Top Three Takeaways

1. Fluids are drugs and must be prescribed accordingly to achieve the desired therapeutic goals promptly and minimize complications.

2. Each body fluid compartment—intracellular, interstitial, and intravascular—may require a different fluid prescription tailored to a patient’s individual needs.

3. Arbitrarily assigning a fluid rate or dose can contribute to patient morbidity and mortality and lead to missed fluid therapy goals.

Overview

Fluid therapy involves administering prescribed fluids to restore a patient’s body fluid homeostasis. As with any medication, the pharmacokinetics and pharmacodynamics of each fluid must be considered to attain therapeutic goals and minimize complications. However, fluid therapy alone cannot fix every abnormality, and using a standardized fluid rate for all patients can result in patient morbidity (see Box 1). To prescribe effective fluid therapy, veterinarians must have a basic understanding of the body’s fluid compartments and how water is distributed among them.

Fluid Distribution and Flow Among the Three Primary Fluid Compartments

The three main body compartments in mammals contain water. The intracellular and extracellular fluid compartments contain 67% and 33% total body water, respectively (Figure 1), and these sections are separated by cell membranes. The extracellular fluid compartment is further divided into interstitial (25% of the total body water, or 75% of the extracellular body water) and vascular (8% of the total body water, or 25% of the extracellular body water) compartments, and capillary walls separate these spaces (Figure 1).

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Fluid intake by any route can affect the body fluid compartments. Administered fluids move between compartments based on:

• Tonicity of the fluid

• Tonicity of the patient’s extracellular compartment

• Size of any macromolecules in the administered fluid

Sodium is the most abundant cation in the extracellular fluid compartment and is the most important molecule that supports extracellular tonicity. A fluid given IV that contains a sodium concentration similar to that of the extracellular fluid compartment will redistribute within 45 min based on a compartment’s percent total body water; i.e., in a normal animal, 25% of the administered fluid will remain in the intravascular space and 75% will move into the interstitial space. Fluid movement across the endothelial membrane depends on the contents of the administered fluid and the condition of the patient’s capillary membrane. The modified Starling hypothesis describes how fluid moves across the capillary membrane (Figure 2). Hydrostatic pressure, colloid osmotic pressure, and vascular permeability influence fluid movement.^1,2

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When increased capillary membrane permeability, elevated intravascular hydrostatic pressure, or decreased plasma colloid osmotic pressure occurs, more isotonic fluid can pass into the interstitium or body cavity and cause tissue edema, effusion, or both.

Administering a hypertonic fluid IV will cause water to move from the interstitial and intracellular spaces into the intravascular space. This can be desirable for rapid intravascular volume resuscitation. However, for this strategy to succeed, the interstitial and intracellular spaces must already be adequately hydrated. When administered IV, a hypotonic fluid will cause water to move from the extracellular space into the intracellular space, which is a suitable approach when treating a solute-free water deficit.

Goal-Directed Fluid Therapy

Goal-directed fluid therapy requires creating a fluid prescription that replaces fluid deficits that may exist in each fluid compartment, using the following steps:

1. Recognize which fluid compartment deficit(s) exists.

2. Understand which fluid type and administration route will best replace each deficit.

3. Calculate the fluid dose and administration rate.

4. Monitor patients for response to therapy and signs of complications.^3–5

Assessing Patients: General Principles Before and During Fluid Therapy

The extracellular fluid compartment (i.e., the vascular and interstitial spaces) must have adequate volume before intracellular fluid compartment deficits can be addressed. Therefore, assess and address alterations in volume homeostasis in the following order:

1. Assess the intravascular fluid space by evaluating:

   • Patient history

   • Perfusion parameters (mentation, heart rate, capillary refill time, mucous membrane color, extremity temperature, skin turgor, and pulse quality)

   • Monitored parameters (blood pressure, electrocardiogram findings)

   • Laboratory test results (Table 1)

   • Diagnostic imaging findings (Tables 1, 2)

2. Assess the interstitial space by evaluating:

   • Patient hydration parameters (Tables 3–5)

3. Assess the intracellular space by evaluating:

   • Patient sodium concentration

   • Solute-free water deficit (FWD) (Box 2)

TABLE 1 Intravascular Volume Assessment

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TABLE 2 Stages and Clinical Signs of Hypovolemic Shock

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TABLE 3 Estimated Interstitial Dehydration (%) Based on Physical Examination Findings^a

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TABLE 4 Extracellular Hydration Status Assessment Parameters and Expected Changes from Baseline in Patients Receiving Hypo- or Over-hydration^a

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TABLE 5 Additional Clinical and Diagnostic Findings That May Indicate Overhydration/Fluid Overload

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Replace deficits and monitor response

To replace extracellular fluid space deficits (i.e., vascular and interstitial fluid space deficits):

• Administer a buffered isotonic crystalloid fluid that contains a sodium concentration similar to the patient’s.

• For rapid intravascular volume replacement, a hypertonic crystalloid, a colloid solution, or both can also be used.⁴

• Closely monitor patient parameters until fluid homeostasis is achieved and maintained (Tables 1, 3–5).

• Monitoring may also be achieved by assessing the relative variation in the caudal vena cava during one respiratory cycle using ultrasonography and calculating the Caudal Vena Cava Collapsibility Index.^6,7 See Table 6 for some conditions that pose additional challenges in addressing individual fluid compartment needs. For more information on addressing fluid therapy challenges, see Section 5, Fluid Therapy in Ill Patients.

TABLE 6 Conditions That Pose Challenges When Addressing Individual Fluid Compartment Needs

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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.

Overview

When developing a replacement or maintenance fluid therapy plan, tailor the fluid type, volume, administration route, and administration rate to each patient. 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.⁸

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.

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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).

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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.

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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).

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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.⁹

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TABLE 7 Endpoints to Monitor for Hypovolemia and Dehydration

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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).¹⁰

TABLE 8 Composition of Commonly Used Crystalloids^1,2–4

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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.¹¹ 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),¹² 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).

Calculating Fluid Requirements

Divide the fluid therapy plan into resuscitation, rehydration, and maintenance rates (Table 9) as follows:$$\mathit{\text{Total}} \mathit{\text{Fluid}} \mathit{\text{Requirement}} = \mathit{\text{resuscitation}} \mathit{\text{rate}} \hspace{1em}+ \mathit{\text{rehydration}} \mathit{\text{rate}} \left(\mathit{\text{include}} \mathit{\text{ongoing}} \mathit{\text{losses}}\right) \hspace{1em}+ \mathit{\text{maintenance}} \mathit{\text{rate}}.$$

TABLE 9 Fluid Therapy Dosing According to Stage of Fluid Requirements

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Several formulas are available to calculate fluid requirements,¹³ and there is no evidence that one is superior to another.¹⁴ Regardless of the formula used, customize the fluid plan to each patient, and adjust it based on patient monitoring findings and ongoing losses.

