Section 3: Fluids for Replacement and Maintenance

When developing a replacement or maintenance fluid therapy plan, tailor the fluid type, volume, administration route, and administration rate to each patient.

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Top Three Takeaways

  1. Calculate fluid requirements based on three main phases of fluid therapy: resuscitation, rehydration, and maintenance. The administration route depends on the severity of the patient’s fluid deficit and their ability to take in fluids orally. When possible, enteral routes should be used.
  2. Use replacement fluids, also called isotonic crystalloids, to treat hypovolemia and dehydration, keeping in mind that each condition requires different strategies. Hypovolemic patients require immediate intravascular volume replacement delivered as one or more small IV or intraosseous (IO) fluid boluses over 15–30 min. Dehydrated patients require sustained fluid delivery over 12–24 hr.
  3. Use maintenance fluids, also referred to as hypotonic crystalloids, to provide daily fluid requirements in patients with inadequate fluid intake. Using isotonic crystalloids to meet maintenance fluid needs can lead to electrolyte derangements in patients.

Keep in mind that evaluating a patient’s fluid balance is not a one-time event. As the patient’s clinical status progresses, adjust the fluid prescription to meet ongoing needs, response to therapy, and the course of the disease.

Hypovolemia and dehydration are two related medical conditions that involve a deficiency of fluids in the body and require replacement fluid therapy. Although these conditions share similarities, there are distinct differences between them, and fluid administration strategies differ.

Patients with an intravascular volume deficit, or hypovolemia, require rapid IV fluid replacement. On the other hand, patients with fluid deficits from inadequate fluid intake and ongoing losses, or dehydration, need slow, sustained fluid deficit replacement, allowing the interstitial and intracellular compartments to reabsorb these fluids. Once patients are adequately hydrated and euvolemic, they may only require maintenance fluid therapy if they are unable to maintain fluid homeostasis through oral ingestion.

Managing Hypovolemia

Hypovolemia refers to a decreased volume of circulating blood, which results in reduced tissue perfusion. It occurs when both fluid and electrolytes are lost, leading to a decrease in total blood volume in the circulatory system (Figure 3). Hypovolemia can be caused by various factors, such as excessive bleeding, severe burns, severe diarrhea or vomiting, kidney disease, or inadequate fluid intake. Parameters to detect hypovolemia are listed in Table 2.

Immediate medical attention is crucial for hypovolemic patients because untreated hypovolemia can lead to serious complications. Treatment entails administering IV fluids to restore the blood volume and addressing the underlying cause.

Correct hypovolemia by administering a buffered isotonic fluid bolus of 5–10 mL/kg (cats) and 15–20 mL/kg (dogs) over 15–30 min. The boluses can be repeated if the desired hemodynamic and perfusion goals have not been achieved and the patient remains hypovolemic (Figure 4).

Managing Dehydration

Dehydration is a condition in which the body loses more fluids than it takes in, resulting in an imbalance of water and electrolytes (Figure 5). Inadequate fluid intake, excessive panting in dogs, vomiting, diarrhea, or medical conditions such as diabetes can cause dehydration. Parameters to detect dehydration are listed in Table 3.

Depending on the degree of dehydration, it can usually be managed by replenishing lost fluids through oral rehydration or subcutaneous (SC) fluid administration. IV fluid administration is preferred in severe cases of dehydration or in patients who cannot tolerate oral fluid administration.

Dehydration can be corrected by calculating the fluid deficit (Box 3) based on the degree of dehydration (Table 3) and administering fluid therapy over 12–24 hr (Figure 6).

Figure 3: Hypovolemia
Infographic showing Hypovolemia results in a decreased volume within the vascular space.

Figure 3

Hypovolemia results in a decreased volume within the vascular space. Acute hypovolemia primarily affects this compartment. As the severity and duration of hypovolemia persist, it can affect other compartments as well.

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Figure 4: Treatment of Hypovolemia
Infographic for Treatment of Hypovolemia

Figure 4

Treatment of hypovolemia requires rapidly delivering fluid into the vascular space to restore the effective circulating volume.

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Figure 5: Dehydration and Euvolemia
Figure 5 Infographic of Dehydration and Euvolemia

FIGURE 5

Dehydration results in a decreased volume within the interstitial space.

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Box 3: Calculating the Fluid Deficit

Fluid Deficit (L) = Body Weight (kg) x % Dehydration (as a decimal)


Figure 6: Treatment of Dehydration
Figure 6 Infographic-for Treatment of Dehydration

Figure 6

Treating dehydration requires slow, sustained delivery of intravascular fluids, which will be slowly absorbed into the interstitial space over 12-24 hours. Subcutaneous and oral routes are not depicted; however, these routes also correct dehydration.

