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Fluid therapy: overview

ISSN 2398-2950



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


  • Total body water is approximately two thirds of body weight, of which two-thirds is intracellular fluid (ICF) and one-third is extracellular fluid (ECF).
  • ECF is comprised of interstitial fluid, CSF, synovial fluid, etc (75%) and plasma (25%).
  • Circulating blood volume in the cat is approximately 60 ml/kg.
  • 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.
  • The interstitium is separated from the intracellular space by cell membranes. These are freely permeable to water but selectively permeable to solutes.
  • Sodium is the most abundant ion of the ECF. Therefore it is known as the osmotic skeleton, 'holding' water in whatever fluid space it is in.

Establishing the deficit


  • A detailed history is very useful. Include specifically:
    • When the patient was 'normal'.
    • Recent food and water intake.
    • Occurrence of vomiting or diarrhea.
    • Urine output.
    • Exposure to heat or trauma.
    • Hemorrhage.
    • Diuretic use.

Clinical examination

  • Loss of intravascular fluid volume:
    • Slow capillary refill time (note this is also affected by systemic inflammation and temperature).
    • Altered pulse quality - may be weak or hyperdynamic.
    • Tachycardia or bradycardia.
    • Reduced urine output.
    • Cool distal extremities.
  • Loss of interstitial fluid volume:
    • Poor skin elasticity. Remember acute hemorrhage will not increase skin tenting in the short term.
    • Enophthalmus.
    • Dry mucous membranes?
  • Loss of intracellular fluid volume:
    • Lethargy/disorientation if associated with significant hypertonicity/hypernatremia Hypernatremia.
  • The nature and severity of clinical signs will vary with the type and rate of fluid loss.
  • Rapid whole blood loss will result in much more severe clinical signs than gastroenteral losses of the same volume. This is because the gastroenteral losses come from the interstitial as well as the vascular spaces. An equivalent fluid loss resulting from decreased water intake will result in the mildest clinical signs because this loss comes from ICF as well as ECF spaces.

Types of fluid

See Parenteral fluids comparison table Parenteral fluids comparison table.


  • Small electrolytes and molecules, ie <500Da dissolved in water (atomic weight of sodium = 22Da, atomic weight of potassium =39Da, molecular weight of water =18Da, molecular weight of lactate = 90Da). These are the mainstay of fluid therapy, being the cheapest and normally safest type of fluid to administer. Fluids can be described with reference to their tonicity (ie the osmoles that could contribute to osmosis) relative to plasma.
  • Replacement crystalloid solutions (eg Hartmanns solution (Lactated Ringers solution or compound sodium lactate) and 0.9% saline) are considered isotonic and contain electrolytes at approximately the same concentration as extracellular fluid. Therefore they can be given rapidly for intravascular volume expansion.
  • Rapidly equilibrate across the intravascular and interstitial fluid spaces. Therefore only 25% of the administered volume remains in the intravascular compartment.
  • If used for maintenance, then replacement crystalloid fluids may require supplementation with potassium to prevent development of hypokalemia Hypokalemia. Replacement crystalloid fluids are unlikely to contain enough potassium to replace ongoing losses.
  • Maintenance crystalloid solutions contain less sodium and more potassium than replacement solutions. They are designed to meet the water and electrolyte requirements of animals that are not taking enough in to meet their daily losses. They should not be used in situations where large amounts of fluid are needed rapidly as this will result in a significant abnormality of electrolyte levels in the ECF. These solutions are hypotonic but are treated separately to the following group. Maintenance crystalloid solutions are rarely used in veterinary medicine.
  • Hypotonic crystalloid solutions include 0.45% saline and 5% dextrose in water (note 5% dextrose in water is isotonic when delivered into the body, but when the dextrose dissipates, effectively you have delivered simple water, with no other electrolytes, ie hypotonic liquid). These solutions provide free water that can distribute throughout the body fluid compartments including the ICF. Therefore they might rehydrate more efficiently than isotonic solutions. They should not be administered rapidly in an attempt to restore intravenous volume as this will not only prove ineffective but may lead to rapid movement of free water into the intracellular space. This will lead to cellular swelling with potentially serious consequences.