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 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.¹³Table 10 provides empirical recommendations for SC fluid therapy.

TABLE 10 Empirical Subcutaneous Fluid Therapy Recommendations

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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.

Top Three Takeaways

1. Most healthy animals undergoing elective surgery do not require fluids in the postoperative period. Instead, early return to eating and drinking is recommended.

2. Patients who have not been eating before anesthesia may need postoperative fluid therapy until they can voluntarily consume enough to meet their needs. For patients who are not expected to eat well after surgery, such as geriatric cats, SC fluids can be considered for use at home.

3. In patients with renal disease (specifically, those in International Renal Interest Society stage 3 or 4), do not attempt to rectify hypotension by using excessive fluid infusion rates.

Overview

Fluid therapy is recommended in patients undergoing general anesthesia primarily to counteract the vasodilation and decreased cardiac output induced by inhaled anesthetics, as well as to uphold catheter patency. Before the publication of the 2013 AAHA/AAFP Fluid Therapy Guidelines for Dogs and Cats, the recommended fluid administration rate during anesthesia was 10 mL/kg/hr, lacking a foundation of evidence.¹⁶ However, excessively high-volume fluid rates predispose anesthetized patients to an increased risk of volume overload and its associated consequences. The 2013 guidelines recommended reducing fluid rates in anesthetized patients to 5 mL/kg/hr in dogs and 3 mL/kg/hr in cats.¹⁷ Although these reduced administration rates have not undergone formal study, they have been widely accepted and implemented in clinical practice.

Goals for Fluid Therapy in Anesthetized Patients

The following section covers important facets of fluid administration before, during, and after anesthesia, including mechanisms to address hypotension and avoid fluid overload. (For more information on the anesthetized patient, see the 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats, available at aaha.org.)

1. Consider potential fluid deficits (e.g., losses from dehydration, fasting, insensible losses, or anticipated fluid losses during surgery).¹⁸ Whenever feasible, aim to correct 80% of a patient’s dehydration deficit within the 24 hr before anesthesia.

2. For most patients, it is unnecessary to withhold water before anesthesia.

3. Place IV catheters in all patients undergoing anesthesia.

4. Administer balanced isotonic crystalloid fluids using the following guidelines:

   1. Initial fluid rate of 5 mL/kg/hr in dogs with normal cardiac and renal function.

   2. Initial fluid rate of 3–5 mL/kg/hr in cats with normal cardiac and renal function.

5. IV fluids can be beneficial for maintaining catheter patency and supporting cardiovascular function. However, euhydrated, euvolemic patients receiving injectable anesthetics for short periods generally do not require IV fluids. Food and water should be offered as soon as possible after recovery, but SC fluids can be given to patients who do not return to eating and drinking immediately. Prioritize the correction of hypovolemia and dehydration before general anesthesia whenever possible.

6. To ensure proper tissue perfusion, maintain a minimum mean arterial blood pressure of 60 mm Hg.

Managing Hypotension

Hypotension is a common complication of general anesthesia involving inhaled anesthetics (Figure 9).^19–21 Both absolute hypovolemia (e.g., due to hemorrhage) and relative hypovolemia (e.g., stemming from trauma, sepsis, or the use of anesthetics and vasodilatory drugs) can also contribute to hypotension. In cases of hypotension during anesthesia, begin by adjusting excessive vaporizer settings and judiciously administering crystalloid fluids.

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1. Assess every patient’s anesthetic depth carefully. Small reductions in inhalant anesthetic administration (i.e., vaporizer settings) can substantially affect blood pressure and make the difference between normotension and hypotension.

2. Monitor the patient’s heart rate. Bradycardia can contribute to hypotension in anesthetized patients. If the heart rate is lower than normal and the patient is hypotensive, consider anticholinergic therapy.

3. Evaluate body temperature. Decreased body temperature or hypothermia can cause hypotension. Open body cavities and breathing in cold gases from the anesthesia machine can both lead to a decrease in body temperature. Increasing body temperature will help improve hypotension.

4. Consider the concurrent administration of additional analgesics, sedatives, or a combination of both to help reduce the requirements for inhalant anesthetics requirements, as well as the use of regional or local anesthetics.

5. Administer balanced isotonic crystalloid fluids when using inhalant anesthetics:

   1. 5 mL/kg/hr for dogs

   2. 3–5 mL/kg/hr for cats

6. If hypotension persists despite adjusting vaporizer settings and providing crystalloid fluids, then consider sympathomimetic drug therapy with inotropes or vasopressors or colloid fluid therapy.

7. If hypotension is due to severe or ongoing hemorrhage, then transfusion of whole blood or packed red blood cells is necessary to maintain both blood volume and appropriate red blood cell mass to deliver oxygen to tissues.

8. Keep in mind that not all cases of hypotension can be corrected with fluid administration, particularly in pediatric patients and those with cardiac disease, sepsis, etc. (see Section 5, Cardiorenal Disorders, Fluid Therapy in Ill Patients).

9. Remember that both hypovolemia and hypervolemia are detrimental to anesthetized patients.

Monitoring

1. Monitor the duration of anesthesia and total volume of administered fluids closely. If fluid rates surpass 20 mL/kg within a single anesthetic episode, reevaluate both fluid administration rates and the intravascular volume status of the patient. Typically, most healthy animals would not require a maintenance rate of 5 mL/kg/hr for extended periods, unless significant blood loss occurs. To determine the total fluid administration during a single anesthetic event, calculate the daily maintenance rate volume (Table 9).

2. Monitor anesthetized patients carefully to detect any indications of excessive fluid administration. Signs of fluid overload include (see Table 5 for additional clinical and diagnostic signs):

   1. Gallop sound or new murmur (especially in cats)

   2. Edematous tissues and chemosis

   3. Swelling of paws

   4. Clear nasal discharge (nasal edema)

   5. Pulmonary crackles

   6. Low oxygen saturation (SpO₂)

   7. No alteration in blood pressure along with other clinical signs (i.e., patients remain nonhypertensive)

   8. Pleural effusion, ascites

3. Stop fluid administration (or use a minimal volume to maintain catheter patency) if patients have signs of excessive fluid administration. Furosemide (1–2 mg/kg IV) may be needed if patients have signs of pulmonary edema (i.e., audible pulmonary crackles, imaging evidence of pulmonary edema, or low SpO₂) or pleural effusion.

