Fluid Therapy Frequently Asked Questions

The term “maintenance fluids” is widely used in small animal medicine but can be confusing as it may either refer to a directed fluid therapy plan provided to a patient, or a particular kind of fluid solution being administered.

Maintenance fluid therapy is a prescription fluid plan designed to preserve a patient’s extracellular volume while maintaining normal electrolyte balance (Moritz and Ayus 2015; Friedman and Ray 2008). The average physiologic fluid requirements are about 40–100 mL/kg/day, depending on the patient’s species and age. Maintenance fluid therapy is indicated for patients who are not eating or drinking. Maintenance fluid therapy is not intended to restore a patient’s fluid losses (e.g., diarrhea, vomiting, burns) and expand extracellular and blood volume (Friedman and Ray 2008). Patients with evidence of fluid loss require replacement fluid therapy as described in the 2024 AAHA Fluid Therapy Guidelines for Dogs and Cats.

When writing a fluid prescription, set the maintenance fluid rate at 40–100 mL/kg/day (based on patient species and age). Arbitrarily setting the fluid administration rate at a multiple of maintenance (e.g., two or three times the maintenance rate) increases the risk of nontargeted therapy and may increase complications associated with fluid therapy.

Keep in mind that “maintenance fluids” may also refer to a specific type of solution used in fluid therapy (Rudloff and Hopper 2021). Maintenance fluids are designed to maintain the body’s daily water requirements. Unlike replacement fluids (lactated Ringer’s solution, Plasma-Lyte A, Normosol-R, 0.9% NaCl), maintenance fluids (Plasma-Lyte 56 and 5% dextrose, Normosol-M and 5% Dextrose, 5% Dextrose in water, 0.45% NaCl) have lower sodium concentrations (and a higher concentration of free water) and higher concentrations of other electrolytes; thus, they are hypotonic (Byers 2017). The volume of maintenance fluids required to replace extracellular fluid deficits would result in cellular swelling, dysfunction, and damage, and therefore should not be used for intravascular volume replacement or interstitial rehydration.

Intraprocedural IV fluids are routinely administered during general anesthesia in veterinary patients. Routine use of intravenous fluids in human patients during general anesthesia remains a topic of discussion, however, and studies in people overall suggest that both excessive and overly restrictive fluid administration during anesthesia is associated with worse outcomes (Malbrain et al. 2020; Jilani et al. 2019).

An important consideration for fluid therapy in anesthetized patients is maintaining the patient’s effective circulating volume while avoiding overhydration or dehydration (Navarro et al. 2015). Fluid therapy also helps maintain IV catheter patency for emergent use if the patient’s status changes. Perioperative fluid choice and therapy should be individualized. While there are no evidence-based recommendations for the ideal fluid rate to administer during general anesthesia, most anesthetized patients should receive routine IV fluids. Recent guidelines recommend an administration rate of 5 mL/kg/h, while closely monitoring the patient for clinical evidence that the patient may need a fluid challenge, (a controlled bolus administration to evaluate the patient’s hemodynamic response), or that the patient may have fluid volume overload (Navarro et al. 2015).

Unique challenges may exist for shelters, mobile surgical operations, community cat programs, and voucher systems where routine intravenous fluid therapy may not be possible during general anesthesia. Because of the brief surgical times generally associated with those practices, fluid administration may not be required for elective surgical procedures (Griffin et al. 2016).  Fluid supplementation is recommended for high-risk patients and patients undergoing procedures where substantial blood loss is anticipated, or when surgical times are prolonged (Griffin et al. 2016).

For more information, refer to the 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats and the American College of Veterinary Anesthesia and Analgesia Recommendations for Monitoring Anesthetized Veterinary Patients. Consult with a veterinary anesthesiologist for patients who have comorbidities that may warrant specialist input regarding their care.

