November 2024
By Danelia de Kock, DVM, BS Math, VetpocketTM Co-founder, “Medical Mathematics for Veterinary Professionals + tables” author

In this VETgirl online veterinary continuing education blog, guest blogger Dr. Danelia de Kock, DVM, BS Math reviews how to calculate intravenous (IV) fluid rates in veterinary medicine. Tune in to learn how to calculate maintenance, dehydration, and ongoing losses fluid rates for dogs and cats, so you can make informed decisions regarding how to formulate an IV fluid therapy plan for your veterinary patients! Be sure to read to the end for our Medical Math Practice Problem (see Figure 4).

How to Calculate Fluid Rates

 

Image courtesy of Pixabay.com

In veterinary medicine, appropriately managing a patient’s intravenous (IV) fluid therapy is crucial for helping ensure their recovery and overall well-being. Not only is it important to use the appropriate fluid rate for a patient, but also to use the appropriate fluid type. In veterinary medicine, we often use balanced isotonic crystalloids as our “maintenance fluids”, but this is a misnomer, as TRUE maintenance fluids are hypotonic in nature. Regardless of what type of fluid is used (e.g., maintenance versus replacement; isotonic, hypotonic, or hypertonic; crystalloid versus colloid), the clinician must be able to appropriately calculate fluid rates.

It is important to remember to always tailor IV fluid therapy to the individual patient’s needs, that fluid therapy guidelines are ever evolving, that fluids are drugs and therefore come with risks, and to avoid adverse effects such as electrolyte derangements or fluid overload.

Fluid rate formulas are based on fluid estimates needed to maintain normal patient fluid balance. Therefore, these formulas provide estimates, which are meant to be used as starting points and as guidelines only, and then adjusted as indicated for each individual patient’s needs.

A patient’s fluid rate needs to be carefully calculated, and the patient regularly monitored to ensure an appropriate volume of fluids is being administered – do not administer too little or too much, as both can result in deleterious outcomes.

Patients with comorbidities such as cardiac or renal disease, and cats, are at an increased risk for fluid overload. Always have these types of discussions with the patient’s owner, allowing all parties involved to be on the same page concerning any potential risks of IV fluid therapy.

Note: In order to solve some of the below formulas, the patient’s body weight needs to be in kilograms (kg). Body weight can be converted from pounds (lb) to kg by using the unit conversion factor 1 kg = 2.2 lb (shortcut: lb divided by 2.2 = kg).

Maintenance Fluid Rate

This is the fluid rate necessary to maintain the daily metabolic needs of a normal patient, including losses such as through normal urine output and through moisture content of feces.

Note that several different maintenance fluid rate formulas exist, and that there isn’t a definitive consensus on which formula to use. Non-linear formulas (such as Figure 1.) are generally considered to be more accurate than linear formulas (e.g., 50-60 mL/kg/day), as linear formulas may underestimate or overestimate fluid needs for patients outside a certain weight range.

An additional, important note is that part of the maintenance fluid rate formula accounts for the average urine output of normal healthy patients which is ~1 to 2 mL/kg/hr or ~24 to 48 mL/kg/day. However, patients with renal disease can have variable urine output rates ranging from polyuria to anuria, so maintenance fluid rate formulas do not apply to patients with renal dysfunction. In these patients, tailoring IV fluid therapy to the individual patient’s needs is especially important.

Figure 1. Courtesy of Danelia de Kock.

Calculator tip: If you have a scientific calculator, use the exponent function “xy“ for solving a number raised to the power of 0.75; to access this function using your smartphone’s calculator, turn your phone sideways when you are in the calculator app. When using a calculator that does not have an exponent function, raising a number to the power of 0.75 is the same as raising the number to the power of 3 and then taking the square root twice. For example, 100.75 = 10 x 10 x 10, then take the square root, and take the square root again.

The daily maintenance fluid rate in mL/day can be converted into the hourly maintenance fluid rate in mL/hour by using the unit conversion factor 1 day = 24 hours (shortcut: mL/day divided by 24 = mL/hour).

Dehydration Fluid Rate

This is the additional fluid rate necessary to rehydrate a patient within a certain period of time, usually over an 8 to 24 hour time period.

Figure 2. Courtesy of Danelia de Kock.

Recall that x% = x/100, for example 5% = 5/100 = 0.05.