4. For anesthetized patients subjected to positive-pressure mechanical ventilation, consider the use of a pulse pressure variability monitor^22,23 or plethysmographic variability index from advanced pulse oximetry to assess fluid responsiveness.^24,25 The monitor is used in a similar fashion to a pulse oximeter and helps evaluate whether cardiac output increases with volume expansion.

5. Return routine surgical patients to normal eating and drinking as soon as possible after anesthesia.

Patients with Renal Disease

To effectively care for patients with renal disease (specifically, those in International Renal Interest Society stage 3 or 4), it is crucial to correct dehydration before anesthesia, optimize cardiac output by using an appropriate anesthetic protocol that supports cardiovascular function (avoid dexmedetomidine if other alternatives are available), and closely monitor and manage blood pressure. Attempting to rectify hypotension by using excessive fluid infusion rates should be avoided.

Top Three Takeaways

1. Do not forget about enteral routes of fluid delivery, including nasogastric, nasoesophageal, or esophageal, when treating sick patients. If tolerated by the patient, water can be mixed with food or administered separately.

2. Do not withhold fluids in patients who are dehydrated or hypovolemic because of concurrent anemia. Monitor them closely to determine whether a transfusion is also indicated.

3. Mitigate electrolyte derangements carefully. Never administer a bolus of fluids supplemented with potassium chloride (KCl). Thoroughly mix the fluids to ensure even dispersion of the KCl before administration. Intentionally manage sodium derangements to avoid potential life-threatening fluid shifts in the brain that can occur with rapid resolution of chronic (>24–48 hr) sodium alterations.

Overview

Fluid therapy in sick patients requires a cautious, balanced approach and the ability to predict problems before they occur. The veterinary team faces complex challenges in fluid therapy when treating patients who present with conditions such as gastrointestinal, renal, or cardiac disease, anemia, electrolyte imbalances, traumatic brain injury (TBI), hypovolemic or vasodilatory shock, edema, thermoregulation disorders, and hypoglycemia.

Nutritional Therapy as a Prompt for Enteral Fluid Therapy

Nutrition is one of the most neglected patient requirements during hospitalization.²⁶ After 72 hr of anorexia, a patient’s metabolism shifts to alternate energy sources, such as ketones and fatty acids, instead of glycogen and glucose.²⁷ Nasoesophageal, nasogastric, or esophageal feeding tubes facilitate caloric intake,²⁸ and water can be mixed with food or administered separately.²⁹ Use a canned diet to increase water intake.

The stomach’s capacity is 5–10 mL/kg³⁰ at the time of starting enteral nutrition, and no consensus on canine and feline gastric emptying times exists (although prolonged emptying times have been reported).³¹ Use conservative calculations of fluid and food requirements for enteral nutrition and adjust based on a patient’s clinical signs of nausea, regurgitation, and vomiting.

Determine enteral water requirements based on daily maintenance rates and divide this amount between IV and enteral supplementation.³² It is important to continue to provide free access to water. Given that enteral nutrition rates typically start at one-third the resting energy requirement³³ to avoid refeeding syndrome, enteral water administration may also aid in allowing the stomach capacity to accommodate increased volumes of subsequent enteral nutrition feedings.

Anemia

Do not withhold fluids in anemic patients. If an anemic patient is also dehydrated or hypovolemic, provide fluid therapy while recognizing that patients with a low hematocrit may require blood products. In healthy patients, fluid resuscitation has not been shown to decrease hemoglobin concentrations.³⁴ Fluid therapy may result in a beneficial increase in microvascular flow and perfusion with an overall increase in oxygen delivery in patients with hypovolemic or distributive shock. However, fluid administration in non–fluid responders or fluid-overloaded patients can lead to a relative, but not absolute, reduction in hemoglobin concentration (“dilutional anemia”), which can cause a paradoxical decrease in oxygen delivery.

Carefully monitor anemic patients and thoroughly assess them for shock, hypovolemia, dehydration, and need for maintenance fluids. These needs should be addressed through an appropriate fluid prescription. These patients may become transfusion dependent when appropriately resuscitated or rehydrated. Transfusion triggers (e.g., heart rate, mucus membrane color, capillary refill time, respiratory rate and effort, pulse quality, blood pressure, mentation, and attitude) should be considered in conjunction with laboratory test results (e.g., packed red blood cell count and trends, hematocrit, hemoglobin levels, and blood lactate) to determine the need for a blood transfusion.^35,36

Azotemia

Azotemic patients have varying fluid requirements that depend on factors such as their hydration status (including both dehydration and fluid overload), urine production levels, acute versus chronic onset, acid-base and electrolyte status, the underlying cause of the azotemia, and the extent of its severity.³⁷ These patients present special monitoring challenges as they might experience xerostomia (dry mouth) secondary to uremia,³⁸ prolonged skin tenting due to reduced skin elasticity associated with aging, concentrated retained urine, and inaccurate relative creatinine levels due to decreased muscle mass.³⁹ Additionally, there may be a lack of a baseline creatinine concentration or previous body weight for comparison. Anecdotal techniques to assess dehydration include evaluating skin turgor over the rib cage, analyzing weight trends and sodium concentrations, fluid and food intake history, and identifying signs of fluid overload (see Section 6, Fluid Overload).

