Fluid Therapy for the Companion Animal

Kimberly Baldwin, LVT
Education Coordinator
Cornell University
Ithaca, NY


The purpose of this lecture is to give the technician an overview of the types of fluids available to include crystalloid and colloids. It will review fluid doses, insensible and sensible losses, clinical signs of dehydration and overhydration, and fluid calculations. We will also discuss oncotic pressure and the components for volume and distribution.

Water is sixty percent of total body weight. This value varies based on lean body weight. The neonate will have a much higher percent of total body water based on lean body weight as compared to the obese adult animal. Out of that sixty- percent of total body water:

40% Intracellular
20% Extracellular, which is broken down further to interstitial, plasma and intravascular

The intracellular and extracellular compartments contain shifting concentrations of positive and negative ions.

Positive ions = (cations)
Negative ions = (anions)

The predominant cations are sodium (extracellular) and potassium, calcium and magnesium (intracellular)

Three predominant ions are chloride and bicarbonate (extracellular) and proteins (intracellular)

The most common abnormalities occur in sodium and potassium concentrations. The concentrations of these ions will be altered by fluid therapy.

The force that determines shifts related to ion concentration is osmotic pressure. Proteins (mainly albumin) regulate intravascular oncotic pressure. Its large molecular weight cannot cross the vascular membrane. Oncotic pressure between the intravascular and interstitial space is maintained because of this. Protein within the intravascular space exerts strong oncotic force that continuously pulls fluid back and holds it in the vascular space. This maintains normal blood pressure and flow. Critical ill animals are susceptible to low oncotic pressure due to protein loss. This occurs through vomiting, diarrhea, fever or excessive urination and disease processes. Decreased oncotic pressure can also be caused by an increased vascular permeability (allowing protein molecules to leak through) due to sepsis, immune mediated disease, or neoplasia. Monitoring total protein and albumin levels is critical in fluid therapy for all patients receiving fluids.


A crystalloid is a water-based solution that easily permeates cell membranes with varying electrolyte compositions. Sodium is there major component for osmosis. This means that over 75% of crystalloid fluids given will leave the intravascular space within 30 minutes after administration
A crystalloid solution can be...

Hypotonic The osmolality is less than serum (extracellular). Most of the fluid goes into the intracellular space. Ex .45% NACL or 2.5 % Dextrose/NACL
Not used in shock but can be used as a maintenance for those who have a high risk of fluid retention or sodium restrictions. Ex. heart failure

Isotonic These fluids have an osmolality closest to serum (extracellular fluid) Ex are .9% NACL, LRS, Plasmalyte. Good for maintenance and shock therapy
NaCL - normal sodium, high chloride, doesn't have potassium, calcium or magnesium
When would you avoid it? Sodium restricted cases, heart failure, hypertension, and metabolic acidosis
Lactated Ringers Solution - has calcium, potassium, lactate
Lactate converts to bicarbonate in the healthy liver
When contraindicated? Liver disease, cancer, hypercalcemic or hyperkalemic
Normosal- similar to LRS but has magnesium. It also has acetate and gluconate instead of lactate. Acetate and gluconate are metabolized by muscle not liver

Hypertonic fluids refer to fluids that have a greater osmolality than the extracellular fluid. An example of this is 7% sodium chloride
7% NACL is used in very small boluses (3- 5ml/kg) to increase intravascular pressure by drawing fluid from the interstitial and intracellular space. It is used for animals in shock.


Colloid solutions refer to a solution that has an osmolality greater than the extracellular space.

The term colloid refers to high molecular wt. This solution does not pass through the vascular membrane. It remains intravascular to assist with oncotic pressure. Fluids remain in the intravascular space as long as the colloidal pressure is greater than that of the tissue. Administration of colloid with a crystalloid during resuscitation or maintenance restores and maintains intravascular pressure.

Three common synthetic colloids are:

Hetastarch This has a higher molecular wt. therefore stays within the vascular space longer 12 - 48hrs.

Dextran has a lower molecular weight but has a larger variation of molecules. Therefore has a shorter duration within the vascular space, 4 - 8hrs, but has a greater oncotic pull to the vascular space.

Oxyglobin is an ultrapurified solution of bovine hemoglobin in modified LRS. It supplies the animal with a substitute oxygen transporting system by circulating in the plasma and transporting oxygen from the lungs to the tissues. Good for hemolytic anemia and trauma patients with decreased oxygen carrying capacity. Oxygen carrying capacity is retained for up to 40hrs.

Natural Colloids

These products will be discussed in depth in the following lecture.

Plasma is the ideal colloid to increase oncotic pressure and assist with hypoprotienemia and "leaky" vessel disorders.

Fresh Frozen Plasma contains plasma proteins, albumin and coagulation factors.

Packed Red Blood Cells contain red blood cells only. This is given to animals who have significant red blood cell anemia (usually PCV of 20% or less) and decreased oxygen carrying capacity.