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Managing Hypovolemia and Dehydration

In cases in which both hypovolemia and dehydration are present (Figure 7), address hypovolemia first, then rehydration (Figure 8). Both fluid prescriptions should include concrete endpoints to monitor to identify when hypovolemia and dehydration have resolved (Table 7). Additionally, ultrasonographic evaluation of cardiac chambers and the caudal vena cava (Caudal Vena Cava Collapsibility Index) may be used as valid assessments of fluid responsiveness.

Figure 7: Severe Dehydration and Hypovolemia
Figure 7 Infographic of Severe Dehydration and Hypovolemia

Figure 7

Dehydration results in a decreased volume within the interstitial space. As dehydration worsens, it can affect the vascular and intracellular compartments as well, leading to dehydration with concurrent hypovolemia.

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Table 7: Endpoints to Monitor for Hypovolemia and Dehydration
Fluid Status Hypovolemia Dehydration
Initial parameters (See Table 2) (See Table 3)
Initial treatment strategy
  • 5-10 mL/kg (cat), 15-20 mL/kg (dog) of a buffered isotonic fluid over 15 minutes
  • Assess perfusion parameters at the end of each bolus.
  • Calculate replacement volume and deliver over 12–24 hours.
  • Assess patient parameters throughout the fluid delivery period with the goal of correcting the full dehydration deficit within 12–24 hours.
End points
  • Improvement in heart rate, CRT, blood pressure, and mentation
  • Improved skin turgor, mucous membranes, and urine specific gravity and increased body weight and urine output
End point treatment strategy
  • If vitals have returned to normal, then assess if dehydration needs to be addressed and continue with a rehydration fluid plan.
  • If vitals have improved but not normalized, repeat the same or lower-volume bolus and reassess.
  • If end points have returned to normal, then assess if oral ingestion is possible. If not, continue with maintenance fluid plan.
  • If dehydration has not completely resolved, recalculate fluid requirements and deliver over an additional 12–24 hours.

CRT, capillary refill time

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Figure 8: Treatment of Severe Dehydration and Hypovolemia
Figure 8 Infographic Treatment of Severe Dehydration and Hypovolemia

Figure 8

Treatment of severe dehydration and hypovolemia requires a two-fold strategy. First, correct hypovolemia by rapidly delivering intravascular fluids and restoring the effective circulating volume. Once the hypovolemia has resolved, address dehydration with the slow and sustained delivery of intravascular fluids administered over 12-24 hours.

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Selecting Fluids for IV Administration

To prescribe appropriate IV fluid therapy, many factors must be considered, including a patient’s age, current and underlying medical conditions (e.g., renal or cardiac impairment, hypoproteinemia), fluid and electrolyte balance, and other specific needs (see Section 5, Fluid Therapy in Ill Patients). These factors influence the choice of IV fluid type and whether adjustments are needed in the fluid’s composition or administration rate.

Crystalloids

Crystalloid solutions are the most common type of fluid used, and these can be classified as replacement or maintenance solutions. The composition of replacement solutions resembles that of the extracellular fluid (Table 8). Maintenance solutions contain less sodium and more potassium than replacement fluids (Table 8).

Maintenance versus replacement fluids

Using the term “maintenance fluids” to refer to a fluid administration rate is a common misnomer. Instead, the term refers to a classification of crystalloid solutions formulated with different electrolyte concentrations to meet a patient’s daily requirements. Replacement fluids (e.g., lactated Ringer’s solution) are intended to replace lost body fluids and electrolytes (Table 8). Replacement fluids are frequently used interchangeably to meet replacement and maintenance needs— where clinicians supplement replacement fluids with potassium or dextrose to approximate maintenance requirements. Using replacement fluids long term instead of maintenance fluids may predispose patients to sodium derangements and hypokalemia. Although there is no evidence that using replacement fluids as maintenance fluids has short term detrimental effects, it is important to refer to these fluids correctly and ensure that patient maintenance needs (electrolyte composition and volume administered) are properly met.

Sodium concentration

Always consider a patient’s sodium concentration. Dogs have a lower average sodium concentration (~145 mEq/L) compared with that of cats (~155 mEq/L), and pediatric patients may have slightly lower sodium concentrations than adults. Although point-of-care blood analysis facilitates obtaining quick results, it may not always be available in cases in which immediate fluid resuscitation is required. In situations in which sodium concentration is unknown, the best fluid choice is a buffered isotonic fluid. Once the sodium concentration is obtained, adjust the fluid choice to better reflect the patient’s needs (Table 8).