In cases in which patients are not hypotensive, the fluid prescription should provide a gradual correction of dehydration, whereas patients with significant renal compromise should receive fluids at slower rates. Fluid therapy is not the mainstay of treating azotemic patients. Rather, it is to support the kidneys by correcting treatable abnormalities associated with renal compromise so that the kidneys can heal themselves. Key aspects entail reducing sodium and chloride load,⁴⁰ managing blood pressure, treating anemia^41,42 and infections, ensuring adequate short-term nutrition (without protein restriction), and addressing primary conditions that may trigger secondary AKI (e.g., acute pancreatitis)⁴³ or acute-on-chronic kidney disease presentations. Fluid requirements for patients with chronic kidney disease vary depending on the severity of polyuria and polydipsia along with other clinical signs.⁴⁴

Heart Disease

The most important consideration for fluid therapy in patients with cardiac disease is to prevent the onset of heart failure.⁴⁵ For cardiac patients, therapeutic goals include increasing myocardial contractility, decreasing preload and afterload, counteracting the pathological effects of the renin-angiotensin-aldosterone system, improving vasodilation, and optimizing diastolic filling.^46,47

Cardiac patients may experience dehydration, relative hypovolemia, electrolyte disturbances, moderate to severe azotemia, and metabolic imbalances caused by heart failure and medications. Fluid therapy is frequently avoided in cardiac patients owing to its potential to increase preload in left-sided heart failure, increase afterload, and decrease venous return in right-sided heart failure (especially in the presence of increased intra-abdominal pressure from ascites).^48–50 Whenever possible, fluid intake should be provided enterally, such as through water and a canned diet. When fluid therapy is necessary, administer 0.45% NaCl with 2.5% dextrose IV at half to daily maintenance rates (See Table 9 for maintenance rates), depending on the patient’s needs and tolerance of supplemental fluids.⁵¹ Hypotension in patients with congestive heart failure should be addressed by considering positive inotropes.

Cardiorenal Disorders

The cardiorenal axis is an important consideration⁵¹ because a pathological state in either the cardiovascular or renal system has the potential to affect the other.⁵² Renal disease treatment focuses on maintaining hydration with enteral water, so less conflict exists with heart disease treatment; however, the administration of additional fluid therapy presents challenges. Cardiac medications may potentially lead to mild azotemia through diuresis (e.g., furosemide)⁵³ and decreased glomerular filtration rate (GFR) (e.g., enalapril and telmisartan).⁵⁴ It is crucial to consistently monitor for fluid overload, severe progressive azotemia, worsening of heart failure, changes in blood pressure, and other relevant indicators to ensure that these patients do not decline as a result of fluid therapy. Even with the best possible therapy, this can be a significant challenge.

Patients with Hypovolemia and Edema

Alterations in Starling’s forces (hydrostatic and oncotic pressure) (Figure 2) may contribute to the development of edema in veterinary patients. Common causes of edema include vasculitis, hypoalbuminemia, heart failure, kidney failure, lymphatic obstruction, and thrombosis.⁵⁵ Fluid therapy in hypovolemic, edematous patients presents a therapeutic challenge, and the underlying cause of the edema must be taken into consideration in order to safely provide fluids.

When edema results from hypoalbuminemia, intravascular volume resuscitation and restoration are paramount, and colloids can be used to raise the colloid oncotic pressure. This can be accomplished with rapid administration of synthetic colloids (e.g., hetastarch and tetrastarch [although their use is controversial; see Section 7, Questions and Controversies in Fluid Therapy]), canine-specific albumin (CSA) in dogs, human serum albumin in dogs or cats (which may predispose patients to allergic reactions or anaphylaxis), and veterinary plasma products (e.g., fresh frozen plasma, frozen plasma, and cryoprecipitate-poor plasma [CPP]).

Although plasma products are less prone to induce allergic reactions, large plasma volumes are needed to significantly change albumin concentrations (∼22 mL/kg plasma to increase serum albumin by 0.5 g/dL), which results in higher costs of care and risk of volume overload. In one study, there was no difference in mean serum albumin concentrations before and after transfusion with fresh frozen plasma (median dose of 15–18 mL/kg).⁵⁶ Thus, although a plasma bolus may be used to treat hypovolemia in edematous patients, its use for this purpose is controversial.⁵⁷

A continuous rate infusion (CRI) of canine CPP, administered at a rate of 1.1–2.2 mL/kg/hr, has been described in a case report for treating hypoalbuminemia.⁵⁸ This may be a more reasonable approach to treating hypoalbuminemia because CPP has a higher albumin concentration than other plasma products,⁵⁹ although CPP is less widely available.⁵⁸

CSA appears to be a relatively safe alternative to synthetic colloids and increases albumin concentrations more efficiently than plasma products.^60,61 In one study, CSA administration improved the shock index in hypovolemic canine patients.⁶¹ In another study, CSA increased arterial blood pressure within 2 hr of administration in dogs with septic peritonitis.⁶⁰

Giving colloids to patients with edema due to vasculitis may be controversial, as colloids may leak into the interstitial space and worsen interstitial edema. In these patients, judicious colloid use combined with lower volumes of crystalloids is recommended. Overall, regardless of the cause of edema, use crystalloids judiciously in all affected patients.

Low-dose diuretics may be considered in patients with edema, but only in normotensive, normovolemic patients.

Traumatic Brain Injury

The primary fluid therapy goal in treating veterinary patients who have TBI is optimizing cerebral perfusion pressure and mean arterial blood pressure. The Brain Trauma Foundation guidelines for human patients recommend maintaining systolic arterial blood pressure between 100 and 110 mm Hg to reduce mortality and improve outcomes.⁶²

Studies that evaluate the ideal fluid choice for veterinary patients with TBI are limited. However, information based on human clinical studies and swine and rodent research studies supports the use of packed red blood cells, plasma, and platelets over crystalloid fluids because of better outcomes in patients with ongoing hemorrhage.^63,64

Osmotherapy—using osmotic agents (e.g., hypertonic saline or mannitol) to reduce intracranial content volume—is common in treating patients who have TBI (Figure 10). Various studies and a meta-analysis in human medicine suggest that both mannitol and hypertonic saline effectively lower intracranial pressure, but there is no evidence to recommend one over the other.^65–67 Hypertonic saline may have the advantage of avoiding diuresis, increasing cardiac preload, and positively impacting cerebral perfusion.⁶⁵ In dogs and cats, mannitol is usually dosed at 0.5–1 g/kg IV given over 15 min with a microfilter, whereas hypertonic saline (NaCl 7.2%) is dosed at 1–6 mL/kg IV given over 15 min. Continuous infusion of hypertonic saline has been described in human patients, but it is not thought to significantly affect outcome.⁶⁸

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Electrolyte Disturbances

Electrolyte imbalances are common in ill patients. Proper fluid selection and, if necessary, supplementation of electrolytes aid in restoring balance.

