IV Fluid Therapy in Dogs and Cats: A Practical Refresher

IV Fluid Therapy in Dogs and Cats: A Practical Refresher

Fluid therapy is one of those things we use constantly in veterinary practice, but working out the right fluid plan still requires more thought than simply choosing a bag and setting a rate.

Is the patient hypovolaemic? Dehydrated? Unable to maintain normal fluid intake? Are there ongoing losses that need replacing?

Thinking about why the patient needs fluids makes it much easier to work out what should actually go into the fluid plan.

Start with the patient, not the fluid rate

There isn't one fluid rate that suits every dog or cat.

A useful way to approach a fluid plan is to consider the patient's requirements separately:

Resuscitation — does the patient have an intravascular volume deficit that needs to be addressed urgently?

Rehydration — is there an existing fluid deficit from dehydration?

Maintenance — can the patient currently meet their normal daily water requirements?

Ongoing losses — are they continuing to lose fluid through vomiting, diarrhoea, urine production, drains or other sources?

The 2024 AAHA Fluid Therapy Guidelines recommend tailoring the type, volume, route and rate of fluids to the individual patient rather than routinely using arbitrary multiples of maintenance. AAHA

Calculating a dehydration deficit

One of the most useful calculations to have close by is the patient's estimated dehydration deficit.

A commonly used calculation is:

Fluid deficit (L) = Body weight (kg) × estimated dehydration as a decimal

For example, a 10 kg dog estimated to be 6% dehydrated has an estimated fluid deficit of:

10 × 0.06 = 0.6 L

or 600 mL.

That deficit then needs to be incorporated into the overall fluid plan and corrected over an appropriate period based on the patient's condition. Current AAHA guidance generally describes correction of dehydration over approximately 12–24 hours, with ongoing reassessment. AAHA

Don't forget maintenance

Once perfusion and dehydration have been considered, a patient who is unable to eat or drink normally may also require maintenance fluids.

AAHA lists several accepted methods for estimating maintenance requirements. Simple starting figures commonly used for adult patients are approximately:

Dogs: 60 mL/kg/day
Cats: 40 mL/kg/day

These are starting estimates rather than a reason to "set and forget". Oral intake, liquid diets, IV medications and other sources of fluid all contribute to the patient's total daily volume. AAHA

Ongoing losses still count

A patient with vomiting, diarrhoea, polyuria or other measurable fluid losses may require additional replacement on top of their calculated deficit and maintenance requirements.

Whenever possible, estimate or measure these losses rather than simply increasing the patient's fluids by an arbitrary amount.

This is where regular reassessment becomes particularly important.

Fluids are a treatment that needs monitoring

Putting a patient on fluids isn't the end of the fluid plan.

Body weight, hydration, perfusion, respiratory rate and effort, urine output, electrolyte changes and the patient's overall response can all help determine whether the original prescription is still appropriate.

Fluid overload can cause significant complications, particularly in patients with cardiac or renal disease and other conditions that alter their ability to handle fluid administration. Current guidance emphasises reassessing fluid balance regularly and adjusting the prescription as the patient's needs change. AAHA

Keep the calculations somewhere easy to find

Fluid therapy involves plenty of calculations and values that you may know perfectly well — until you're standing beside a sick patient at the end of a busy day trying to remember exactly which formula you wanted.

That's why we made the VetVital IV Fluid Therapy Veterinary Reference Card.

It keeps commonly used fluid therapy information and calculations together in a compact, waterproof card that can stay on your lanyard and be checked when you need it.

SHOP THE IV FLUID THERAPY REFERENCE CARD

VetVital reference cards and resources are designed as quick clinical reference tools for veterinary professionals and students. Fluid therapy should always be tailored to the individual patient using current guidelines, appropriate monitoring and professional clinical judgement.

References:
2024 AAHA Fluid Therapy Guidelines for Dogs and Cats.
Merck Veterinary Manual — Fluid Therapy in Animals.