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Section 4: Fluid Therapy and Anesthesia

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.

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Dog with iv receiving fluid therapy
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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.

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. 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. 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). 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:
    • Initial fluid rate of 5 mL/kg/hr in dogs with normal cardiac and renal function.
    • 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).,, 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.

  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:
    • 5 mL/kg/hr for dogs
    • 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):
    • Gallop sound or new murmur (especially in cats)
    • Edematous tissues and chemosis
    • Swelling of paws
    • Clear nasal discharge (nasal edema)
    • Pulmonary crackles
    • Low oxygen saturation (SpO2)
    • No alteration in blood pressure along with other clinical signs (i.e., patients remain nonhypertensive)
    • 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, or plethysmographic variability index from advanced pulse oximetry to assess fluid responsiveness., 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.

Figure 9: Fluid Therapy During Anesthesia
Figure 9 Infographic-of Fluid Therapy During Anesthesia

Figure 9

Fluid Therapy During Anesthesia

CRI, continuous rate infusion

Download PDF of Figure 9

Box 4: Special Fluid Therapy Needs During Anesthesia
  1. Hypoglycemia
    • Add dextrose for pediatric toy dog breeds OR in response to hypoglycemia in dogs or cats.
      • Add 50 mL 50% dextrose to 1 L fluid bag to create a 2.5% dextrose solution.
  2. Acute surgical blood loss
    • Increase fluid rate (up to 10 mL/kg/hr) while surgically correcting the issue (e.g., dropped pedicle).
    • Consider that it takes three times as much crystalloid fluid volume to replace one volume of blood lost.
    • If blood products are unavailable, and the patient’s perfusion is compromised, consider a colloid to aid perfusion.
  3. Hypoproteinemia
    • Use canine albumin when total protein is <2.0 g/dL (a dose of 450 mg/kg canine albumin will increase the serum albumin by 0.5 g/dL). Start at a rate of 0.5 to 1 mL/kg for 30 min and increase the rate if no adverse effects are noted. Deliver over 3–4 hr. For more information, see Mazzaferro EM, Edwards T. Update on albumin therapy in critical illness. Vet Clin North Am Small Anim Pract 2020;50(6):1289–305.
    • Use a colloid with a crystalloid when total protein is <4.0 g/dL (hetastarch 1–5 mL/kg/hr in the anesthetized patient).
    • Use fresh frozen or frozen plasma when available.
    • Consider that it takes ~20–25 mL/kg to raise the albumin by 0.5 g/dL, and for large-breed dogs, this could be cost prohibitive. For specific doses, see Beer KS, Silverstein DC. Controversies in the use of fresh frozen plasma in critically ill small animal patients. J Vet Emerg Crit Care (San Antonio) 2015;25(1):101–6.

Use of synthetic colloids in patients is currently controversial because of safety and efficacy considerations and lack of evidence-based consensus. For more information on colloids, see Section 7, Questions and Controversies in Fluid Therapy.

Citations
  1. Bednarski, RM. Dogs and cats. In: Tranquilli WJ, Thurmon JC, Grimm KA, eds. Lumb & Jones Veterinary Anesthesia and Analgesia. 4th ed. Hoboken (NJ): Blackwell Publishing; 2007:705–15.
  2. Davis H, Jensen T, Johnson A, et al; American Association of Feline Practicioners; American Animal Hospital Association. 2013 AAHA/AAFP fluid therapy guidelines for dogs and cats. J Am Anim Hosp Assoc 2013;49(3):149–59.
  3. Muir WW. A new way to monitor & individualize your fluid therapy plan. Vet Med 2013;108(2). Available at https://www.dvm360.com/view/new-way-monitor-and-individualize-your-fluid-therapy-plan. Accessed July 13, 2023.
  4. Kudnig ST, Mama K. Perioperative fluid therapy. J Am Vet Med Assoc  2002;221:1112–21.
  5. Gaynor JS, Dunlop CI, Wagner AE, et al. Complications and mortality associated with anesthesia in dogs and cats. J Am Anim Hosp Assoc 1999;35:13–7.
  6. Redondo JI, Rubio M, Soler G, et al. Normal values and incidence of cardiorespiratory complications in dogs during general anaesthesia. A review of 1281 cases. J Vet Med A Physiol Pathol Clin Med 2007;54:470–77.
  7. Fantoni DT, Ida KK, Gimenes AM, et al. Pulse pressure variation as a guide for volume expansion in dogs undergoing orthopedic surgery. Vet Anaesth Analg 2017;44:710–8.
  8. Celeita-Rodriguez N, Teixeira-Neto FJ, Garofalo NA, et al. Comparison of the diagnostic accuracy of dynamic and static preload indexes to predict fluid responsiveness in mechanically ventilated isoflurane anesthetized dogs. Vet Anaesth Analg 2019;46:276–88.
  9. Klein AV, Teixeira-Neto FJ, Garofalo NA, et al. Changes in pulse pressure variation and plethysmographic variability index caused by hypotension-inducing hemorrhage followed by volume replacement in isoflurane-anesthetized dogs. Am J Vet Res 2016;77(3):280–7.
  10. Skouropoulou D, Lacitignola L, Di Bella C, et al. Intraoperative assessment of fluid responsiveness in normotensive dogs under isoflurane anaesthesia. Vet Sci 2021;8(2):26.

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