Fluid therapy: Determining routes and rates of fluid administration

Fluid therapy is a common treatment prescribed by veterinarians. Sometimes it may seem so common that we don’t stop to think about the nuances of administration—but it’s crucial that we do so to prevent adverse effects, such as fluid overload.

As the  2024 AAHA Fluid Therapy Guidelines for Dogs and Cats remind us, “Fluids are drugs that are prescribed to patients, and like any medication, they must be used in a way that achieves therapeutic goals and minimizes complications.”

In other words, we must think of fluid therapy the same way we’d think of a drug prescription. That means avoiding practices like putting patients on “twice maintenance” and, instead, tailoring the fluid therapy to treat that individual patient’s vascular, interstitial, or intracellular fluid deficits. Additionally, it’s imperative that any patient receiving IV fluids is monitored closely throughout their treatment.

When using fluids in practice, it is important to select appropriate rates and routes of administration to achieve therapeutic goals—and the 2024 Fluid Therapy Guidelines provides practical guidance along with helpful diagrams to illustrate some of the points below (and more).

Patient assessment 

Patient assessment is critical to determine the type and degree of fluid deficit in a patient.

Initial assessment should include a thorough history to assess route and degree of fluid losses (i.e., vomiting, diarrhea, blood loss, etc.) and a physical examination with special focus on perfusion parameters (heart rate, CRT, mucus membrane color, pulse quality, extremity temperature and body temperature).

Additional data can be gathered to detect and differentiate between hypovolemia (a decrease in fluid in the vascular space) and dehydration (a fluid whole-body fluid deficit and electrolyte imbalance).

Blood pressure, electrocardiogram, laboratory testing, and diagnostic imaging can be used to further assess intravascular volume and thereby detect hypovolemia.

A patient’s degree of dehydration, caused by losing more fluids than the body is taking in, can be estimated by evaluating skin turgor and mucus membrane moisture. Because dehydration can affect multiple fluid compartments starting with the interstitial space, there are additional ways to evaluate dehydration, particularly if it worsens. As this condition worsens, patients may have retraction of the globes within the orbits and dull corneas, for example.

Dehydration and hypovolemia can be present at the same time, but they don’t have to be. Patients with greater than 10% dehydration, for example, will begin to show evidence of hypovolemia. Hypovolemia can be present in some circumstances without dehydration, such as with severe bleeding.

Patients on fluid therapy must be monitored and regularly reassessed, and all team members involved should work to avoid fluid overload through prevention and early recognition.

Choosing a route of administration

Fluid can be administered through several routes: enteral, subcutaneous, intravenous, and intraosseous. The route of administration should be chosen based on whether the patient has hypovolemia, dehydration, or both.

While correcting hypovolemia and dehydration require different routes of administration, isotonic crystalloids (also known as replacement crystalloids) should be used to treat both conditions. When both hypovolemia and dehydration are present, hypovolemia should be addressed first.

Hypovolemia requires rapid replacement of fluids through intravascular or intraosseous routes. Small boluses are given over 15 to 30 minutes. After each bolus, perfusion parameters are reassessed.

Dehydration should be corrected more slowly, ideally over a 12-24 hour period. Oral rehydration or subcutaneous routes are generally preferred. For severe cases of dehydration or those where patients cannot tolerate oral fluids, intravenous fluids can be administered.

Determining fluid rates

Intravenous fluid therapy can be divided into three phases: resuscitation, rehydration, and maintenance. Table 9 in the guidelines summarizes these phases, including the formula for calculation and the time over which the fluids should be administered.

Resuscitation is administered as small boluses of 5-10 mL/kg for cats and 15-20 mL/kg for dogs. These boluses are administered over approximately 15 minutes and may be repeated until target perfusion parameters are met. Once resuscitation is completed, rehydration can begin.

The rehydration phase focuses on replacing the total fluid deficit and ongoing losses over a 12- to 24-hour period. The total fluid deficit in liters is calculated by multiplying the body weight in kilograms by the percent dehydration (as a decimal). Ongoing losses through vomiting, diarrhea, or other routes should be added to this deficit.

Maintenance fluids are the amount needed for normal function. Standard maintenance rates are 60 mL/kg/day for dogs and 40 mL/kg/day for cats. Pediatric patients require increased rates, with puppies requiring 3 times the adult dose and kittens requiring 2.5 times the adult dose. When calculating the volume of fluids a patient is receiving, the total volume of enteral water, intravenous drugs, and liquid diets should be subtracted from the maintenance fluid route that is administered intravenously.

There is limited evidence-based guidance for the volume of subcutaneous fluids to administer. The guidelines recommend 20-30 mL/kg administered 1 to 2 times daily. Depending on volume, fluids should be administered over multiple sites with a maximum volume of 10-20 mL/kg per site.

Take home points

Patient assessment should be used to guide selection of fluid administration route and rate of administration. Patients should be repeatedly assessed during therapy to allow for adjustments in the therapy plan.

The 2024 AAHA Fluid Therapy Guidelines for Dogs and Cats offer a complete guide to the use of fluids in practice, including considerations for patients undergoing anesthesia or with illness, which may affect how fluid therapy is administered.

Photo credit: © AAHA

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