How I Choose a Fluid
ACVIM 2008
Steven L. Marks, BVSc, MS, MRCVS, DACVIM
Raleigh, NC, USA


Fluid therapy is one of the cornerstones of medical management. Fluids are used to correct dehydration, treat shock, treat hypotension, diurese, correct electrolyte abnormalities, provide an avenue for other therapeutic agents to name a few. There are different types of fluids available and they have different advantages and disadvantages depending on the clinical setting for which they are used. The following discussion will provide a general overview of fluids and fluid selection and emphasize their clinical indications.


Crystalloids are solutions that contain electrolyte components able to enter the intravascular, intracellular and interstitial spaces. Their impact on the intravascular space is short lived and their primary effect is on the intracellular and interstitial compartments. The major advantages of crystalloid solutions are their availability and their cost effect. The major disadvantage of crystalloid solutions is that only approximately 25% of the volume administered remains in the vascular space 1 hour after administration. Crystalloids can further be divided into replacement solutions and maintenance solutions.

Replacement Solutions

Replacement solutions are designed for rapid volume expansion, to correct dehydration, diuresis and management of different types of shock states. The consistent characteristic of all replacement solutions is their high sodium and chloride content when compared to maintenance solutions. These solutions are designed to approximate the solutes found in plasma.

Lactated Ringers Solution

Lactated Ringers solution has been a mainstay of fluid therapy for the veterinary clinician. In many cases this crystalloid has been the default fluid selection. The reason this fluid is commonly chosen is that it is a buffered, isotonic electrolyte solution that contains high concentrations of sodium and chloride, small amounts of potassium and calcium and provides lactate as a buffer. The amount of potassium and calcium is not enough for maintenance; therefore these components must be supplemented.

Saline (0.9%)

Physiologic saline is a commonly used crystalloid. Among the isotonic solutions, it contains the highest concentration of sodium and chloride. It also is a fluid that is compatible with many other therapeutics including transfusion products and can be used as a carrier for these other agents. It is also a fluid that can easily be supplemented with potassium and magnesium if required by the patient.

Normosol R

Normosol R is a solution very similar to Lactated Ringers solution. It contains a similar concentration of sodium, chloride and potassium. In addition it contains magnesium but at a lower concentration than is considered maintenance. The major difference in these solutions is the buffer agent. Rather than lactate being used as a precursor to bicarbonate, acetate and gluconate are found. These compounds can be converted to bicarbonate in striated muscle and do not require metabolism via the liver.


Plasmalyte is very similar to Normosol R but has acetate and lactate as bicarbonate precursors and about double the amount of potassium. These solutions are often used in similar clinical circumstances.

Maintenance Solutions

Maintenance solutions are a subclass of crystalloid solutions that contain electrolytes in concentrations to approximate body losses. Compared with replacement solutions they contain much less sodium and chloride and much higher concentrations of potassium. These fluids are typically used after volume depletion has been corrected and euvolemia is established.


Normosol-M is a maintenance solution. As you can see from the table this solution contains significantly less sodium and chloride when compared to the replacement crystalloid solutions. It also contains significantly more potassium and glucose in order to maintain the isotonicity. The disadvantage of using this solution in critically ill patients is the possibility of the contribution to hyperglycemia of injury.

Half Strength Saline

Half strength saline or 0.45% saline contains exactly ½ the sodium and chloride as 0.9% saline. Although this solution is relatively hypotonic it can safely be used as a maintenance solution if supplemented with potassium.

2.5% Dextrose and 0.45% Saline

The addition of 2.5% dextrose to this low sodium solution allows it to be isotonic and can work well as a maintenance solution similar to 0.45% saline if supplemented with potassium. The disadvantage of this solution for critically ill patients is the presence of dextrose and the possibility of it contributing to the hyperglycemia of injury.

Composition of commonly used crystalloid solutions.








0.9% saline





















Plasmalyte 148














0.45% saline







2.5 % Dex 0.45% saline







Hypertonic Saline (75%)

Hypertonic saline is a hypertonic solution and is considered a replacement solution. It is primarily used for rapid volume expansion. There are several proposed mechanisms of action but pulling fluid into the intravascular space is the primary mechanism. The limitation with this product is its short lived affect. This effect can be enhanced and prolonged by adding a colloid such as Hetastarch and obtaining a synergistic effect. Hypertonic saline can be administered as a bolus of 4-7 ml/kg in the dog or 2-4 ml/kg in the cat. When used in combination with Hetastarch may be used as 7 ml of hypertonic saline (23.4 %) diluted with 43 ml of Hetastarch in the same syringe and administered at a dosage of 3-5 ml/kg IV.