Hypokalemia

Dogs and cats in poor health often experience hypokalemia. This may be due to increased urinary or gastrointestinal loss, prolonged anorexia, alkalemia, an aldosterone-secreting tumor, or treatment with potassium-poor replacement fluids.⁶⁹ Clinical signs of hypokalemia may include weakness, cardiac arrhythmias, and respiratory muscle impairment.⁷⁰

Treat hypokalemia with KCl supplementation in IV fluids (Table 11). Determine the KCl dosage based on the patient’s serum potassium concentration. For IV KCl administration, calculate the total mEq/kg/hr to be administered, ensuring that the supplementation rate does not exceed 0.5 mEq/kg/hr, as rapid administration can be fatal.⁷⁰ Never bolus fluids supplemented with KCl. Before administration, invert the bag and thoroughly mix the fluids to ensure uniform dispersion of the KCl.⁷¹ Exercise caution with highly supplemented fluids or KCl CRIs to prevent inadvertent bolus or over administration to the patient. Use appropriate fluid pump settings or syringe pumps as safeguards and educate staff on the risks of over administration. In cases of persistent hypokalemia even after KCl supplementation, check magnesium levels to determine whether magnesium supplementation is needed.⁷²

TABLE 11 Guidelines for Potassium Supplementation in Fluids

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Sodium imbalances

Sodium concentration disturbances are common. These cases should be managed with deliberate care to prevent potentially life-threatening fluid shifts in the brain that may occur after rapid correction, faster than 0.5 mEq/kg/hr, of chronic (greater than 24–48 hr) sodium alterations.

Hyponatremia. Patients with acute euvolemic hyponatremia (e.g., primary polydipsia) may present with neurologic signs and should be treated with 2–6 mL/kg of 3–7.5% hypertonic saline over 10–15 min.^73–75 In symptomatic chronic hyponatremia, a similar approach using hypertonic saline should be taken. Once neurological clinical signs resolve, treatment should continue similarly to that in asymptomatic chronic hyponatremia patients. In asymptomatic chronic hyponatremia patients, isotonic crystalloids (Table 12C) that have a sodium content ∼10 mEq/L higher than the patient’s sodium should be selected. A rate of correction should be calculated that does not exceed 0.5 mEq/L/hr or a maximum sodium increase of 10–12 mEq/L/day to prevent osmotic demyelination syndrome (Tables 12A, 12D).⁷⁶

TABLE 12A Approach to Fluid Therapy in Hyponatremic Patients

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TABLE 12B Common Causes of Acute and Chronic Hyponatremia in Dogs and Cats

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TABLE 12C Sodium Concentration of Isotonic Crystalloids

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TABLE 12D Calculating Expected Changes in Sodium Concentration

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Patients with hypovolemic hyponatremia should be resuscitated with a crystalloid that contains a similar sodium content to the patient’s current sodium concentration.^74,75 Depending on the severity of the hyponatremia, commercial isotonic crystalloids may not be available with similar sodium contents. In these cases, custom fluids can be tailored by adding sterile water to an isotonic crystalloid to achieve the desired sodium content. These fluids should be used exclusively for bolus administration until hypovolemia is resolved. Hypotonic fluids should not be used in patients with hyponatremia.⁷⁵

Hypernatremia. The strategies to treat hypernatremia are similar to those taken with hyponatremia regarding the considerations of chronicity and rate of correction of 0.5 mEq/L/hr. Sodium concentration drops that are faster than this rate in patients with chronic hypernatremia may cause abrupt fluid shifts that lead to cerebral edema.⁷⁷ However, patients with acute hypernatremia can undergo rapid sodium concentration correction without the risk of cerebral edema, by using hypotonic IV fluids. Calculate the free water deficit and administer fluids at an appropriate rate while monitoring sodium concentrations frequently to allow for a safe resolution of hypernatremia (Table 13A).

TABLE 13A Approach to Fluid Therapy in Hypernatremic Patients

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TABLE 13B Common Causes of Acute and Chronic Hypernatremia in Dogs and Cats

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Hypochloremia. Relative shifts in chloride usually occur with shifts in sodium. To assess true chloride disturbances in the face of sodium derangements, chloride should be corrected using the following equation:$$\text{Corrected} \text{Cl} = \left(\text{normal} \text{Na/measured} \text{Na}\right)\text{×measured} \text{Cl}$$In patients with true hypochloremia that have developed metabolic alkalosis, 0.9% NaCl has historically been the fluid of choice because of its high chloride concentration (154 mEq/L). Recently, concerns have been raised about the effect of 0.9% NaCl on kidney function owing to the potential risk of causing hyperchloremia, which has been shown to cause renal vasoconstriction and reduced renal blood flow.^78,79 To safely use 0.9% NaCl in hypochloremic renal patients, recheck chloride concentrations frequently, and once chloride levels have been corrected, use a buffered isotonic crystalloid (e.g., lactated Ringer’s solution or Plasma-Lyte) instead.

Hypocalcemia. Use calcium gluconate to treat patients with clinical signs of hypocalcemia (e.g., weakness, tachycardia, and tremors). Lactated Ringer’s solution contains a very small amount of calcium, and it will not resolve hypocalcemia.

Hypercalcemia. Historically, 0.9% NaCl has been recommended for calciuresis in hypercalcemic patients, but its effects are generally mild. Because of concerns regarding kidney injury with 0.9% NaCl, consider using a balanced isotonic crystalloid instead in patients at risk of or with current kidney disease.

Vasodilatory States

Patients may exhibit signs of poor perfusion or shock (such as tachycardia, hypotension, weak peripheral pulses, or elevated lactate concentrations) due to generalized vasodilation, also referred to as vasodilatory or maldistributive shock. In vasodilatory shock, there is excessive dilation of blood vessels, leading to a significant decrease in blood pressure and inadequate perfusion of vital organs.^80,81 Common clinical conditions that can lead to vasodilation include acute pancreatitis, anaphylaxis, sepsis, trauma, and parvoviral enteritis. Historically, vasodilatory shock treatment has revolved around rapid administration of fluids, the administration of broad-spectrum antibiotics, establishing source control, and using pharmacologic interventions to maintain adequate mean arterial blood pressure.^80,82

Because distinguishing between hypovolemic and vasodilatory shock based solely on physical examination findings is challenging, and because patients with vasodilatory shock may have some degree of hypovolemia, a fluid challenge is a reasonable treatment approach in these situations. Poor or limited response to fluid boluses may suggest vasodilation. In cases in which there is a poor response to fluid therapy, vasopressor therapy should be considered.⁸² In patients with suspected anaphylaxis, it is crucial to administer epinephrine, a vasopressor, as soon as possible.⁸³

