May 2023

In Part 1 of a two-part VETgirl online veterinary continuing education blog, Dr. Garret Pachtinger, DACVECC, provides a practical, case-based approach to fluids. After all, you use fluid therapy EVERY. SINGLE. DAY in veterinary medicine, whether you’re a small animal, exotics, large animal or equine veterinarian! But, it’s not just “set it and forget it” and it’s a whole lot more than just water and salt. So, read on to find out more! Don’t forget to tune back in a few weeks for Part 2.

Fluid therapy is one of the most commonly used therapies for the small animal practitioner. Despite a large amount of research the general consensus is that there is not one fluid type that is better than another for resuscitation. This is often why there is debate as to what fluids a practice should purchase to have on the shelf.  Moreover, the type of fluid desired may vary based on the underlying disease process.

The reason that fluid therapy is so important in medicine is that living organisms are comprised predominantly of…fluid!. Total body water content is approximately 60% of body weight in a non-obese, adult dog or cat. Total body water is further distributed between two major compartments: the intracellular (ICF) and extracellular (ECF) fluid.

Total Body Water (TBW) Fluid Compartments

The ICF compartment is the larger of the two compartments and comprises 66% of the total body water and 40% of body weight. It is separated from the ECF compartment by a cell membrane that is permeable to water but impermeable to most solutes.  The ECF comprises the remaining 33% of the TBW and 20% of body weight. The ECF is subdivided into the plasma (25% of ECF) and interstitial (75% of ECF) fluid compartments.

The need for fluid therapy is often divided into 2 main categories:

  1. Restoring the patient’s intravascular volume (hypovolemia)
  2. Replacement of extravascular fluid (dehydration)

There are 4 types of hypoperfusion commonly recognized in veterinary practice:

  1. Hypovolemia (i.e., loss of intravascular volume)
  2. Maldistributive / Septic (i.e., loss of vascular tone, fluid shifting, third spacing)
  3. Cardiogenic (i.e., myocardial dysfunction leading to lack of cardiac output and perfusion)
  4. Obstructive (i.e., decreased venous return to the right side of the heart as a result of obstruction, e.g., due to gastric dilatation and volvulus or pericardial effusion)

It is important to distinguish which type of hypoperfusion is present as their initial treatment as well as long term therapy will differ based on the underlying disease process.  As compared to cardiogenic causes, when clinical signs of hypovolemia are present (pale mucous membranes, prolonged capillary refill time, dull mentation, poor pulse quality, cold extremities, and tachycardia (or bradycardia in cats)  intravascular fluids must be replaced for emergency resuscitation.

When thinking about replacement of fluids, often the first fluid we consider is an isotonic crystalloid. Crystalloid fluids are mixtures of sodium chloride and other physiologically active solutes (K+, Ca2+ or Mg2+, glucose and buffer). Sodium is the major solute in the extracellular space, the majority of the extracellular space is extravascular, and as a result sodium will reside primarily outside the vascular space. Common crystalloids veterinarians have on the shelf include Lactated Ringer’s, Normosol-R, and Plasma-Lyte 148.  These fluids are classified as isotonic solutions as they have an electrolyte concentration and osmolarity that is similar to the ECF (plasma). These fluids rapidly distribute between the intravascular and interstitial compartments. After thirty minutes, 75% of the volume of fluids infused into the intravascular space shifts into the interstitial space.  When using isotonic crystalloids, the estimated shock volumes of fluids are 90 ml/kg in dogs, and 60ml/kg for cats.  The author initially replaces 1/4 to 1/3 of the calculated volume as rapidly as possible, the reassess perfusion parameters, notably heart rate, mucous membrane color, CRT, pulse quality, blood pressure, and eventually urine output.