Patients may require placement of an IV catheter but may not need intravenous fluids (e.g., a patient hospitalized for seizure monitoring who is otherwise healthy and voluntarily eating and drinking). Studies in human patients have demonstrated that flushing an intravenous catheter once or twice a day with saline effectively keeps a peripheral catheter patent and does not increase catheter complications when IV fluids are not administered (Schreiber et al. 2015; Keogh et al. 2016; Belbase et al. 2023). A study in healthy dogs demonstrated that either saline or heparinized saline flushes given every six hours for up to 42 hours kept 18-gauge cephalic catheters patent. Smaller gauge catheters were not evaluated in that study (Ueda et al. 2013).

Thus, peripheral intravenous catheters are expected to remain patent with intermittent flushing (every 6 to 12 hours) if the patient is not receiving intravenous fluids. More frequent flushing may be required for patients with a higher risk of catheter complications (e.g., hypercoagulability, skin infections, etc.).

Limited studies in veterinary patients suggest that heparinized flushes are not necessary to maintain central and peripheral venous catheter patency (Ueda et al. 2013; Vose et al. 2019). Many studies in human patients also demonstrate that heparin is not required in flushes to maintain catheter patency, although it was not a consistent finding in every study (Shah et al. 2005; Kumar et al. 2013; Sotnikova et al. 2020).

For most veterinary patents, routine use of heparin is likely not required, although additional studies are needed. Consider heparin for patients with a higher risk of hypercoagulability (e.g., immune-mediated hemolytic anemia, sepsis, severe acute pancreatitis); however, most of these patients receive ongoing infusions of intravenous fluids, which help maintain catheter patency.

In human patients, potential complications of heparinized flushes include drug interactions, over heparinization of small patients, and heparin-induced thrombocytopenia and bleeding.

Heparin locks are used to prevent catheter thrombosis in central venous catheters between hemodialysis sessions, because most dialysis sessions are performed every 24–48 hours. A high concentration of heparin (1000–5000 U/mL) is infused into the catheter lumen. The volume of heparin infused is usually equal to twice the internal volume of the catheter system (Hadaway 2006; Yevzlin et al. 2007; Chalhoub et al. 2011). The use of heparin locks has also been described in vascular access ports for blood collection in feline donors and in dogs undergoing bone marrow transplantation (Abrams-Ogg et al. 1992; Aubert et al. 2011). Heparin locks are associated with higher risks of life-threatening bleeding complications due to inadvertently introducing a large concentration of heparin into the patient’s systemic circulation (Novak et al. 2008; Tan et al. 2021).

Other than in the circumstances described above and where patients are supervised by a trained team that frequently uses heparin locks, do not routinely use heparin locks in small animal patients.

Air embolism is a rarely encountered but dreaded complication of intravenous fluid therapy that can cause harm including death. Air embolism has been reported in veterinary patients (Mouser and Wilson 2015; McCarthy et al. 2016). Air introduced into the bloodstream may occlude blood flow in the pulmonary circulation and cause respiratory distress, seizures, altered mentation, and death (Mouser and Wilson 2015; McCarthy et al. 2016). In anesthetized patients, reduced end-tidal CO2 as measured by capnography may be the earliest indicator of air embolism. It is important to note that reduced oxygen saturation measured by pulse oximetry is considered a late sign of vascular air embolism. Anesthetized patients may be more susceptible to the consequences of air emboli because they are unable to ambulate.

The severity of clinical signs of venous air embolism relates to the volume of air delivered and its rate of entrapment in the circulatory system (Mouser and Wilson 2015). In dogs, the reported lethal volume of air injected as a single bolus ranges from 7.5–15 mL/kg, although this depends on how rapidly it was administered (Munson and Merrick 1966; Alvaran et al. 1978; Mouser and Wilson 2015).

Veterinary professionals should ensure that all connections are tight when setting up fluids for patients. To ensure there is no air in the system, visually inspect the infusion lines immediately after set up and when infusion is initiated. Do not ignore air alarms triggered by the fluid infusion pump and inspect the lines before restarting the infusion pump.