Colloids are solutions that contain larger molecular weigh particles generally suspended in a crystalloid. These particles allow this solution to exert oncotic pressure and remain in the vascular space. The duration of effect is somewhat based on particle size and is much longer than crystalloid solutions.


Hetastarch is one of the most common synthetic colloids used in veterinary medicine. It contains hydroxyethyl starch which is primarily made of amylopectin. It can be used at a maintenance dosage of 20 ml/kg/day or to effect as an intravenous bolus at a dosage of 5 ml/kg. It has the advantage of having a long half life and can be used in combination with isotonic or hypertonic crystalloid solutions.


Dextran 70 is a polymer of glucose and may cause coagulation defects due to the coating of platelets. It can be used at dosages of 20 ml/kg and can be used as a single agent or in combination with isotonic or hypertonic crystalloid solutions.


Oxyglobin is a hemoglobin based oxygen carrier (HBOC). In addition to providing the patient with oxygen carrying capacity, it has excellent colloid properties. In theory this product is ideal when volume is required and a transfusion of red blood cells in beneficial. Oxyglobin can be administered at a dosage of 10-30 ml/kg to effect.

Transfusion Products

Fresh Whole Blood

Fresh whole blood is the gold standard of transfusion products. This product contains RBC, clotting factors, albumin and platelets. It is primarily provided to patients that require red blood cells and or clotting factors. In many cases it is the easiest product to obtain and may be the only product that will provide functional platelets. Typically dosages of 20 ml/kg are administered to raise the patient PCV 10%.

Packed Red Blood Cells (pRBC)

Packed red blood cells are part of the armament of blood component therapy. Made from fresh whole blood this product is refrigerated and can be stored for 21-35 days. The dosage used in approximately 10-15 ml/kg in order raise the patient PCV approximately 10%. This product is used when a patient is anemic and requires oxygen carrying capacity.

Fresh Frozen Plasma

Fresh frozen plasma is the component made when pRBC are separated from fresh whole blood. This product is primarily used to provide clotting factors but can also be used for oncotic pressure although this is an inefficient way to provide albumin per unit. The typical dosage is 10 ml/kg up to three times daily but may vary depending on the clinical condition of the patient.

Frozen Plasma

Frozen plasma is fresh frozen that is outdated i.e., older than one year or that was collected and not frozen within 4-6 hours. It is a good source of albumin and can be used for oncotic pressure. It has a shelf life of approximately 5 years and is used at a dosage of 10 ml/kg.

Fluid Selection

The goal of fluid selection is to match the fluid with the patient needs. Crystalloids are primarily used for rapid volume expansion. The advantages of these fluids have been discussed. These solutions should primarily be chosen based on their sodium and chloride concentrations and whether or not they contain a buffer. These solutions are readily available and inexpensive; however large volumes must be administered. The colloids are also used for rapid volume expansion but are more expensive and require less volume for effect. The colloids can be used to effect or as a 24 hr infusion to improve oncotic pressure. Isotonic crystalloids and colloids can be used together for volume expansion. The transfusion products can also be used based on patient needs. Oxygen carrying capacity requires components that contain red cells, oncotic pressure requires products that contain albumin and coagulopathies require components that contain clotting factors.

Overall it is the clinician's responsibility to understand what fluid choices are available and which best meet the patient's needs.


1.  Driessen B, et al. J Vet Emerg Crit Care 2006;16:276.

2.  Humm K, et al. JVIM 2007;21:656.

3.  Mathews KA. Vet Clin North Amer Small Anim Pract 1998;28:483.

4.  Prittie J. J Vet Emerg Crit Care 2006;16:329.

5.  Silverstein D, et al. J Vet Emerg Crit Care 2005;15:185.

6.  Vincent JL, et al. Crit Care Med 2006;34:1333.

Speaker Information
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Steven Marks, BVSc, MS, MRCVS, DACVIM
North Carolina State University
Raleigh, NC