The Basic of Fluid Therapy

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Body Water Compartments

Fluid therapy in small animal practices is an integral part of many patients' care and/or treatment plans. The primary goal of fluid therapy is to restore or maintain intravascular volume, tissue perfusion and to reverse dehydration. It is a pharmacological intervention and therefore needs to be specifically individualized to each patient and closely monitored and re-evaluated as the patient's status changes.

Before discussing how to formulate an individualized treatment plan for our patients, let’s look at the fluid compartments in the body.

Body Water Compartments

Approximately 60% of an animal's weight in kilograms is water. Of this 60%, approximately two-thirds is intracellular fluid (ICF) and one-third is extracellular fluid. The extracellular fluid volume can be broken down further, with three quarters as interstitial fluid and the remaining quarter as intravascular fluid.


Figure 1. Fluid compartments in the body (Hughston, 2016).

Approach to fluid therapy plan

Approach to fluid therapy plan

There are many factors to consider when creating and implementing a fluid therapy plan for your patient.

Initial Patient Assessment;

  • Patient history
  • Chief complaint
  • Physical examination findings

Completing a thorough patient assessment should provide an insight as to the process in which the fluid has been lost.

There are two ways fluid loss can occur in pets, dehydration or hypovolaemia. This is dependent upon the speed and the compartment from which the fluid is lost from their body. Dehydration is a fluid deficiency associated with all fluid compartments, resulting in an overall increase in electrolyte concentrations. Whereas hypovolaemia is a reduction in intravascular volume with either whole blood or plasma water loss. Understanding this will help to formulate the initial stages of a fluid therapy plan. 

An infusion plan has different phases depending upon what the goal for your patient is. Fluid resuscitation/rehydration, optimization, stabilisation, evacuation (Hansen, 2021).

  • Resuscitation; correcting perfusion abnormalities. Rehydration; correcting hydration abnormalities.
  • Optimisation; ongoing administration of replacement fluids that are working to optimize circulation.
  • Stabilisation; is a recovery stage in which the patient is hemodynamically stable and the infusion therapy moves toward optimizing electrolyte balance and replacing ongoing losses.
  • Evacuation - the cessation of assisted fluid therapy, as self-sufficiency via oral intake is adequate.

Developing this treatment plan is multi-faceted and will involve the consideration of a number of other variables as well.


Route of administration

There are multiple ways to deliver fluid to a patient and this can be determined by the speed at which the fluids are required and the total volume to be administered. 

Route of administration

Table 1. Determining Route of Fluid Administration








- Correction of mild-moderate dehydration.






- Maintenance of not severely ill patients.

- Use isotonic fluids.

- Best to administer by gravity flow through an 18- to 20-gauge needle (for an adult-sized cat; use a smaller needle for pediatric patients).

- Do not deposit more than 10-12 ml/kg per injection site.

-  Fluid should be deposited dorsally along the area bordered by the scapulae anteriorly and the iliac crests posteriorly.

- The average 5- to 6-kg cat can receive 150-200 cc once or twice daily.

-  Avoid hypertonic/hypotonic fluids.

- Not conducive for patients >10kg.

- Do not deposit fluid under infected skin.

- Not useful in hypovolemic shock.


- IV fluid therapy is the preferred route to treat severe hypovolemia and/or dehydration.

- The best route for correcting hypotension.

- Provides for rapid delivery at the most precise dosage.

- Most effective for medium and large dogs.

- Prepare a sterile site for needle or cannula intravenous insertion.


- Use isotonic fluids for volume repletion.


- Maintain complete sterility of IV cannula and infusion system.

- Avoid intravenous overload caused by excess fluid delivery.

- Avoid catheter sepsis and phlebitis.

- Avoid catheter displacement and the inadvertent extravascular placement of the fluid infusion.


- When intravenous access is unavailable.


- Particularly useful in small animals.


- Provides direct access to the vascular space


- IO fluid administration is comparable in effectiveness to the IV route. (Cameron et al. 1989). 

- Prepare a sterile site 1 cm distal to tibial tuberosity, proximal media tibia, or trochanteric fossa of femur

- Make a small nick skin incision.

- Insert either an 18-20 gauge hypodermic needle, a spinal needle or a small bone marrow needle.

- Secure with tape and bandage.

-  Avoid growth plates.



- Use a needle proportional to bone size to avoid trauma.



- When intravenous access is unavailable.


- Provides a vehicle for delivering ample volumes of fluid over a short time period.


- Relatively rapid absorption

- Use isotonic fluids.

- Use needle gauges 16-20, depending on the patient's size.

- Prepare a sterile injection site just lateral to the midline and midway between the umbilicus and the pelvic brim

- Hypertonic fluids will worsen the dehydration.

- Do not use if the patient has abdominal sepsis, ascites, or peritonitis.

- Do not use with pending abdominal surgery.

Per Os

- For anorectic patients with short term illness.

- More conducive to pets over <20kg.

- Very conducive for neonates.

- Can use a stomach tube, pharyngotomy tube, small dosing syringe or a small baby bottle and nipple depending on patient size and underlying condition.

- Warm fluids to body temperature.

- Aspiration pneumonia.

- Not useful in hypovolemic shock.

- Not to be used with a vomiting patient.

- Avoid air administration.

Table modified from (Schaer,1989) and (Schaer, 2005).

Type of fluid

As mentioned in the table above, another consideration is the temperature the fluids are delivered during the infusion therapy. Warming fluids to body temperature has been shown to be useful in large volume resuscitation, however, this may have limited usefulness during low IV infusion rates (Davis, 2013).