These resources provide guidance on the collection and administration of blood products:

1. Manual of Veterinary Transfusion Medicine and Blood Banking. Yagi K, Holowaychuk M, eds. Manual of Veterinary Transfusion Medicine and Blood Banking. Hoboken, NJ: John Wiley & Sons; 2016.
2. The American College of Veterinary Internal Medicine Consensus Statement Update on Canine and Feline Blood Donor Screening for Blood-Borne Pathogens Update on Canine and Feline Blood Donor Screening for Blood-Borne Pathogens
3. Transfusion for Military Working Dogs Guidelines
4. 2021 ISFM Consensus Guidelines on the Collection and Administration of Blood and Blood Products in Cats

The Association of Veterinary Hematology and Transfusion Medicine published a three-part consensus statement to define transfusion reactions and develop guidelines to prevent transfusion reactions, monitor transfusion administration, and help diagnose and treat transfusion-associated reactions in dogs and cats. The Association of Veterinary Hematology and Transfusion Medicine (AVHTM) Transfusion Reaction Small Animal Consensus Statement (TRACS) Part 1: Definitions and clinical signs is available here; Part 2: Prevention and monitoring is available here; and Part 3: Diagnosis and treatment is available here.

Acetate-containing fluids (Plasma-Lyte A and Normosol-R) are commonly used in veterinary medicine. Historically, there have been concerns that vasodilation may occur after rapid administration of acetate-containing fluids, leading to hypotension and hemodynamic instability. These concerns have been shared in case reports and animal studies (Gianolli and Kutter 2023).

However, acetate-containing fluids have been widely used as a rapid bolus to treat hypovolemia in veterinary patients (Vézina-Audette et al. 2022), and patients have experienced positive responses to the fluid bolus (Silverstein, Kleiner et al. 2012). Case-controlled studies in human patients have also demonstrated the efficacy of acetate-containing fluids in treating hypovolemic shock (Pfortmueller et al. 2017).

Thus, it is unlikely that acetate-containing fluids will lead to or exacerbate hypotension in dogs and cats, and they may be used to treat hypovolemia in veterinary patients.

Fluid overload is a common concern in patients with underlying cardiac or renal disease. Echocardiographic findings may help assess fluid intolerance risk in patients with heart disease, but administering subcutaneous fluids may not be safer than administering intravenous fluids.

When concerned about a patient’s heart or kidney function, use subcutaneous fluids cautiously and consider dose reductions (Abbott 2006).

Medications have historically been administered into the subcutaneous fluid bubble, but keep in mind that reduced or altered drug efficacy may occur. A recent study demonstrated that dilution of maropitant citrate in lactated Ringer’s solution prolongs the subcutaneous absorption of the drug and reduces maximum plasma concentration (Yee et al. 2023). Studies of other drugs are still needed, but it is likely best to administer subcutaneous medications as directed by the manufacturer and administer the medication in a subcutaneous bubble only when the manufacturer explicitly recommends it.

Intravenous fluids should not be withheld in anemic patients in need of volume expansion and rehydration. It is reasonable to resuscitate and rehydrate anemic patients with fluid therapy, while closely monitoring patients for clinical signs that indicate a blood transfusion is needed (e.g., tachypnea, hypotension, tachycardia, weakness, etc.) (Turner et al., 2022).

In human patients, dilutional anemia (when excess fluid volume is administered) can generate microcirculatory disturbances in the capillaries primarily in patients who have cardiac disease, but patients who have renal disease also experience negative impacts that reduce renal function (Ince 2015). Hemoglobin is aggressively monitored in human patients to determine transfusion threshold. Beyond maintaining normal perfusion pressures, if hemodilution drops the blood volume below the hemoglobin threshold, then this leads physicians to transfuse (Perel 2017).

In veterinary medicine the hematocrit tends to be the focus for transfusion threshold along with clinical signs, although dialysis clinicians may pay more attention to hemoglobin concentrations. When making decisions regarding aggressiveness of fluid therapy, blood pressure should be initially maintained with intravenous fluids and vasopressors, and the patient’s hematocrit can be considered secondarily.  Due to the lack of an established hematocrit threshold, recommending where to draw that line is unclear.