Table 8: Composition of Commonly Used Crystalloids
Osmolarity (mOsm/L) pH Na (mEq/L) Cl (mEq/L) K (mEq/L) Mg (mEq/L) Ca (mEq/L) Dextrose (g/L) Buffers
REPLACEMENT CRYSTALLOID FLUIDS
Hypertonic Crystalloids
3% NaCl 1027 5.0 513 513 0 0 0 0 None
5% NaCl 1711 5.0 856 856 0 0 0 0 None
7.5% NaCl 2566 5.0 1293 1293 0 0 0 0 None
23.4% NaCl 8008 5.0 4004 4004 0 0 0 0 None
Isotonic Crystalloids
0.9% NaCl 308 5.0 154 154 0 0 0 0 None
Plasma-Lyte A 294 7.4 140 98 5 3 0 0 Acetate, Gluconate
Plasma-Lyte 148 294 5.5 140 98 5 3 0 0 Acetate, Gluconate
Normosol R 294 5.5 140 98 5 3 0 0 Acetate, Gluconate
Lactated Ringer’s 275 6.5 130 109 4 0 3 0 Lactate
MAINTENANCE CRYSTALLOID FLUIDS
Hypotonic Crystalloids
Plasma-Lyte 56 in 5% dextrose 363 3.5-6 40 40 13 3 0 50 Acetate
0.45% NaCl 154 5.6 77 77 0 0 0 0 None
0.45 NaCl in 2.5% dextrose 280 4.5 77 77 0 0 0 25 None
5% dextrose in water 252 4.0 0 0 0 0 0 50 None
Normosol M in 5% dextrose 363 5.0 40 40 13 3 0 50 None
  1. Rudloff E, Hopper K. 2021. Crystalloid and colloid compositions and their impact. Frontiers in Veterinary Science. 8:639848.
  2. Strandvik GF. 2009. Hypertonic saline in critical care: a review of the literature and guidelines for use in hypotensive states and raised intracranial pressure. Anaesthesia. 64(9):990-1003
  3. Holden D, et al. 2023. Hypertonic saline use in neurocritical care for treating cerebral edema: A review of optimal formulation, dosing, safety, administration and storage. American Journal of Health-System Pharmacy. 80(6):331-342.
  4. Carr CJ, et al. 2021. An audit and comparison of pH, measured concentration, and particulate matter in mannitol and hypertonic saline solutions. Frontiers in Neurology. 12:667842.

Download Table 8 PDF


Calculating Fluid Requirements

Divide the fluid therapy plan into resuscitation, rehydration, and maintenance rates (Table 9) as follows:


Total Fluid Requirement = resuscitation rate
+ rehydration rate (include ongoing losses)
+ maintenance rate.


Several formulas are available to calculate fluid requirements, and there is no evidence that one is superior to another. Regardless of the formula used, customize the fluid plan to each patient, and adjust it based on patient monitoring findings and ongoing losses.

Table 9: Fluid Therapy Dosing According to Stage of Fluid Requirements
Stage Formula Rate of Administration Comments
Resuscitation Cat: 5–10 mL/kg
Dog: 15–20 mL/kg
15 min Assess perfusion parameters after bolus.
May repeat bolus as needed.
Rehydration Total fluid deficit (L) = Body weight (kg) × % Dehydration (as a decimal) Over 12–24 hr Ongoing losses should be assessed through inputs and outputs and incorporated into the fluid plan.
Maintenance Dog:
a. 60 mL/kg/day
b. 132 × BW (kg)0.75
c. 30 × BW (kg) + 70 = mL/kg/day
Cat:
a. 40 mL/kg/day
b. 80 × BW (kg)0.75
c. 30 × BW (kg) + 70 = mL/kg/day
Pediatric:
Dog: 3 × adult dose
Cat: 2.5 × adult dose
Over 24 hr Also incorporate enteral water, liquid diets, and IV medications into the total volume of the fluid plan.

BW, body weight

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Selecting Fluid Administration Routes

The choice of fluid administration route depends on the severity of the fluid deficit and the patient’s ability to take fluids orally or via a feeding tube. Hypovolemia always requires IV or IO fluid delivery. However, dehydration can be corrected through IV, SC, or enteral fluid administration, or a combination of these routes.

Selecting IV and IO Routes

The IV and IO administration routes are preferred in patients with severe fluid deficits, acute fluid losses, and perfusion deficits, as well as in patients with minimal or no oral or enteral fluid intake. These routes allow for rapid dispersion of fluid and electrolytes, ensuring precise dosage and safe delivery of large fluid volumes and hypertonic fluids.