The fluid deficit can be converted into mL by using the unit conversion factor 1 L = 1,000 mL (shortcut: L multiplied by 1,000 = mL).

The dehydration fluid rate in mL/hour is calculated by dividing the fluid deficit in mL by the time period (in hours) to correct the fluid deficit.

Ongoing Losses Fluid Rate

This is the additional fluid rate necessary to account for abnormal losses occurring through conditions such as vomiting, diarrhea, polyuria, excessive panting, sweating, and blood loss.

Ongoing losses need to be measured or estimated, which can sometimes prove challenging. Since a patient’s ongoing losses will change with the patient’s response to treatments, I recommend reassessing the patient’s ongoing losses every 4 to 6 hours and adjusting the fluid rate as indicated. Alternatively, you can estimate the patient’s anticipated total daily losses, divide by 24 hours, and add this to the patient’s hourly rate. When calculating ongoing losses, you can measure the weight of soiled bedding materials using the equivalents of 1 kg = 1 L or 1 g = 1 mL, but for fluids containing solids (i.e., diarrhea) this number would need to be adjusted further, perhaps by visual inspection, for an estimation of solids content.

The ongoing losses fluid rate in mL/hour is calculated by dividing the estimated ongoing losses in mL by the time period (in hours) within which the ongoing losses are to be corrected.

Total Fluid Rate

The maintenance, dehydration, and ongoing losses fluid rates are added together to obtain the total fluid rate for your patient.

Figure 3. Courtesy of Danelia de Kock.

Patients who are at an increased risk for fluid overload need to be monitored extremely closely while on IV fluid therapy, and rates adjusted accordingly.

Remember to take into account volumes from IV medications (including CRIs), IV flushes, enteral fluids and liquid diets, subcutaneous fluids, etc. as these will also provide fluid volume to the patient.

Monitoring and Adjustments

It is imperative to monitor patients regularly while on IV fluid therapy, especially patients on high fluid rates and/or patients with comorbidities such as cardiac or renal disease. Key parameters to monitor include: mentation, temperature, heart rate, respiratory rate, mucous membrane palpation and color, capillary refill time, skin turgor, blood pressure, electrolytes, acid-base balance, urine specific gravity, creatinine, blood urea nitrogen, packed cell volume/total protein, fluid ins and outs, and body weight.

Clinical signs that a patient may be experiencing fluid overload include: polyuria (in the absence of renal impairment or diuretic administration), gaining unintended weight, edema (e.g., subcutaneous, pulmonary), shivering, restlessness, pulmonary crackles, tachypnea, cough, labored breathing, chemosis, or clear nasal discharge. Fluid overload is life-threatening!

Conclusion

The key to quality IV fluid therapy and achieving the best possible outcomes is to tailor IV fluid therapy to the individual patient’s needs by obtaining a good patient history, performing a thorough physical exam, performing diagnostics, careful calculations, frequent monitoring, and ongoing adjustments as indicated to help avoid adverse effects such as electrolyte derangements or fluid overload. It is imperative to take into account a patient’s individual circumstances and not to rely on the numbers alone.

Figure 4. The Medical Math Practice Problem is an excerpt from: de Kock, Danelia. Chapter 11. Medical Mathematics for Veterinary Professionals + tables. VetpocketTM 2020, page 239.

Abbreviations:
AAHA: American Animal Hospital Association
BWkg: body weight in kilogram
CRI: Constant rate infusion
IV: intravenous
kg: kilogram
L: liter
lb: pound

References:
1. Pardo M, Spencer E, Odunayo A, et al. 2024 AAHA Fluid Therapy Guidelines for Dogs and Cats. J Am Anim Hosp Assoc 2024 Jul 1:60(4):131-163.
2. Davis H, Jensen T, Johnson A, et al. 2013 AAHA/AAFP Fluid Therapy Guidelines for Dogs and Cats. J Am Anim Hosp Assoc 2013 May-Jun;49(3):149-59.
3. Langston C, Gordon D. Effects of IV Fluids in Dogs and Cats with Kidney Failure. Front Vet Sci 2021;8:659960. Published 2021 Apr 20.
4. de Kock D. Medical Mathematics for Veterinary Professionals + tables. VetpocketTM 2020.

Please note that the opinions in this blog are expressed by the speaker and not directly endorsed by VETgirl.

  1. Sierra Reyes says:

    This was a really good and simple way to explain everything! As someone who isn’t the best at math, I really appreciated this!

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