Thermoregulation Disturbances

Hypothermia. Whenever possible, use warm fluids for resuscitation or rehydration in hypothermic patients for active core rewarming. The task force believes warm fluids will at least mitigate worsening hypothermia compared with the administration of room temperature fluids. It has been recommended to warm IV fluids to 40–42°C (104–107.6°F).⁸⁴ Fluids can be warmed using numerous methods, including immersion of IV tubing in warm water, microwaving of the fluid bag, in-line fluid warmers, and prewarming of fluids in a convection oven.⁸⁵ However, the effect of warm fluids in producing an increase in body temperature is controversial.^86–88

Hypothermia is commonly observed in cats in shock. Aggressive fluid resuscitation with the aim of restoring normal blood pressure in hypotensive, hypothermic cats will often lead to interstitial fluid overload, pulmonary edema, and pleural effusion. Administering conservative fluid volumes (e.g., 5 mL/kg IV boluses at a time) concurrent with active rewarming is an essential part of shock resuscitation therapy in cats.⁸⁹

Hyperthermia. Fluid therapy is important in treating hyperthermia in dogs and cats. Administering room temperature fluids may assist with cooling and maximize volume expansion.⁹⁰

Hypoglycemia

Administer dextrose to patients exhibiting clinical signs related to hypoglycemia, such as lethargy, weakness, ataxia, or seizures. For a bolus, use 0.5–1 mL/kg (0.25–0.5 g/kg) of 50% dextrose, which should be diluted at a ratio of 1:2–1:4 and administered over 2–5 min.⁹¹ In most cases, the bolus should be followed by a CRI of 1.25–5% dextrose (Table 14) until the patient can maintain normoglycemia. The concentration of dextrose instilled in fluids depends on the severity of the patient’s hypoglycemia and fluid administration rate. It is advisable to administer dextrose concentrations exceeding 5% through a central line to avoid phlebitis.

TABLE 14 How to Formulate Dextrose-Containing Fluids

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Note that in patients with insulinoma, dextrose administration may perpetuate hypoglycemia. Therefore, dextrose should only be given when patients present with clinical indications of hypoglycemia, and the lowest effective concentration should be used to prevent the manifestation of clinical signs.⁹²

Top Three Takeaways

1. First, do no harm. Fluid overload is a potentially life-threatening complication for which no universally effective therapy exists.

2. Excessive iatrogenic fluid administration is the most common cause of fluid overload. Prevention and vigilant monitoring are crucial to mitigate the potential risks associated with fluid therapy.

3. Patients with impaired renal function are at an increased risk of fluid overload because of their kidneys’ inability to increase urine output and effectively excrete excess fluid.

Overview

Fluid overload, or fluid intolerance, is a clinical spectrum that spans from hypervolemia to life-threatening edema and cavitary effusions. The guidelines task force has proposed that fluid intolerance may be the more appropriate term for this condition, as this term more accurately describes how the amount of fluid needed to overload a patient is dependent on their tolerance for a given amount of fluids. “Fluid intolerance” also encompasses both iatrogenic overload and overload due to underlying comorbidities. However, given that “fluid overload” is still widely used and recognized within the veterinary medical profession, it will be the primary term used in these guidelines to refer to this condition.

All patients receiving fluid therapy are susceptible to fluid intolerance/fluid overload; however, excessive administration is the most common cause of fluid overload. Hypervolemia might be more common than perceived, as early signs of increased body weight and positive fluid balance are difficult to assess. Typically, hypervolemia is recognized when advanced signs such as edema and effusions occur.⁹³ It is important to understand the harms of fluid overload and the need to prevent this complication, given the limited effectiveness of treatments.

Recognizing and Managing Fluid Overload

As discussed earlier, fluid movement is influenced by hydrostatic pressure, oncotic pressure, and vascular permeability. Patients who receive excess fluids during resuscitation, maintenance, or anesthesia first develop hypervolemia, which causes an increase in hydrostatic pressure. A decrease in oncotic pressure can occur with dilution of plasma proteins associated with excessive fluid administration or protein-losing disease. Hypervolemia, as well as inflammation and injury, can damage the glycocalyx and increase vascular permeability. Together, these changes in fluid dynamics promote movement of fluid out of vessels into the interstitial space. As the interstitial space expands, it becomes more compliant, accommodating larger quantities of interstitial fluid, further exacerbating the problem of fluid overload.

Changes in fluid dynamics favoring accumulation of interstitial fluid are often seen in combination with the body’s physiological response to stress. Hypovolemia, illness, and injury lead to water and sodium retention and increased thirst. This response exacerbates the risk of fluid overload as resuscitation fluids are administered to critically ill patients when their bodies are primed to retain fluid.⁹⁴

By the time clinical edema or effusions are noted, internal edema will also be present that can negatively impact organ function (Figure 11).⁹⁴ Once edema has occurred, it is very difficult to reverse even with ultrafiltration. Thus, prevention and early recognition are keys to minimizing the morbidity and mortality associated with fluid overload.

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Although all patients receiving IV fluids are at risk of fluid overload, those with impaired renal function, heart disease, or liver disease or those receiving large fluid volumes are at highest risk (Box 5). Pets with oligo-anuric renal failure are particularly susceptible to fluid overload because they are unable to excrete excess fluid. Yet these patients often receive aggressive fluid therapy necessitating close monitoring for weight gain, hypertension (in presence of AKI/CKD), or early signs of edema.^93,94,95

To reduce the risk of fluid overload:

• Use a low-volume strategy for fluid resuscitation.

• Tailor ongoing fluid therapy to meet each patient’s needs.

• Include all fluids delivered as medications, infusions, flushes, and enteral feedings to improve the accuracy of fluid balance calculations. Cats and small dogs with smaller blood volumes can quickly become fluid overloaded if clinicians are not mindful of the volumes administered.

• Limit the total fluid volume delivered in anesthetized patients during lengthy procedures. As a general rule, patients receiving anesthetic fluids should receive no more than the daily recommended total of 20 mL/kg/24 hr.⁹⁴

• Consider enteral water supplementation.

To recognize hypervolemia sooner, weigh patients frequently (every 6–12 hr) to identify a 10% increase in body weight compared with the initial baseline.^93,94 Regularly document all fluid inputs and outputs to help identify positive fluid balance. Additional signs of fluid overload are summarized in Table 15.

