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Section 7: Questions and Controversies in Fluid Therapy

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.

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Dog with iv receiving fluid therapy

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.

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.,,,, 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.,,,,,, ,,,,,, ,,,,, ,, A newer understanding of the function of the endothelial surface layer and revision of the Starling hypothesis, 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.

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:

  • 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. Synthetic colloids should be avoided or used with caution in patients with pre-existing azotemia, coagulopathy, anemia, and thrombocytopenia. Some veterinary clinicians also consider systemic hypertension and sepsis to be contraindications for synthetic colloid use.

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,, Species specific albumin products are the recommended transfusion products in veterinary patients with severe or worsening hypoalbuminemia., 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. Although plasma products contain varying amounts of albumin, complications of plasma transfusions appear to be less risky than xenotransfusion with human albumin products.,,,, 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.,

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. 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., 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). 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.

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.,,, 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., Experimental studies in rats and pigs suggest there may be a neuroprotective effect associated with hypertonic saline administered during CPR,,, 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. 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.

Citations
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