Fresh Whole Blood contains RBC's, platelets, plasma proteins, and coagulation factors. Indicated for patients with massive hemorrhage or severe liver disease. Thrombocytopenia is not an indication for whole blood unless administered to a small animal (10kgs or less)

Component therapy (Blood products) should be administered over 4 - 6hrs
Initially 2ml/pd/hr. A complete TPR and clinical assessment of the animal should be completed before starting component therapy. The technician should then complete a TPR after 1/2hr of administration. Total dose to be given should be divided among four - six hours and a rate should be set accordingly. Monitor patient for transfusion reaction throughout transfusion. Watch for tachypnea, tachycardia, fever, lethargy, facial swelling, vomiting, diarrhea, restlessness etc

Fluid Therapy Doses

Shock doses for Crystalloid
Canine  =  40 - 90/kg/hr
Feline  =  20 - 60 /kg/hr
Maintenance  =  30ml/lb/day

Colloid shock dose  =  3 - 5 ml/kg/hr* or higher
Colloid maintenance  =  10 - 15ml/kg/day
Oxyglobin  =  10 - 30mg/kg over several hours. Rate should not exceed 10ml/kg/hr. Lower oxyglobin doses should be used in cats due to the risk of volume overload.

When administrating shock fluids of any type the animal should be bolused fluids for 15 minutes. Necessary parameters should be evaluated and compared to initial presentation before proceeding with aggressive fluid therapy. At minimal a TPR, CRT, heart and lungs ascultated, and PCV/TP should be completed

When deciding a fluid rate for a patient, maintenance, dehydration and contemporary or ongoing losses need to be considered.

Maintenance fluids are estimated at 30ml/lb/day.
They are made up of insensible and sensible losses.

Insensible loss is not readily measured. It is estimated as 10ml/lb/day. It is lost in respiration evaporation and passage of normal feces Insensible losses equal 1/3 of maintenance fluids.
Insensible losses will increase if the patient is panting or febrile.
Some clinicians will increase maintenance by 10% for each 1.8F rise of body temp.

Sensible loss is measured as urine production. Normal urine production is 1ml/lb/hr. Sensible losses equal 2/3 of maintenance fluids.

Contemporary or ongoing losses refer to excessive vomiting or diarrhea. It is recommended to estimate volume lost and double that amount to add to fluid rate. This is only done if the animal has multiple bouts of fluid loss. The ongoing losses can be piggybacked into maintenance fluids to simplify fluid rates.

An example of the use of ins and outs would be when a 60lb canine mix came in with chronic renal failure and needs diuresing.
Ins and outs are not started until the initial dehydration is corrected.

Insensible losses would be calculated at 10/lb/day
10 x 60 = 600 a day or 25ml/hr. A urinary catheter would be placed and urine would be measured every one - two hours. The urine produced would be divided by how often it was measured (1 - 2hrs) and added to maintenance fluids for that same time period.

Contemporary or ongoing losses (vomiting or diarrhea) would be estimated as it occurred and added (depending on how often fluids need to be changed) to insensible losses.

Evaluating Dehydration

5% dehydration  =  minimal clinical signs, possible dry mm

6 - 8% dehydration  =  mild to moderate skin turger, dry mm

10 - 12% dehydration  =  marked decrease in skin turger, dry mm, weak and rapid pulse, slow capillary refill time, change in mentation-slow to respond

Percent dehydration is replaced over a 24hr period. This will vary based on % dehydration, clinician, and disease process.

To calculate mls needed to replace dehydration the technician should complete the following formula

% dehydration in decimal x bdy wt in kgs = number of liters deficit. Multiple this by 1000 and it will give you ml dose.

Ex. A 10lb(4.5KG) cat is 8% dehydrated
.08 x 4.5kg = .36L x 1000 = 360ml needed to be replaced

All animals on fluids must be monitored closely for overhydration. Accessing skin turger, TPR, mm color and moistness, urine output, and auscultation of heart and lungs sounds are routine observations of the patient requiring fluid therapy.

Clinical signs of overhydration include:

Nasal discharge




Pulmonary edema

Easy calculations when a syringe or fluid pump is not available

Drops per minute for total volume to be administered over a specific number of hours

Drops per minute =

   total volume

x drops per ml


Example. The 4.5kg DSH is 8% dehydrated and the clinician wants you to replace it over 12hrs.
.08 x 4.5kg =.36L x 1000 = 360ml
12hrs x 60 minutes = 720
360 divided by 720 = .5 x 60(drops per ml) = 30 drops per minute. 60(sec/min) divided by 30(drops per minute) = one drop every 2 seconds

Calculating the drops per minute to be administered.

Rate (drops/min) = ml/hr x drops/ml calibrated divided by 60(minutes/hr)

Example. The dog needs 75ml/hr
75 x 10(drip set) = 750 divided by 60(minutes/hr) = 12.5 drops/min 60(seconds) divided by 12.5(drops/min) = 1 drop every 4.8 seconds.