TABLE 15 Clinical, Radiographic, and Ultrasonographic Findings Associated with Fluid Overload

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Once edema has started, it can become life-threatening and be very difficult to resolve. See Box 6 for common misconceptions that can lead to fluid overload. There is no definitive therapy. General therapeutic approaches include sodium and water restriction, discontinuing IV fluids, administering diuretics, and increasing patient mobility (Figure 12). The interventions and success of those interventions are limited. Thus, the best approach is to avoid fluid overload.^93,94

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Top Three Takeaways

1. Fluids are drugs with complex interactions in the body and require an understanding of how and when to prescribe them. Clinicians must carefully consider hydration and volume status, electrolyte and acid-base derangements, and underlying comorbidities when prescribing fluid therapy.

2. Questions concerning the safety of synthetic colloids, the efficacy of plasma to treat hypoalbuminemia, whether potassium-containing fluids are safe in treating hyperkalemic patients, and whether IV fluids are indicated during cardiopulmonary resuscitation (CPR) are answered here.

3. The RECOVER Advanced Life Support guidelines for veterinary CPR advocate a careful approach to the fluid prescription in patients with cardiopulmonary arrest, owing to the possibility of reducing oxygen delivery to myocardial and cerebral tissues in patients without hypovolemia or distributive shock.

Overview

The scarcity of large-scale, blinded, prospective studies on fluid therapy in veterinary medicine gives rise to questions and controversies regarding its application in practice. Clinical decisions often must be extrapolated from the existing limited data, further contributing to the ongoing discussions and uncertainties surrounding fluid therapy.

Are Synthetic Colloids Safe?

Colloids rapidly expand the intravascular space and have a longer duration of action than crystalloids. Their use has long been considered “volume-sparing” as compared with the volume of crystalloids required to match their effect. Synthetic colloids (e.g., hydroxyethyl starches [most common], gelatins, and dextrans) are more readily available and less costly than natural colloid alternatives such as blood products or species-specific albumin. Despite their initial attractiveness as resuscitation fluids, the use of synthetic colloids in critically ill small animal patients is currently controversial because of safety and efficacy concerns.^96–100 Complications associated with their use in human patients include the development of AKI and coagulopathy. Colloids are dissolved in NaCl, which delivers a high chloride load to the distal tubule and accounts for some of the renal effects seen in humans. There is limited evidence that these same complications may occur in critically ill dogs and cats, although most studies are retrospective evaluations or small experimental studies.^101–120 A newer understanding of the function of the endothelial surface layer¹²¹ and revision of the Starling hypothesis^122,123 have raised concerns that extravasation of these synthetic macromolecules increases tissue edema and ultimately reduces oxygen delivery in states of increased vascular permeability, such as occurs with systemic inflammatory response syndrome, sepsis, trauma, and other damage to the endothelial surface layer.¹²⁴

There is no evidence-based consensus on the optimal use of synthetic colloids in small animals. Very little evidence exists on whether one synthetic colloid is safer or more effective than another. Clinicians must carefully weigh risk versus benefit for individual patients.

Consider the following before administering synthetic colloids:¹⁰⁰

• - Avoid synthetic colloids if the patient is responsive to crystalloid or alternative therapies.

• - Synthetic colloids should not be used as a substitute for albumin. In cases of severe hypoalbuminemia, natural colloids like plasma products or albumin should be considered.

• - In complex disease processes, such as sepsis or septic shock, treatment is multimodal and may involve natural colloids and/or vasopressors.

• - Current evidence suggests a minimal risk of synthetic colloid-associated AKI in dogs and cats when administered in small doses (<20 mL/kg) for short durations.

• - Administering synthetic colloids to hemodynamically stable patients and those with pre-existing azotemia is not advised.

• - Synthetic colloids may be considered for short-term need for colloid osmotic pressure support (e.g., during anesthesia for a portosystemic shunt ligation in a patient with hypoalbuminemia).

Current recommendations for synthetic colloid administration include minimizing dose and duration while closely monitoring kidney function (up to 90 days after exposure), coagulation parameters, hematocrit, and platelet count.¹⁰⁰ Synthetic colloids should be avoided or used with caution in patients with pre-existing azotemia, coagulopathy, anemia, and thrombocytopenia.¹⁰⁰ Some veterinary clinicians also consider systemic hypertension and sepsis to be contraindications for synthetic colloid use.⁹⁹

Can Plasma Be Used to Address Hypoalbuminemia?

A serum albumin concentration of <2.0 g/dL is a negative prognostic indicator associated with increased patient morbidity and mortality in small animals due to development of peripheral edema, impaired tissue perfusion and oxygenation, hypercoagulability, altered protein-bound drug metabolism, and poor wound healing. In general, natural colloids (e.g., albumin and plasma) are preferred over synthetic colloids to address hypoalbuminemia, but they remain controversial because of a lack of evidence linking correction of the albumin concentration with improved outcomes in critically ill veterinary patients^56,125,126 Species-specific albumin products are the recommended transfusion products in veterinary patients with severe or worsening hypoalbuminemia.^61,125 Because of the limited availability and cost of species-specific albumin, plasma products (e.g., fresh frozen plasma, frozen plasma, and CPP) are sometimes used to treat hypoalbuminemia in dogs and cats.¹²⁷ Although plasma products contain varying amounts of albumin,⁵⁹ complications of plasma transfusions appear to be less risky than xenotransfusion with human albumin products.^{125,128–131} The volume requirement and cost of plasma to raise serum albumin concentration is a serious limitation in its use for treating hypoalbuminemia. For many patients, nutritional support is more effective, less challenging, less risky, and less costly than attempting to increase serum albumin directly with natural colloids.^132,133

Are Potassium-Containing “Balanced” Isotonic Crystalloids Harmful in Hyperkalemic Patients?

Hyperkalemia is a common electrolyte disturbance in small animal patients with conditions such as urethral or bilateral ureteral obstruction, ruptured bladder, hypoadrenocorticism (dogs), anuric or oliguric renal failure, and others.¹³⁴ Many of these patients require fluid resuscitation, and clinicians must choose between administering potassium-free fluid (e.g., 0.9% NaCl) or balanced isotonic electrolyte solution (e.g., lactated Ringer’s solution, Normosol-R, and Plasma-Lyte A).

