dealing with head trauma or when utilizing the limited volume resuscitation volume fluid therapy technique. One dose equals 4xs the volume of isotonic saline so a much smaller volume is needed. If interstitial dehydration or hypernatremia secondary to a free water deficit are present, the use of hypertonic solutions is contraindicated.

Difference in Crystalloids:
Because of the vast amount of crystalloid choices it may get confusing as to what to use when. A good rule of thumb is to first look at serum sodium levels. If the pet has normal sodium you can consider using Lactated Ringer’s, Normosol-R® or Plasmalyte-A®. If the pet has a low sodium then you may want to consider 0.9% NaCl because it contains more sodium. If the patient has had a persistent decrease in sodium then you may want to consider LRS or Normosol (LRS has a lower concentration of sodium than Normosol). During chronic hyponatremia, the brain adapts to prevent cerebral edema. With rapid correction of serum sodium concentration, osmotic shifts and cerebral dehydration can occur.

The second item that should be looked at when deciding what crystalloid to use is the patient’s potassium level. Patients that have a normal or low potassium should likely be put on Lactated Ringer’s, Normosol-R® or Plasmalyte-A®. If the patient has a high potassium then 0.9% NaCl, which contains no potassium, should be considered. It is important to remember that the choice of what crystalloids to use it not an exact science.

Colloids are high molecular weight fluids that do not pass readily through the capillary membranes. They help to increase oncotic pressure because they keep fluids in the intravascular space. The particles draw sodium and water around to their core structure within the vascular space, thus contributing to the water holding property of a colloid. About 50-80% of the infused volume stays in the intravascular space. Colloids are most always be administered with a crystalloid fluid, to replenish both intravascular and interstitial fluid deficits. They are used to increase blood pressure and maintain colloid osmotic pressure.

There are two types of colloids:

• Synthetic
• Natural

Synthetic colloids: We have accessed to hydroxyethyl starches
There are three types of hydroxyethyl starches (HES)

• Hetastarch
• Pentastarch
• Tetrastarch

The two that most veterinary practitioners have access to are hetastarch and tetrastarch. Generally the difference between the three is the degree of substitution (the number of substituted glucose molecules divided by the number of glucose molecules present) and the molecular weight (hetastarch has the highest molecular weight, tetrastarch the lowest). HES is an effective volume expander by increasing or maintaining serum oncotic pressure, but it is not possible to predict how much the pressure will be increased within a particular patient.

There has been a lot of concerns about using synthetic colloids in veterinary medicine. Synthetic colloids have been shown in people to cause several adverse side effects including acute kidney injury, higher morbidity rates, prolonged hospitalization and coagulopathies. The veterinary based studies are limited with studies showing benefits and concerns of using these products. If using colloids, do so with caution until more veterinary based studies come forth.

Natural colloids are defined as being made from the body and are organic in nature (pRBC, whole blood, FFP, albumin) Veterinary medicine now advocates component therapy when using a blood product as opposed to using whole blood.

Whole Blood/Packed Red Blood Cells (PRBC): Both are used to increase a patient’s red blood cell count. Whole blood is the only natural colloid that retains most of its clotting factors for up to 24 hours however, platelets start to deteriorate within minutes of the sample being collected. The decision to transfuse any patient should not be based just on hematocrit only. Both the hematocrit and patient’s clinical status should be evaluated together. The benefit to transfusing either whole blood or packed cells is that it will result in an increase in oxygen carrying capacity. Packed red blood cells (PRBCS) are considered the treatment of choice when dealing with anemic patients. **NOTE: In this VETgirl podcast, Amy mentioned that packed red blood cells (pRBC) are colloids. Please note this is an error, as pRBCs carry very little plasma proteins, and while considered a life-saving transfusion, are not considered in the “colloid” group of fluid therapy.

Plasma Components: Fresh frozen plasma (FFP) contains water, electrolytes, albumin, globulin and coagulation factors. It does not contain any platelets. The storage life of FFP is approximately 12 months after which it loses its labile coagulation factors and can be labeled as frozen plasma (FP) and stored for an additional 4 years. Fresh frozen plasma is a common natural colloid used to treat coagulation disorders (disseminated intravascular coagulation, liver disease) as well as certain other diseases such as pancreatitis and peritonitis.

Other Fluid Choices: There are a few other fluid choices that veterinary practitioners have available.

Cryoprecipitate is plasma component that contains a high concentration of clotting factors von Willebrand’s factor, factor VIII, XIII and fibrinogen. It has a shelf life of approximately one year and is typically used in patients with von Willebrand’s disease, hemophilia A or a fibrinogen deficiency. Animal Blood Resources International (ARBI),, sells both canine and feline lyophilized (freeze dried) cryoprecipitate.

Platelet rich plasma (PRP) contains concentrated platelets and all clotting factors which are harvested from whole blood that is less than 8 hours old. Platelet rich plasma is indicated in patients that have a decreased platelet count that require surgery or have clinical bleeding. It is not indicated in cases of immune-mediated thrombocytopenia because the patient’s body will destroy any new platelets within minutes.

Albumin: Serum albumin is an important fluid choice because it maintains oncotic pressure and increases albumin. There are a variety of reasons that a decrease in albumin can occur including: liver disease, kidney failure, sepsis, pancreatitis, and malnutrition. Human serum albumin (HSA) has been used in both dogs in cats. Unfortunately, veterinary patients will create anti-HSA antibodies approximately 4-6 weeks post transfusion, thus making the administration of albumin only a one time occurrence. In 2008 a JAVMA study was published which proved that five out of 68 negative control dogs (7%) had detectable antibodies to HSA. Since then it has been well documented that type III hypersensitivity reaction can occur in dogs 8-16 days post administration of HSA. Limb swelling, facial swelling, edema, vasculitis and hemorrhage were noted. That said it is still an option for patients who are at risk of dying from hypoalbuminemia.

Most veterinary studies have focused on 25% HSA. In patients with immune-mediated diseases where a decrease in albumin has occurred, such as protein losing nephropathy and enteropathy, a HSA transfusion is not recommended. When choosing to use this product it is important the benefits outweigh the risks. Animal Blood Resources International (ARBI),, does offer canine serum albumin, but it is almost always backorder making it near impossible to purchase.

  1. I never really thought of difference between the fluids and why, I always just followed treatment but now I have a better understanding of the different types and purposes.

  2. Thank you for all that wonderful information. Refreshed my mind with things I knew and also exposed me to new things.

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