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Fluids and Anesthesia

One of the most common uses of fluid therapy is for patient support during the perianesthetic period. Decisions regarding whether to provide fluids during anesthesia and the type and volume used depend on many factors, including the patient’s signalment, physical condition, and the length and type of the procedure. Advantages of providing perianesthetic fluid therapy for healthy animals include the following:
 
  • Correction of normal ongoing fluid losses, support of cardiovascular function, and ability to maintain whole body fluid volume during long anesthetic periods
  • Countering of potential negative physiologic effects associated with the anesthetic agents (e.g., hypotension, vasodilation)
  • Continuous flow of fluids through an IV catheter prevents clot formation in the catheter and allows the veterinary team to quickly identify problems with the catheter prior to needing it in an emergency

 

TABLE 2

Determining the Route of Fluid Administration

 
Patient parameter Route of fluid administration
Gastrointestinal tract is functional and no contraindications exist (e.g., vomiting) Per os
Anticipated dehydration or mild fluid volume disturbances in an outpatient setting Subcutaneous. Caution: use isotonic crystalloids only. Do not use dextrose, hypotonic (i.e., D5W), or hypertonic solutions. Subcutaneous fluids are best used to prevent losses and are not adequate for replacement therapy in anything other than very mild dehydration
Hospitalized patients not eating or drinking normally, anesthetized patients, patients who need rapid and/or large volume fluid administration (e.g., to treat dehydration, shock, hyperthermia, or hypotension) IV or intraosseous
Critical care setting. Used in patients with a need for rapid and/or large volume fluid administration, administration of hypertonic fluids and/or monitoring of central venous pressure Central IV

D5W, 5% dextrose in water.

 

When fluids are provided, continual monitoring of the assessment parameters is essential (Table 1). The primary risk of providing excessive IV fluids in healthy patients is the potential for vascular overload. Current recommendations are to deliver , 10 mL/kg/hr to avoid adverse effects associated with hypervolemia, particularly in cats (due to their smaller blood volume), and all patients anticipated to be under general anesthesia for long periods of time (Table 4).6 8 In the absence of evidence-based anesthesia fluid rates for animals, the authors suggest initially starting at 3 mL/kg/hr in cats and 5 mL/kg/hr in dogs. Preoperative volume loading of normovolemic patients is not recommended.

The paradigm of “crystalloid fluids at 10 mL/kg/hr, with higher volumes for anesthesia-induced hypotension” is not evidence-based and should be reassessed. Those high fluid rates may actually lead to worsened outcomes, including increased body weight and lung water; decreased pulmonary function; coagulation deficits; reduced gut motility; reduced tissue oxygenation; increased infection rate; increased body weight; and positive fluid balance, with decreases in packed cell volume, total protein concentration, and body temperature.9,10 Note that infusion of 10–30 mL/kg/hr LRS to isoflurane-anesthetized dogs did not change either urine production or O2 delivery to tissues.11 A fluidconsuming “third space” has never been reliably shown, and, in humans, blood volume was unchanged after overnight fasting.12

TABLE 3

Recommended Maintenance Fluid Rates (mL/kg/hr)49

 
Cats Dogs
Formula: 80 x body weight (kg)0.75 Formula: 132 x body weight (kg)0.75
Rule of thumb: 2–3 mL/kg/hr Rule of thumb: 2–6 mL/kg/hr

Preanesthetic Fluids and Preparing the Sick Patient
Correct fluid and electrolyte abnormalities in the sick patient as much as possible before anesthesia by balancing the need for preanesthetic fluid correction with the condition requiring surgery. For example, patients with uremia benefit from preanesthetic fluid administration.13 Further, develop a plan for how fluids will be used in an anesthesia-related emergency based on individual comoribund conditions, such as hypertrophic cardiomyopathy and oliguric/polyuric renal disease.

Monitoring and Responding to Hypotension
During Anesthesia Blood pressure (BP) is the parameter often used to estimate tissue perfusion, although its accuracy as an indicator of blood flow is not certain.11,14,15 Hypotension under anesthesia is a frequent occurrence, even in healthy anesthetized veterinary patients. Assess excessive anesthetic depth first because it is a common cause of hypotension.7,16 Exercise caution when using fluid therapy as the sole method to correct anesthesia-related hypotension as high rates of fluids can exacerbate complications rather than prevent them.10,11

TABLE 4

Recommendations for Anesthetic Fluid Rates

  • Provide the maintenance rate plus any necessary replacement rate at < 10 mL/kg/hr
  • Adjust amount and type of fluids based on patient assessment and monitoring
  • The rate is lower in cats than in dogs, and lower in patients with cardiovascular and renal disease
  • Reduce fluid administration rate if anesthetic procedure lasts > 1 hr
  • A typical guideline would be to reduce the anesthetic fluid rate by 25% q hr until maintenance rates are reached, provided the patient remains stable

Rule of thumb for cats for initial rate: 3 mL/kg/hr
Rule of thumb for dogs for initial rate: 5 mL/kg/hr

If relative hypovolemia due to peripheral vasodilation is contributing to hypotension in the anesthetized patient, proceed as described in the following list:

  • Decrease anesthetic depth and/or inhalant concentration.
  • Provide an IV bolus of an isotonic crystalloid such as LRS (3–10 mL/kg). Repeat once if needed.
  • If response is inadequate, consider IV administration of a colloid such as hetastarch. Slowly administer 5–10 mL/kg for dogs and 1–5 mL/kg for cats, titrating to effect to minimize the risk of vascular overload (measure BP every 3–5 min).9 Colloids are more likely to increase BP than crystalloids.15
  • If response to crystalloid and/or colloid boluses is inadequate and patient is not hypovolemic, techniques other than fluid therapy may be needed (e.g., vasopressors or, balanced anesthetic techniques).9
  • Caution: Do not use hypotonic solutions to correct hypovolemia or as a fluid bolus because this can lead to hyponatremia and water intoxication.

Postanesthetic Fluid Therapy
Postanesthetic fluid administration varies based on intra-anesthetic complications and comorbid conditions. Patients that may benefit from fluid therapy after anesthesia include geriatric patients and patients with either renal disease or ongoing fluid losses from gastrointestinal disease. Details regarding anesthesia management may be found in the AAHA Anesthesia Guidelines for Dogs and Cats.17

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