The IO route is typically used in patients for whom IV access is not possible and serves as a bridge for resuscitation until an IV catheter can be placed. However, IO infusion rates are more limited than IV infusion rates; for example, the humerus and femur sites allow IO catheter infusion rates of up to 1 mL/kg/min.

Selecting the SC Route

The SC route is preferred for outpatient fluid therapy or for patients receiving fluids via multiple routes (e.g., IV fluids during day hospitalization and SC fluids during the night). However, evidence-based information is lacking regarding ideal patient selection for SC fluid therapy, the optimal SC infusion volume and treatment frequency, and the possible adverse effects of SC fluids. Table 10 provides empirical recommendations for SC fluid therapy.

To estimate the patient’s percent dehydration and calculate maintenance fluid requirements, follow the recommendations outlined above. It is important to avoid prescribing SC fluids for euhydrated patients because there is no evidence that such therapy is beneficial and it may be detrimental in patients that have challenges in body fluid homeostasis (e.g., underlying cardiac disease and hypoproteinemia). Use a new fluid administration set and fluid bag for each individual patient.

Selecting Enteral Administration

Whenever possible, use the enteral administration route. Enteral fluids are often underused and should be an integral part of the fluid therapy plan if patients can tolerate oral liquids. Placement of feeding tubes can aid in providing enteral fluids in anorexic patients and should be considered as part of an overall fluid therapy plan.

Enteral fluids can be used exclusively to correct mild dehydration in patients with inadequate fluid intake or to supplement parenteral administration routes. First, estimate the patient’s percent dehydration and then calculate maintenance fluid requirements as usual. Liquid
diets and oral water can be delivered through voluntary intake or feeding tubes (e.g., nasogastric and esophagostomy). Enteral administration
may be particularly useful in dehydrated patients who cannot tolerate IV fluids (i.e., patients with cardiac disease or hypoproteinemia). If a feeding tube is present, deliver enteral fluids as intermittent boluses or as a continuous infusion with a fluid pump. Take care to
label oral fluids appropriately to avoid connection errors that may
lead to accidental administration of water or a liquid diet into a
patient’s vascular space.

Table 10: Empirical Subcutaneous Fluid Therapy Recommendations
Subcutaneous Fluid Dose Frequency Type of Fluid Comments
20–30 mL/kg Once or twice a day • Lactated Ringer’s, Plasma-Lyte, or Normosol R
• 0.9% NaCl has a low pH and may be painful. Avoid SC use.
• Deliver to multiple sites depending on volume and skin elasticity. Maximum amount is 10–20 mL/kg per site.

SC, subcutaneous

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Citations
  1. Tello LH, Pardo MA. Fluid and electrolyte therapy during vomiting and diarrhea. Vet Clin North Am Small Anim Pract 2022;52(3):673–88.
  2. Harris BA, Hofmeister EH, Gicking JC. A survey of emergency and critical care veterinarians regarding IV fluid bolus therapy and monitoring practices in small animals. J Vet Emerg Crit Care (San Antonio) 2021;31(5):564–73.
  3. DiBartola SP, Bateman S. Introduction to fluid therapy. In: DiBartola SP, ed. Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice. 4th ed. St. Louis: Elsevier Saunders; 2012:331–50.
  4. Hansen B, Vigani A.Maintenance fluid therapy: isotonic versus hypotonic solutions. Vet Clin North AmSmall AnimPract 2017;47(2):383–95.
  5. Wellman ML, DiBartola SP, Kohn CW. Applied physiology of body fluids in dogs and cats. In: DiBartola SP, ed. Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice. 4th ed. St. Louis: Elsevier Saunders; 2012:15.
  6. Hopper K, Garcia Rojas A, Barter L. An online survey of small animal veterinarians regarding current fluid therapy practices in dogs and cats. J Am VetMed Assoc 2018;252(5):553–9.
  7. Mazzaferro E, Powell LL. Fluid therapy for the emergent small animal patient: crystalloids, colloids, and albumin products. Vet Clin North Am Small Anim Pract 2022;52(3):781–96.
  8. Lange J, Boysen SR, Bentley A, et al. Intraosseous catheter flow rates and ease of placement at various sites in canine cadavers. Front Vet Sci 2019;(19):6:312.
  9. Hopper K, Garcia Rojas A, Barter L. An online survey of small animal veterinarians regarding current fluid therapy practices in dogs and cats. J Am VetMed Assoc 2018;252(5):553–9.
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