Hypertonic saline

  • 7.5% saline, osmolarity = 2567 mOsm/l (compared to plasma = 311-322 mOsm/l in cats). Therefore water is drawn from the interstitial compartment into the vascular compartment, and from the intracellular compartment into the interstitial compartment.
  • Infusion of a limited volume of hypertonic saline (eg 2-4 ml/kg over 10-20 mins) expands the vascular fluid volume and may restore hemodynamic parameters and tissue perfusion in cats with acute circulatory collapse.
  • Hypertonic saline improves cardiovascular function by expanding circulating volume, increasing preload and therefore cardiac output, if hypovolemia was the problem.
  • The hemodynamic effects of hypertonic saline are not sustained. Isotonic crystalloid solutions must be administered subsequently to maintain the beneficial effects and to prevent cellular dehydration.


  • Colloid solutions contain large particles (ie >10000 Da) that do not readily leave the vascular space. Therefore they (and the water that accompanies them) remain in the vascular compartment longer than crystalloids and may draw water from the interstitium into the vascular space.
  • Main indications for colloids:
    • Rapid vascular volume expansion.
    • Maintenance of  intravascular volume.
    • Maintenance of an adequate colloid osmotic pressure in the setting of hypoproteinemia.

Types of colloid solutions

  • Blood: see transfusion Blood transfusion.
  • Plasma:
    • Plasma proteins provide oncotic pressure to keep water in the intravascular space and potentially limit tissue edema. Feline plasma is increasingly available but may be cost prohibitive and does carry a risk of transfusion reaction.
    • 75% of transfused albumin disappears from the vascular space within 48 hours. Therefore large volumes of plasma are required to achieve significant or longlived elevations in blood albumin levels.
  • Albumin:
    • Human serum albumin can be used to increase plasma albumin levels and treat severe cardiovascular instability in cats and dogs. Side effects can be severe and/or delayed however, and less information is available for its use in cats. 
    • A lyophilized feline albumin product has been available historically but little information is available on its use.

Hydroxyethyl starches (HES)

  • Synthetic mixture of polysaccharides.
  • 130kDa molecular weighted tetrastarch solutions safest and most available.
  • The properties of hydroxyethyl starch solutions vary according to the molecular size of the polysaccharides involved, as well as the nature of bonding between the saccharide molecules.
  • Availability of different products varies geographically.
  • More persistent than dextrans and gelatins.
  • Anaphylactic reactions and coagulopathies may be less common than with dextrans although are still of concern, along with acute kidney injury.
  • Rate of side effects increase with increasing size and concentration of the HES molecules. The recommended dose should always be present on the fluid label and should be checked prior to administration.
  • Often avoided nowadays (irrespective of limited availability) in favor of balanced crystalloids, natural colloids and vasopressor therapy in critically unwell patients.
  • Currently unavailable (UK) but expected to be re-introduced with controlled access restrictions. Caution exercised since 2013 by both the Food and Drug Administration and European Medicines Agency on their use.

Gelatin solutions

  • Bovine collagen derived, succinylated gelatine, eg Gelafusine® in sodium chloride.
  • Molecular weights of approximately 30kDa.
  • Used more readily in face of hydroxyethyl starch bans but still infrequently.
  • Smaller than most HES molecules with much shorter half-life.
  • Cheaper than other colloids, side-effects in humans more frequent than with HES. 


  • Glucopolysacharrides divided to molecular weights of 40-70kDa.
  • Anaphylaxis, kidney injury, thrombocytopathia are risks.
  • No longer recommended nor readily available anymore.

