Intravenous fluids are used to treat a number of different conditions, including shock, hypovolemia, and dehydration. In general small animal practice, however, one of the most common use of IV fluids is to treat dehydration or ongoing fluid losses in a hospitalized patient. 

Providing optimal fluid therapy requires careful selection of fluid type, as well as determining an appropriate fluid rate. Additionally, it’s important to be aware of the risks that can accompany IV fluid therapy, so that you can minimize the likelihood of complications and detect complications promptly if they do occur.

Selecting Appropriate Fluids

Fluids can be divided into a number of different categories, depending on their composition and intended use. The primary distinctions made in IV fluid therapy include balanced vs. unbalanced fluid solutions and crystalloid vs. colloid fluids. 

Balanced fluids (for example, lactated Ringer’s solution) have a composition that resembles the body’s extracellular fluid, while unbalanced fluids (such as normal saline) do not. Therefore, balanced fluids are often used to correct dehydration and/or to provide maintenance fluid therapy, while unbalanced fluids are typically reserved for specific medical situations. 

Colloids contain high-molecular-weight substances that will remain in the circulation (such as hetastarch), while crystalloids (such as 0.9% saline, lactated Ringer’s solution, etc.) contain electrolytes and other solutes that are capable of leaving the circulation to enter other body fluid compartments. Therefore, colloids are typically used when emergency circulatory support is necessary, such as in cases of hypovolemia or hemorrhagic shock. Crystalloids are more commonly used for dehydration and/or maintenance fluid therapy. 

Most veterinary clinics stock three different fluid types, which are used on the majority of cases requiring IV fluids: 

• Replacement solution (lactated Ringer’s solution, Normosol-R, or Plasmalyte)
• 0.9% saline (NaCl)
• 5% dextrose in water 

When selecting which type of fluid to administer to a given patient, it’s important to take into account the patient’s electrolyte and acid-base status. In general, fluid losses should be replaced with a fluid that is similar in composition to what was lost. 

Replacement solutions are typically the first choice for most patients. The composition of replacement fluids is very similar to the composition of the body’s extracellular fluids. 

In patients with hypochloremia, hypercalcemia, or hypernatremia, 0.9% NaCl may be selected. Dextrose (5%) in water is also a good option for pets with hypernatremia, or pets with heart disease or other conditions that may make them more susceptible to problems associated with hypernatremia. 

Supplemental potassium is often added to IV fluids, in order to prevent hypokalemia. Potassium supplementation should be based on bloodwork findings. 

Calculating a Fluid Rate

When calculating a fluid rate for your patient, there are three key factors that you must take into account: 

• Maintenance: fluids required to maintain a pet at its current status
• Rehydration: fluids required to correct dehydration
• Replace ongoing losses: fluids required to offset vomiting, diarrhea, or other losses

Let’s walk through these calculations with an example. Imagine that a 10 kg dog presents for vomiting and diarrhea associated with pancreatitis. The dog is 8% dehydrated and you estimate that the dog is losing approximately 200 ml/day of fluid through vomiting and diarrhea. What fluid rate should be administered for the first 24 hours of hospitalization? 

Calculate the patient’s maintenance fluid rate.

The normal maintenance fluid rate for small animal patients is 50-60 ml/kg/day. Therefore, you multiply 10 kg by 60 ml/kg/day and determine that the patient needs 600 ml/day of IV fluids to meet maintenance requirements. 

Calculate the fluid volume needed for rehydration. 

In your physical exam, you determined that the dog is 8% dehydrated. Therefore, you multiply 10 kg (the dog’s weight) by 0.08 (the percent dehydration) and determine that the dog has a fluid deficit of 0.8 kg. Because 1 kg of fluid equals 1 L of fluid, you now know that the dog has a fluid deficit of 0.8 L, or 800 mL. 

Typically, we aim to correct fluid deficits over the course of the first 24 hours of hospitalization. Therefore, this patient should receive an additional 800 mL of IV fluids during the first 24 hours of hospitalization, to correct dehydration.

Calculate ongoing losses. 

When a patient first comes into the hospital, you may only be able to guess at the amount of ongoing fluid losses the patient is experiencing. Therefore, it’s important to monitor the pet closely during hospitalization so that you can determine whether adjustments are needed to this component of your fluid therapy. 

In this example, you estimate that the dog is losing approximately 200 ml/day of fluids through vomiting and diarrhea. 

Finally, add these all together. 

From the calculations above, you determined that the dog’s fluid needs include: 

• Maintenance: 600 ml/day
• Rehydration: 800 ml in the first 24 hours
• Ongoing losses: 200 ml/day 

Adding these values together tells you that the dog requires 1600 ml of IV fluids during the first 24 hours of hospitalization. Divide 1600 ml/day by 24 hours/day, to obtain the dog’s initial fluid rate of 66.7 ml/hr. 

It is important to monitor hospitalized patients closely, as fluid requirements often decrease rapidly. After the first 24 hours of hospitalization, the pet’s dehydration will likely be corrected and the fluid rate can be decreased to 800 ml/day or 33 ml/hr. As vomiting and diarrhea are controlled with medication, the necessary fluid rate will further decrease to 600 ml/day or 25 ml/hr, prior to eventual discharge.

Potential Risks of Fluid Therapy 

While fluid therapy is an essential component of treating many patients, it is not completely risk-free! It’s important to be aware of the risks associated with fluid therapy, so that you can plan for these risks and monitor patients appropriately. 

Pulmonary edema is the most common and significant effect of IV fluid overload. This is more likely to occur in pets with heart disease, although it can occur in any patient if fluid rates are too high. Additionally, increased vascular permeability (associated with some disease states) can also increase the risk of pulmonary edema. All pets receiving IV fluid therapy should be carefully monitored for signs of pulmonary edema, including the monitoring of resting respiratory rate and periodic thoracic auscultation.

Electrolyte abnormalities may also occur as a result of fluid therapy. Commercially-available fluid replacement solutions, such as lactated Ringer’s solution, are relatively high in sodium. This may pose risks in pets receiving extremely high rates of IV fluids. Rapid sodium shifts may cause neurologic effects, such as seizures and/or obtundation. 

Phlebitis is also a potential complication of IV fluid therapy, especially when hyperosmolar fluids are utilized. Catheter sites should be checked regularly in all patients.