Contributors: John Dodam, Elisa Mazzaferro, Claire Waters, Simon Cook

 Species: Canine   |   Classification: Miscellaneous

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) = //www.vetlexicon.com67% 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) = //www.vetlexicon.com90 ml/kg bodyweight.
  • Daily water requirements often reported as //www.vetlexicon.com 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

  • The next step should be deciding if the animal has lost a particular type of fluid or component that might help determine which fluid is best to use. This is most relevant in choosing blood products for administration. The history and physical exam are most useful for this.​

Type of fluid loss

  • Dehydration can be hypertonic, isotonic or hypotonic, but you will usually require a minimum database (eg PCV Hematology: packed cell volume, TS, electrolytes, urinalysis) to determine the specific type accurately.

Isotonic dehydration/mixed water and electrolyte loss (isotonic fluid loss)

  • Water loss is comparable to electrolyte loss.
Causes
  • Diarrhea, vomiting, (hemorrhage). Although vomiting can equate to hypertonic dehydration as sodium and bicarbonate are retained in response to loss of HCl; the net effect is water loss.
Effects
  • Isotonic water/electrolyte losses are essentially from the ECF with no change to ICF volume.
Sequelae
  • Clinical evidence of dehydration - loss of skin elasticity, dry mucous membranes, cardiovascular instability if severe, oliguria (renal compensation), increasing PCV (likely rapidly).

Hypertonic dehydration/hypotonic fluid loss/primary water loss

  • Water loss is greater than salt loss.
Causes
  • Decreased daily fluid intake, eg unable to drink, anorexia associated with pain/systemic illness.
Effects
  • Primary water loss is initially from ECF → increased osmolarity → water moves from ICF into ECF and maintains osmotic equilibrium → loss borne by intracellular fluid.
Sequelae
  • Large ICF water reserve means may take time for signs to develop: thirst, decreased skin turgor but this is less affected by ICF loss, slowly increasing PCV Hematology: packed cell volume, increased brain osmolarity and associated CNS signs (depression, disorientation), circulatory collapse is possible Fluid therapy: acute circulatory collapse.

Hypotonic dehydration/hypertonic fluid loss, or isotonic fluid loss with water replacement

  • Electrolytes are lost in excess of water, and/or water is replaced more so than electrolytes. Net result is development of hypotonicity.
Causes
  • Water intake or hypotonic fluid administration replacing isotonic losses.
Effects
  • Intracellular fluid volume will increase but extracellular fluid compartment is depleted.
Sequelae
  • Hyponatremia Hyponatremia. May require isotonic fluid for correction of ECF volume depletion but hypertonic fluid for correction of hypotonicity/hyponatremia.

Hemorrhage

  • Losses entirely from circulation initially.
  • Circulating blood volume is smallest part of ECF volume.
  • Sudden large loss of blood → cardiovascular collapse.
  • Non-fatal blood loss → some shift of ECF fluid from interstitium into vascular system.
  • Replace like with like (in terms of volume and type of fluid lost) where possible, although often crystalloids will suffice in first instance.

Types of fluid

Crystalloids

See  Parenteral fluids comparison table Parenteral fluids comparison table​.
  • Hypotonic crystalloids (maintenance fluids) (plasmalyte-56 and normosol–M, or 0.45% sodium chloride +/- additional KCl):
    • Useful for replenishing free water deficits as they replace what is lost, ie water with fewer total electrolytes.
    • Contain relatively high amounts of potassium, and will dissipate throughout both the intracellular and extracellular spaces, compared to isotonic crystalloids that have very little ‘electrolyte free water.’
    • They should not be used for volume resuscitation and may cause hyponatremia, for example, if not used carefully.
    • D5W (5% dextrose in water) is pure water, made isotonic by the addition of dextrose. After infusion, glucose is absorbed and metabolized, with the net effect of having delivered pure water, safely, intravenously. (To infuse pure water without the local isotonic environment would cause cell lysis.)
  • Isotonic crystalloids (replacement fluids) (0.9% sodium chloride, compound sodium lactate //www.vetlexicon.com lactated ringers solution //www.vetlexicon.com Hartmann’s solution):
    • Currently the mainstay of fluid therapy in veterinary medicine, with a historical trend towards use of balanced crystalloids (compound sodium lactate) as opposed to 0.9% sodium chloride).
    • Compound sodium lactate has less sodium/chloride than 0.9% NaCl, with the addition of a buffer in lactate, potassium, and calcium.
    • Although the original intention was for these to be ‘replacement’ fluids, they are often used, seemingly without ill-effect, as both replacement and maintenance fluids in veterinary medicine.
  •  Hypertonic crystalloids (eg 7.2% sodium chloride):
    • Use: volume resuscitation of large breed dogs, and head trauma patients (cats and dogs).
    • Draws interstitial (and intracellular) water into the vasculature, restores circulating volume (cardiac output and peripheral perfusion)
    • Small volume required: doses usually described as 3-5 ml/kg but it is possible to dose to effect (especially in TBI), ie 1-2 ml/kg over 5-10 minutes…re-assess…another 1-2 ml/kg of a 7.2-7.5% solution. Unlikely to require >5 ml/kg.
    • Used mostly in large animals otherwise requiring large volumes isotonic crystalloids, or in head trauma patients requiring fluid resuscitation and/or alleviation of raised intracranial pressure.
    • Usually followed with isotonic crystalloids after 30 minutes to prevent cellular dehydration.