Fluid administration

Route of fluid administration

  • Fastest and most reliable route of administration.
  • An over-the-needle catheter should be placed and secured. The cephalic vein Cephalic catheterization is used routinely, otherwise the jugular Jugular catheterization, and saphenous veins (lateral or medial Medial saphenous catheterization) are relatively easy to catheterize and secure.
  • For safety, an infusion pump should be used or the cat should be monitored frequently to ensure the fluids are not going in too fast or extravascularly.
  • Catheter associated complications such as phlebitis, obstruction and leakage are more likely to occur after 3 days.
  • This route of fluid administration is ineffective in patients that are hypovolemic or whose circulation is compromised as absorption of the fluid is very limited and much too slow.
  • It may be used to provide maintenance requirements with an isotonic solution once peripheral circulation is reestablished.
  • Give 50-100 ml at two or three different sites as required.
  • In non-critical patients without gastrointestinal disease this may be the best route of fluid administration.
  • This is an alternative method of delivering fluids into the vascular space.
  • It should be used in critical patients when an intravenous catheter cannot be placed and can be extremely useful in pediatric patients.
  • The tibial tuberosity, trochanteric fossa of the femur or greater tubercle of the humerus may be used.
  • Infiltrate the area with lidocaine before needle or intraosseous catheter insertion.

How much fluid to give?

  • Initial fluid administration should be targeted to correct deficits in circulating volume Fluid therapy: for acute circulatory collapse Fluid therapy: for hemorrhage.
  • Once hypovolemia has been corrected and cardiovascular instabilities stabilized, total body fluid deficits may be addressed.
  • The volume of fluid that needs to be given is equal to the estimated deficit + any ongoing abnormal losses (such as vomiting, drains, large wounds or burns) + normal daily maintenance requirements.
  • The fluid deficit (liters) = % dehydration (rough estimate from clinical examination/history) x body weight (kg) /100.
  • Ongoing abnormal losses can be estimated by measuring urine output (urinary catheters, weighing litter trays, ultrasonographic volume quantification), fecal output and drain production.
  • Maintenance fluid requirements are often approximated at 40-60 ml/kg/day for cats but are likely less, especially in hospitalized patients.
  • This calculation produces a very rough estimate of how much fluid the cat requires. It should be reassessed at least once daily and altered according to response to treatment.
  • Parameters to monitor include:

Rate of fluid administration

  • Fluids given to expand vascular volume following acute hemorrhage can be injected rapidly to replace the volume that has been lost .
  • Hypovolemic cats with shock Shock have often been prescribed fluids at up to 60 ml/kg/hr for 1 h without concerns for pulmonary edema if they have normal cardiac function. However, rather than administer this 'shock dose' of fluids (60 ml/kg) over 1 h, it is preferable to administer a portion of this volume (5-10 ml/kg) over a shorter period (10-20 mins) and then reassess the animal. Repeat 'boluses' may be required depending upon response. The concept of a shock rate should be abandoned.
  • A cat that has become dehydrated over days due to reduced water intake or gastrointestinal losses should receive fluids at a slower rate to allow time for water to equilibrate throughout the body fluid compartments.
  • In practice, most animals requiring fluid therapy are suffering from a degree of hypovolemia and some degree of interstitial and cellular dehydration. The volume deficit should be corrected rapidly to maintain tissue perfusion, whereas the total body fluid deficit can be corrected more slowly (over 24-48 h).
    Cats are more likely to suffer severe effects such as pulmonary edema from excessive fluid administration than dogs.
  • Prevent iatrogenic overhydration by careful and frequent monitoring of cats receiving intravenous fluids, ie bodyweight twice daily, hydration assessment, gauging of urine output, documentation of respiratory rate, point of care ultrasonography.
  • Central venous pressure can be used as a trend. Overhydration or failure of the heart to cope with the fluid load will result in a steady increase in CVP. It is less frequently used now due to lack of benefit in targeting CVP endpoints in human medicine.
Remember whilst these details discuss theoretical requirements, there is increasing evidence associating overzealous fluid administration (or a positive fluid balance) with worsened outcomes and there are many mechanisms by which this is likely the case (consider interstitial edema with reduced organ function, specifially congestive heart failure, glycocalyx and endothelial dysfunction...). This is especially relevant in critically ill patients. It is likely safer to deliver less fluid than more fluid although definitive evidence of this in veterinary patients is lacking at this stage.

Specific conditions requiring fluid therapy

Follow the diagnostic tree for Poor perfusion: stabilization & fluid resuscitation Poor perfusion: stabilization & fluid resuscitation.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Semler M W, Rice T W (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.