Type of fluid

Whilst there are a number of types of fluid available, each product has its own role in treating specific conditions and pathologies. One of the easiest ways to distinguish fluids is based on their intended purpose; maintenance or replacement therapy. In saying this, the most commonly used solution is lactated Ringer’s or Hartmann’s solution (Robinson, Dipecvaa and Mrcvs, n.d.). This is suitable in the majority of cases due to its similarity in composition to plasma. IV Fluid composition can be broken down into two main categories; Crystalloid solutions containing water, electrolytes, and/or glucose and Colloids; mostly albumin and blood products. One of the most simple guidelines when considering which fluid to use in infusion therapy is to replace like with like. 

Table 2. Composition of Common Veterinary Fluids


Fluid Type
                                                                      Component (unit) Buffer(s)Primary Use
pHSodium (mEq/L)Chloride (mEq/L)Potassium (mEq/L)Magnesium (mEq/L)Calcium (mEq/L)Osmolarity (mOsm/L)
0.9% Saline5.5154154000308NoneReplacement
0.45% Saline5.67777000154NoneMaintenance
Plasmalyte A7.414098530294Acetate (27 mEq/L) Gluconate (23mEq/L)Replacement
Plasmalyte 565.040401330363NoneMaintenance
Normosol-R7.414098530294Acetate (27 mEq/L) Gluconate (23 mEq/L)Replacement
Normosol-L5.04040133363363Acetate (16mEq/L)Maintenance
Lactated Ringer's solution (LRS)6.5130109402.7273Lactate (28 mEq/L)Replacement

Table from (Hughston, 2016).


Rate of fluid therapy

Rate of fluid therapy

The initial rate at which the fluid will need to be delivered is heavily dependent on the status of the patient. Below are a few guidelines to calculate the required rate for various fluid therapies.

Volume of fluid needed to correct dehydration in pets is (Schaer, 2005);

Volume (ml) of fluid needed = % dehydration x body weight (kg) 1000

Treating Hypovolemic Shock (Schaer, 2005);

Initial Rapid infusion Dogs 20-40ml/kg IV for 15 minutes. (About half the volume for cats).

Followed by, 70-90ml/kg (dogs) 30-50ml/kg (cats) over one hour.

Then finally a maintenance rate of 10-12ml/kg/hr for dogs and 5-6ml/kg/hr for cats.

Maintenance Rates (Davis, 2013);

Cat IV Fluid Rate 2-3ml/kg/hr

Dog IV Fluid Rate 2-6ml/kg/hr


Monitoring and evaluating fluid therapy plan

Monitoring and evaluating fluid therapy plan

The patient’s condition and physiology can change rapidly during a fluid therapy plan, and therefore it is imperative to monitor and evaluate their needs and tailor or discontinue their treatment as required. Below is an indicator of some of the parameters that can be utilised to monitor the status of the patient whilst undergoing infusion therapy.

Table 3. Evaluation and Monitoring Paraments that May Be Used for Patients Receiving Fluid Therapy.

Pulse rate and quality

Capillary refill time

Mucous membrane color

Respiratory rate and effort

Lung sounds

Skin turgor

Body weight

Urine output

Mental status

Extremity temperature

Packed cell volume/total solids

Total protein

Serum lactate

Urine specific gravity

Blood urea nitrogen




Venous or arterial blood gases

O2 saturation

BP, blood pressure.

Table modified from (Davis et al., 2013)




Fluid therapy, whether that is via intravenous infusion or another commonly used method, is an integral part of treating a variety of medical conditions in a small animal practice. As discussed, there are a number of key factors that dictate how to formulate a treatment plan, however, an important aspect to remember is that it is tailored to each and every patient individually. Providing successful fluid therapy is a highly individualised and dynamic process and continuing to broaden your knowledge and understanding of the topic is important.

Hansen, B. (2021). Fluid Overload. Frontiers in Veterinary Science, 8.
‌Cameron JL, Fontanarosa PB, Passalaqua AM (1989) A comparative study of peripheral to central circulation delivery times between intraosseous and intravenous injection using a radionuclide technique in normovolemic and hypovolemic canines. The Journal of emergency medicine 7, 123-127.
Davis, H., Jensen, T., Johnson, A., Knowles, P., Meyer, R., Rucinsky, R. and Shafford, H. (2013). 2013 AAHA/AAFP Fluid Therapy Guidelines for Dogs and Cats*. Journal of the American Animal Hospital Association, 49(3), pp.149–159.
‌Hughston, L. (2016). The Basics of Fluid Therapy. [online] Today’s Veterinary Nurse. Available at:
Lukas G, Brindle SD, Greengard P (1971) The route of absorption of intraperitoneally administered compounds. The Journal of pharmacology and experimental therapeutics 178, 562-564.
Peck, DVM, DACVECC, C., 2022. Current Concepts in Fluid Therapy. [online] MSPCA-Angell. Available at: <> [Accessed 12 March 2022].
Robinson, B., Dipecvaa, B. and Mrcvs, F. (n.d.). Drenched Dogs and Crispy Cats? A Case Based Approach to Fluid Therapy. [online] Available at: [Accessed 16 Mar. 2022].
‌Schaer, M. (1989). General Principles of Fluid Therapy in Small Animal Medicine. Veterinary Clinics of North America: Small Animal Practice, 19(2), pp.203–213.