Colloids

  • Contain larger molecules (over 10 000Da (10kDa)) that remain in the circulation longer than simple solutes (atomic weight of sodium = 22Da, atomic weight of potassium = 39Da, molecular weight of water = 18Da, molecular weight of lactate = 90Da) retaining intravascular volume.
  • Risks applicable to all colloids: allergies, anaphylaxis, volume overload, (acute kidney injury).

Options

Plasma (Fresh frozen or stored)

  • Increasingly readily available. Not benign - likely a higher immunological transfusion risk than packed red blood cells. Can be used for volume resuscitation and as longer infusions in patients requiring large volumes of crystalloids to maintain perfusion and cater for losses. Also used for correction of coagulopathies.
  • Fresh frozen plasma contains all hemostatic proteins.
  • Stored plasma is deficient in factors V, VIII and von Willebrand Factor compared to fresh frozen plasma, but contains the vitamin K dependant clotting factors.

Hydroxyethyl starches

  • Potato/Maize starch derived.
  • 130kDa molecular weighted tetrastarch solutions safest and most available.
  • This means the average molecular weight of the hydroxyethyl starch molecules is 130kDa on infusion (although immediately hydrolyzed intravenously to 70-80kDa), with 0.4 hydroxyethtyl starch groups per glucose molecule (tetrastarch).
  • Concentration of solution dictates the ‘volume’ effect, ie the oncotic pull, with high concentrations (7-10%) less safe than 4-6% solutions. 6% = 6 grams per 100ml. (NB. glycocalyx and endothelial integrity are more important that oncotic pressure).
  • Used for volume expansion and maintenance of higher colloid osmotic pressure
  • Risks - well established increase in risk of acute kidney injury in critically ill, septic humans.
  • Examples: Voluven® (130/0.4, 6% solution in saline solution), Volulyte® (130/0.4 6% solution in balanced crystalloid (additional Mg++, K+ and acetate))
  • Often avoided nowadays (irrespective of availability) in favor of 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.
  • If used in human medicine, only for volume expansion when crystalloids are insufficient. Contra-indicated in sepsis, AKI, burns, overloaded and critically ill patients.

Gelatins

  • 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.

Dextrans

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

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 include anaphylaxis. Available in 4-25% solutions. Can not be used on more than one occasion.
  • Lyophilized canine albumin is available in the United States and has been documented to increase albumin levels in small numbers of dogs. Robust safety information is not currently available, however. Repeated infusion appears safe thus far.
  • (Cryoprecipitate contains vWF, fibrinogen, FVIII and FXIII, with cryosupernatant retaining the rest. (Cryoprecipitate and cryosupernatant are divided from partially thawed fresh frozen plasma, and subsequently refrozen).)

Fluid administration

Routes of fluid adminsitration

  • Oral - preferred but often not possible.
  • Intravenous - second best, even for maintenance Infusion: fluid pump.
  • Subcutaneous isotonic fluids are an option on an outpatient basis. Remain at injection site, can be uncomfortable +/- risk of infection/cellulitis. This route can only be used for isotonic solutions and additional electrolyte supplementation is not feasible via this route.
  • Intraosseous - in small or very collapsed animals.
  • Intraperitoneal - large surface area for absorption. Consider in neonates if intravenous or intraosseous access has not been possible.

Intravenous catheters

Use over-the-needle or Seldinger technique inserted catheters Seldinger (over the wire) technique. Use another vein if accidental perivascular injection, hematoma or difficulty placing an intravenous line at one site.

Infection avoidance

  • Must use aseptic technique Surgery: asepsis: normal surgical site preparation + handle catheter aseptically.
  • Catheter associated infections, phlebitis and endocarditis are all possible complications.
  • Maintenance: flush with normal saline q 4hours, bandage whether used for access or not, more likely to see complications after 3 days.
  • If signs of phlebitis, pyrexia of unknown origin, leakage - remove catheter and replace with new one at another site.

Oral

  • Use: for maintenance if no vomiting and adequate intestinal function.
    Not fast enough in collapsed animal.
  • Can use non-sterile balanced electrolyte solutions.

Deciding rate of fluid administration

  • Firstly, remember fluids are drugs - the very delivery, and the rate, ought to be justified with frequent (ie twice daily) re-assessments of the patients ongoing requirements or lack thereof.

