Canis ISSN: 2398-2942

Fluid therapy

Contributor(s): John Dodam, Elisa Mazzaferro, Claire Waters, Simon Cook

Introduction

Goals

  • Understanding the indications and benefits of fluid therapy, in addition to risks.
  • Establishing blood components that are lost or lacking.
  • Appreciating and recalling the different fluids available.
  • Recognizing different delivery routes.
  • Formulating a fluid therapy plan.

Physiology

  • Total body water (TBW) = ~67% of bodyweight (33% solids).
  • TBW divided into intracellular fluid (ICF, 67% of total) and extracellular fluid (ECF, remaining 33%).
  • Plasma/intravascular = 25% of the extracellular fluid (interstitial fluid and CSF/synovial fluid etc = remaining 75%).
  • In health, the vascular space is separated from the interstitial space by the vascular endothelium and glycocalyx. Combined, these are permeable to water and small solutes, but impermeable to plasma proteins. The plasma proteins exert an oncotic pressure to maintain water within the vascular space but an intact glycocalyx is the most important factor in regulating fluid shift between compartments.
  • The interstitium is separated from the intracellular space by cell membranes. These are freely permeable to water but selectively permeable to solutes.
  • Circulating blood volume (plasma + red blood cells) = ~90 ml/kg bodyweight.
  • Daily water requirements often reported as ~ 40-60ml/kg/day bodyweight (likely much less, especially in hospitalized patients without excess ongoing losses, but higher in neonates and pediatrics).
  • Sodium is the most abundant electrolyte in extracellular fluid; it is the skeleton of body water - water will not stay if sodium is not there to 'hold' it.
  • Sodium is very important in conjunction with albumin for water retention within the intravascular space (Gibbs-Donnan effect).

Defining need for fluids

  • It is important to consider both the need for fluid therapy, and the aims of fluid therapy.
  • Does the patient have one or more of the following?
    • Dehydration.
    • Hypovolemia.
    • Electrolyte imbalances.
    • Acid base/metabolic derangements.
    • Excessive ongoing losses or failure to retain (ie polyuria, voluminous diarrhea, cavitary fluid accumulation/removal).
    • An inability to tolerate fluids or nutrition per os.
    • A requirement for promoting diuresis.
    • Marked anemia.
    • Coagulopathy.
  • If not, it is unlikely that your patient will benefit from fluid/blood product therapy. Obviously many patients will present with these abnormalities but these questions should be asked on a re-assessment basis also, meaning fluid therapy can be withdrawn as soon as possible.

Establishing the deficit

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Types of fluid

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Fluid administration

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Deciding rate of fluid administration

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Specific conditions requiring fluid therapy

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Further Reading

Publications

Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Semler M (2016) Saline is not the first choice for crystalloid resuscitation fluids. Crit Care Med 44(8), 1541-1544 PubMed.
  • Young P (2016) Saline is the solution for crystalloid resuscitation. Crit Care Med 44(8), 1538-1540 PubMed.
  • Myburgh J A & Mythen M G (2013) Resuscitation fluids. New Engl J Med 369(25), 2462-2463 PubMed.

Other sources of information

  • Liu D T & Silverstein D (2014) Crystalloids, colloids, and haemoglobin-based oxygen-carrying solutions. In: Small Animal Critical Care Medicine, 2nd Edition. Ed. Silverstein and Hopper. 2
  • Rudloff E (2014) Assessment of hydration. In: Small Animal Critical Care Medicine, 2nd Edition. Ed. Silverstein and Hopper. 
  • Hopper K, Silverstein D & Bateman S (2012) Shock syndromes. In: Fluid, Electrolyte, and Acid-base Disorders in Small Animal Practice, 4th Edition. Ed. Dibartola. 


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