Peritoneal fluid administration can be used to resuscitate or rehydrate veterinary patients. Direct peritoneal resuscitation involves instilling hypertonic fluid into the abdominal cavity of hypovolemic patients, in addition to intravenous resuscitation. It reduces organ ischemia and serum concentrations of inflammatory cytokines and causes arterial vasodilation of the intraperitoneal organs to improve blood flow (Weaver et al., 2016). It may be helpful in animals with septic peritonitis, pancreatitis, and gastric-dilatation-volvulus, and it may be used preoperatively or intraoperatively. A protocol described in animals involves instilling warm lactated Ringer’s solution with 2.5% dextrose at a dosage of 10 mL/kg every six hours, with active or passive drainage after the dwell time. More studies on direct peritoneal resuscitation in animals are pending (Waldrop 2022).

Peritoneal fluid therapy may also be used to treat dehydration in dogs and cats, and its use has been described in puppies and kittens (Macintire, 2008). It can be useful to increase body temperature and treat hypothermia. A 2017 veterinary review article suggested that isotonic crystalloids be warmed to 104℉–109℉ for intraperitoneal instillation (Brodeur, Wright et al. 2017).

Hypertonic saline, a hyperosmolar fluid, is commonly administered through a peripheral catheter in veterinary patients. Historically, its administration through a peripheral catheter in human patients has been discouraged because of the risk of infiltration and extravasation. However, recent studies in human patients demonstrate that administering hypertonic saline through a peripheral catheter is safe.

In one study, three complications were observed in 20 human patients given 3% hypertonic saline through a peripheral IV catheter (Perez and Figueroa 2017). Another study demonstrated that only three of 103 patients who received 3% hypertonic saline peripherally experienced adverse side effects (Jannotta et al. 2021). In a study of 526 pediatric human patients, no complications occurred with peripheral administration of hypertonic saline (Pohl et al. 2022). In human patients who received a high infusion rate for more than six hours, complications were limited to phlebitis, erythema, hyperchloremia, and hyperkalemia.

Studies in veterinary patients have not been performed, but 7.2% hypertonic saline is commonly used in dogs and cats and complications associated with its use have not been reported. Administration of hypertonic saline through a peripheral catheter is expected to be safe in dogs and cats (Ajito et al., 1999).

Subcutaneous fluid therapy is commonly administered in cats with CKD, but the ideal frequency is undetermined because it has not been directly studied. A web-based survey of 468 owners of cats with CKD revealed that of the 399 owners who gave subcutaneous fluids, 57 (14%) gave fluids once or twice weekly, 121 (30%) gave fluids three or four times weekly, 156 (39%) gave fluids once daily, 37 (9%) gave fluids twice daily, and 28 (7%) gave fluids on another schedule (Cooley, Quimby et al. 2018). One veterinary review article recommended subcutaneous fluid administration up to every three days for managing CKD in dogs and cats (Polzin 2013). Another review article recommended subcutaneous fluid administration every 24–48 hours in patients with CKD, although this recommendation was made on the premise that supporting evidence is lacking in veterinary literature (Langston 2017). While no evidence exists to recommend weekly or biweekly subcutaneous fluid therapy in patients with CKD, the task force members believe that subcutaneous fluid therapy given once daily to every third day is reasonable. For patients with CKD and heart disease, fluid therapy needs to be balanced with the added volume load on the heart.

Patients may require intravenous fluid therapy and a blood transfusion at the same time, and it is a widely accepted practice in veterinary and human medicine.

Historically, it has been controversial to administer lactated Ringer’s solution concurrently with blood products because lactated Ringer’s solution contains calcium, which may precipitate with citrate in the anticoagulant used in blood products. In 1975, a human in vitro study concluded that lactated Ringer’s solution mixed with citrated blood led to clot formation and later hemolysis, so the recommendation was to avoid this combination (Ryden and Oberman 1975) and use 0.9% NaCl with blood products. These recommendations were updated in 1998 after another study concluded that citrated packed red blood cells and lactated Ringer’s solution infusion rates did not cause clot formation (Lorenzo et al. 1998). This finding was also verified with the use of human AS-3-preserved packed red blood cells (Albert et al. 2009).