Based on signalment, physical exam, aim of fluid therapy and ongoing losses

  • Estimate deficit volume from animal's history: length of illness, extent of vomiting, volume and frequency of diarrhea, physical examination.
  • If available (rarely) known bodyweight before illness enables accurate calculation of deficit.
  • Assess the clinical condition and response to treatment.
  • Measure PCV and total refractometric solids.
  • Urinary losses decrease after 1-2 days without intake - kidneys reduce the water lost.
  • Daily requirement is much higher in very young animal (4-6 ml/kg/hr).

Measure actual losses

  • Blood loss during surgery: count swabs, measure volume in suction, etc.
  • Diarrhea or gastrointestinal disease: count number of times vomited/defecated and estimate volume of each.
  • Effusions withdrawn from body - measure volume.
  • Difficult to estimate fluid volume lost into bowel lumen but can be subjectively gauged by patient side ultrasonography.
  • Daily requirement increases if kidneys cannot concentrate urine.
Summate these in ml/kg/hr in order to tailor your fluid therapy to your patient’s needs. Aim to deliver rehydration fluids over 12-36 hours.

Mixed water and electrolyte (isotonic) loss

  • Severe or acute fluid loss: may require fluid resuscitation by bolus therapy, ie 5-20 ml/kg over 10-20 minutes, repeated to effect.
  • Use balanced crystalloids such as compound sodium lactate (Hartmann’s), Plasmalyte 148 or Normosol R, as opposed to 0.9% saline for most situations.
  • Remaining deficit can be targeted to be replaced over 12-36 hours.
  • Replace further losses as they occur.
  • If crystalloids are to be continued then fluids designed specifically for maintenance with higher potassium levels, or supplementation with KCl of replacement solutions such as Hartmann's, will likely be needed to prevent hypokalemia occuring.

Primary water loss

  • Free water deficit can be calculated using the formula:
    • Free water deficit (liters) = (Current Na/Normal Na)-1) x (Bodyweight in kg x 0.6)
  • In a hypernatremic patient, plasma sodium should not change more than 12 mmol/day, ie a patient with a plasma sodium of 182 mmol/L that you are correcting, should not reach 170 mmol/L in less than 24 hours due to the risk of cerebral edema developing.

Colloids

  • If using for volume resuscitation and crystalloids are no longer effective, consider a bolus of 2-5 ml/kg initially, dosed to effect thereafter or used as an infusion of 0.5-2 ml/kg/hr depending on patient needs and ongoing losses.

Resuscitation from hemorrhagic shock

  • The approach to resuscitation from hemorrhagic shock Fluid therapy: for hemorrhage should be to provide intravenous fluid therapy and achieve hemostasis, with equal importance attributed to each. The practice of hypotensive resuscitation is rarely practicable in veterinary medicine, where cavitary hemorrhage from trauma requiring surgical management is not common, and furthermore, immediate surgical management is seldom practiced. This may well change in the future.
  • Intravenous fluid resuscitation should be provided in bolus format with frequent re-assessment and the concept of shock doses/shock rates should be abandoned. This approach serves to deliver individualized fluid therapy and resuscitation, and avoiding overzealous fluid administration.
  • A good starting point would be delivery of a 10 ml/kg bolus of isotonic, balanced crystalloid solution, ie compound sodium lactate over 10-20 minutes, prior to reassessment of perfusion parameters. More unstable patients may require more aggressive administration and pressure bags in large breeds, but bolus therapy and re-assessment should still underline the approach.
  • A need for blood products is decided upon with time and initial response to treatment, or a high likelihood of requiring them later after extensive crystalloid resuscitation. As a general rule, patients are unlikely to benefit from red blood cell products if they are stabilized with a remaining PCV of 18-20% and adequate circulating blood volume. Plasma may be considered sooner if a coagulopathy is suspected/diagnosed.

Useful calculations

  • Fluid deficit (ml) = bodyweight (kg) x % dehydration x 10
  • Fluid deficit (liters) = bodyweight (kg) x (% dehydration/100)
  • pRBC volumes to infuse:
    • (Roughly 1ml/kg/% rise in PCV desired).
  • pRBC volumes to infuse in dogs:
    • Dogs: 90 ml x bodyweight (kg) x (desired PCV – Patient PCV)/pRBC PCV.
  • Free water deficit (liters) = (Current Na/Normal Na)-1) x (Bodyweight in kg x 0.6).
  • Assessing % dehydration:
    • Use clinical examination to calculate % bodyweight lost as water (dehydration):
      • 5%: subtle/undetectable clinical signs, ie tacky mucous membranes.
      • 10%: obvious clinical signs: tacky mucous membranes, sunken eyes, skin tent.
      • 15%: animal in hypovolemic shock.
      • Could aim to give 80% of calculated deficit and maintenance over 24 hours. If try to replace sooner, will urinate out excess.
      • Fluid deficit (ml) = Bodyweight (kg) x % dehydration x 10.
      • For example 20 kg dog with 10% dehydration would require 2 liters of fluid plus daily requirement if correctable over 24 hours.
      • 5% dehydration is a reasonable assumption in a dog with a short history of vomiting/diarrhea/inappetence even if clinical signs of dehydration are not apparent.