Balanced isotonic electrolyte solutions for resuscitation contain only a small amount of potassium (4–5 mEq/L) (Table 8). Several studies have concluded that potassium-containing isotonic fluids are not detrimental for fluid resuscitation and rehydration in hyperkalemic cats with urethral obstruction.^135,136 Balanced isotonic fluids correct acid-base imbalances faster than an acidifying fluid such as 0.9% NaCl. Balanced isotonic crystalloids also help prevent excessively rapid sodium correction in hyponatremic patients who have hyperkalemia (e.g., Addisonian crisis).¹³⁷ It is key to identify and treat the underlying cause of hyperkalemia. Further management of severe hyperkalemia is indicated to prevent cardiac conduction disturbances and other life-threatening complications when serum potassium is >7 mmol/L.¹³⁸

Are IV Fluids Indicated During CPR?

Fluid therapy is indicated during CPR if it will help mitigate hypovolemia that led to the cardiopulmonary arrest. IV fluid therapy in the form of crystalloid boluses increases preload and cardiac input in small animal patients with hypovolemia or inappropriate vasodilation (e.g., anaphylaxis, sepsis, and systemic inflammatory response syndrome). Increasing the intravascular volume in euvolemic patients, however, may ultimately decrease perfusion pressures and reduce oxygen delivery to myocardial and cerebral tissues.^139–142 For this reason, the RECOVER Advanced Life Support guidelines for veterinary CPR advocate a careful approach to the fluid prescription in patients with cardiopulmonary arrest. These guidelines recommend fluids for specific indications only, such as patients with or likely to have hypovolemia or distributive shock.^143,144 Experimental studies in rats and pigs suggest there may be a neuroprotective effect associated with hypertonic saline administered during CPR,^145–147 but clinical evidence in dogs and cats with cardiopulmonary arrest is lacking. Synthetic colloids should be avoided in these patients because of potential risks and the absence of a survival benefit to justify the risks.¹⁴⁴ Patients with significant hemorrhage may require blood products. Patients with derangements in potassium, calcium, magnesium, glucose, or acid-base status may benefit from directed therapy to help correct life-threatening abnormalities. Patients with acute lipid-soluble toxicoses may benefit from IV lipid emulsion therapy.

Top Three Takeaways

1. Using skilled veterinary technicians to administer and monitor fluid therapy allows the veterinarian to focus on the tasks they alone must perform (i.e., diagnosing, prescribing—including the fluid therapy prescription—prognosing, and surgery), leading to increased efficiency and oversight of patients.

2. Monitoring during IV fluid therapy is critical to prevent fluid overload. When patient monitoring or referral to a hospital that provides monitoring is not available, consider an alternate route of delivery.

3. Infusion pumps are frequently used; however, they may not always be the optimal method for fluid administration.

Overview

IV fluids can be administered in a variety of ways. The veterinary nursing care team must be trained in how to use each method to avoid fluid overload in patients and related potential hazards. These factors help determine the best mode for IV fluid delivery:

• Volume to be infused

• Total duration of infusion

• Level of monitoring available during administration

• Type of fluid administered

Fluid Administration

IV infusion pumps are most commonly used but are not always the ideal mode of administration. For example, a large dog who needs a rapid crystalloid infusion may need an infusion rate that exceeds the 1–2 L/hr that standard pumps provide. In this case, a pressure bag would be the preferred method to deliver the large fluid volume in a short time. For animals receiving a small volume of fluids, a syringe pump may be more accurate than a standard infusion pump that does not have micro settings (Table 16).

TABLE 16 Intravenous Fluid Delivery Modes

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The IV administration set is another consideration in choosing equipment. Smaller animals may benefit from a micro drip set (60 drops/mL), which provides more precise fluid delivery. This can be especially helpful when using the gravity method to deliver fluids in small patients. Macro drip sets come in a variety of drip factors; those most used in veterinary medicine are 10 drops/mL and 15 drops/mL. Fluid pumps are also usually designed for use with a specific type or size of tubing, so it is important to match these to ensure adequate fluid delivery (Table 16).

Use a new IV line and bag for each patient¹⁴⁸ (see Table 17 for IV catheter care and placement). Prime IV lines with the fluid to be infused before use to avoid air embolism. A T-port can ease medication administration, and a Y-port can be used when administering more than one compatible infusion.

TABLE 17 Peripheral Intravenous Catheter Placement and Care Checklist

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IV fluid bags should not be used for saline flushes because of the risk of contamination. Commercially prefilled syringes are preferred to decrease the chances of introducing bacterial contamination that could lead to infection.^149–151

If IV access is not achievable, consider IO catheters early in treatment to avoid fluid resuscitation delays. IO devices are able to quickly drill through the bone but require training. When no device is available, needles or peripheral IV catheters can be used (see https://www.youtube.com/watch?v=10twNYP1pB0 for instructions on placing an IO catheter).

Fluid Monitoring

Trained nursing staff should continually monitor IV fluid administration (Table 18). Without adequate monitoring, severe consequences can occur and compromise patient care. Use multiparameter monitoring to gain a broad picture of the patient’s volume and hydration status and avoid fluid-related complications (Table 19).

TABLE 18 Monitoring Fluid Delivery

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TABLE 19 Evaluation and Monitoring Parameters That May Be Used for Patients Receiving Fluid Therapy^a

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Monitor patients at high risk for fluid overload at least every 2 hr (see Tables 19 and 20 for methods). Weigh these patients 2–3 times a day, check their respiratory rate every 1–2 hr, measure fluid inputs and urine output, and watch for signs of edema. If available, perform focused ultrasonography for fluid responsiveness, which includes scanning for cavity effusion and B-lines in the lungs and evaluating the vena cava. For patients at risk of recurrence of hypovolemic shock, evaluating the inferior vena cava may be a better indicator of recurrence than other more commonly monitored parameters such as heart rate and blood pressure.¹⁵²

TABLE 20 Methods to Monitor Fluid Outputs and Inputs

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In smaller patients, especially pediatrics, keep in mind that serial blood draws can decrease blood volume.

Fully utilizing veterinary technicians, who can not only set up and run fluid therapy but also identify the signs of fluid-related complications, allows the veterinarian to delegate this nursing task after prescribing fluid therapy orders. The stronger a veterinary technician’s understanding of the “why” of fluid therapy, something that is addressed in veterinary technology and nursing programs, the more they can be optimized to their fullest potential, allowing for higher efficiency of the veterinary team.

Many veterinary practices are either unable to provide 24 hr care or geographically unable to refer patients to a 24 hr facility. When continual patient monitoring is not possible, the task force recommends giving IV fluids during the clinic’s open hours and SC fluids before leaving for the night. A slightly higher IV fluid rate can be used during the day unless contraindicated because of the patient’s disease state. Patient monitoring must be provided whenever administering IV fluids.