A 2002 article on perioperative fluid therapy in veterinary patients cited the 1975 in vitro study and recommended avoiding administering citrated blood products through the same IV line as lactated Ringer’s solution (Kudnig and Mama 2002). However, given the evidence presented in human patients, lactated Ringer’s solution is thought to be safe to administer with blood products using the same IV catheter.

Similarly, IV fluids can be administered concurrently with other citrated blood products—including fresh frozen plasma, frozen plasma, and cryoprecipitate. Follow the manufacturer’s recommendations when administering canine albumin or human albumin. When administering IV fluids with blood products, consider the total volume administered to avoid volume overload.

In addition, several mechanisms related to IV fluid administration can affect coagulation function in veterinary patients. However, the main effect of IV fluids on the coagulation cascade is dose dependent (Boyd, Brainard et al. 2021). Thus, it is reasonable to anticipate there will not be a diminished efficacy of plasma products if crystalloids are given concurrently.

Intravenous fluids should not be used routinely in every patient undergoing CPR. In euvolemic patients, isotonic or hypertonic fluid administration may compromise tissue perfusion pressures. For hypovolemic patients and patients with severe electrolyte derangements, intravenous fluids may be used during CPR (Fletcher and Boller 2021). In general, use fluid therapy only if it is specifically indicated in an individual patient.

A common anecdotal statement is that cats are more sensitive than dogs to fluid overload because cats have a lower mL/kg blood volume than dogs. However, fluid rates and doses for cats differ from those for dogs and typically compensate for this.

One reason cats may be more sensitive to fluid intolerance is the administration of large volumes of fluids to a hypovolemic cat during fluid resuscitation. Cats with hypovolemia are classically hypothermic and hypotensive. Hypothermic cats are thought to have generalized vasodilation although vasoconstriction occurs as they approach normothermia. Normotension may not be observed in hypothermic cats due to peripheral vasodilation, and clinicians may inadvertently continue to administer large volumes of fluids to try to improve the blood pressure. Thus, active heat support should be provided to the hypothermic cat to encourage vasoconstriction and large boluses of fluids should be minimized until the cat is normothermic.

Other possible reasons for fluid intolerance sensitivity in cats include occult hypertrophic cardiomyopathy and underappreciated early chronic kidney disease (CKD is present in 1–3% of cats versus 0.5–1% of dogs). As of the time of writing, major veterinary clinical laboratories had not updated reference ranges to be in line with the International Renal Interest Society (IRIS) guidelines for staging CKD, making it easier to miss a CKD diagnosis. Given that IRIS staging of CKD is based on the kidneys’ GFR capabilities, it stands to reason that undetected underlying kidney disease puts patients at a higher risk of fluid overload.

In a randomized, double-blinded study of human patients who were at risk of hyperkalemia, potassium-containing balanced crystalloid fluid administration was not associated with an increased risk of potassium imbalance and led to less hyperkalemia and metabolic acidosis than normal saline administration (O’Malley, Frumento et al. 2005). In fact, compared to Plasma-Lyte, 0.9% NaCl was later shown to decrease renal perfusion and GFR in humans (Chowdhury, Cox et al. 2012). In adult human patients with hyperkalemia, balanced crystalloid fluid administration was not associated with worsening hyperkalemia or increased risk of renal replacement therapy (Toporek et al. 2021).

A study in cats with naturally occurring urethral obstruction showed no significant differences in the rate of reduction of blood potassium whether 0.9% NaCl or potassium-containing balanced isotonic crystalloids were used. However, cats treated with balanced isotonic crystalloids had a faster improvement in blood pH (Drobatz 2008). Although they contain potassium, use balanced isotonic crystalloids when possible, instead of 0.9% NaCl in hyperkalemic dogs and cats.