Aside from isotonic crystalloids, synthetic colloids are another option considered in hypovolemic patients. Synthetic colloids such as Hetastarch, Vetstarch®, and dextrans are fluids that contain are large molecules (molecular weight >10,000daltons) that do not readily cross the vascular membrane.  Synthetic colloids are hyperoncotic to the normal patient and therefore pull fluid into the vascular space from the interstitial space as well as keep fluid in the vascular space. As compared to isotonic crystalloids, synthetic colloids cause an increase in blood volume that is greater than that of the infused volume.

While the goal standard is to measure the oncotic pressure using a colloid osmometer, general principles for the use of synthetic colloid therapy include:

  • When crystalloids are not effective in improving or maintaining blood volume
  • Peripheral edema develops
  • In support of colloid osmotic pressure
  • Capillary permeability problems
  • Total protein < 3.5 g/dL
  • Albumin < 1.5 g/dL

Common colloid bolus doses are 10–20 ml/kg in dogs and 5–10 ml/kg in cats followed by rapid and frequent reassessment. Similar to isotonic crystalloids above, the author uses 1/3 to ¼ of the volume above as an initial bolus followed by patient re-assessment.

Besides isotonic crystalloids and synthetic colloids, another alternative fluid therapy is hypertonic crystalloids, specifically hypertonic saline. Hypertonic saline is considered for rapid expansion of the intravascular compartment and used in patients that have a normal hydration status. Hypertonic saline is contraindicated for a patient that is dehydrated or hypernatremic. Hypertonic saline has a potent effect, drawing fluids from other compartments into the intravascular space due to its potent osmotic forces. The typical dose recommended for rapid resuscitation is 4-7 ml/kg of 7.5% HS over 20 minutes. Additionally, hypertonic saline is theorized to have other beneficial properties including improved myocardial contractility, activation of a neurogenic reflex leading to peripheral vasodilation, improving microcirculatory flow by preventing capillary collapse, a reduction of endothelium cell swelling and alterations in function of polymorphonuclear cells (PMN) and endothelial cells. Complications include bradycardia, bronchoconstriction, sodium fluctuations, fluid overload and pulmonary edema, phlebitis and ventricular arrhythmias.

To prolong the effect of fluid resuscitation, the author also considers the combined use of a hypertonic saline/synthetic colloid.  To achieve this fluid mixture, 1:2.5 ratio of 23.4% hypertonic saline (sodium chloride) and Hetastarch or Vetstarch are used.  To easily make this solution, 17ml of 23.4% hypertonic saline and 43ml of the colloid are mixed in a 60ml syringe.  3-5ml are then used as a bolus in the canine patient and 2-3ml are used as a bolus in the feline patient, followed by re-assessment.

But wait! There’s so much more about fluid therapy. Tune into part 2 in a few weeks to learn more on fluid therapy!

References (Please note the references listed are for both Part 1 and 2):

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  2. Chan DL. Colloids: Current recommendations. Vet Clin No Amer Small Anim Pract 2008;38(3):587-93.
  3. Choi PT, Yip G, Quinonez LG, et al. Crystalloids vs colloids in fluid resuscitation: a systematic review. Crit Care Med 1999;27(1):200-10.
  4. Cohn LA, Kerl ME, Lenox CE, et al. Response of healthy dogs to infusions of human serum albumin. Am J Vet Res 2007;68:657-663
  5. Cornelius LM. Fluid therapy in small animal practice.J Am Vet Med Assoc. 1980;176:110.
  6. Cornelius LM, Finco DR, Culver DH. Physiologic effects of rapid infusion of Ringer’s lactate solution into dogs.Am J Vet Res. 1978;39:1185.
  7. Cunha MG, Freitas GC, Carregaro AB,et al. Renal and cardiorespiratory effects of treatment with lactated Ringer’s solution or physiologic saline (0.9% NaCl) solution in cats with experimentally-induced urethral obstruction. Am J Vet Res. 2010;71:840–846.
  8. DiBartola SP, Bateman S. Introduction to fluid therapy. In: DiBartola SP.Fluid, Electrolyte, and Acid Base Disorders in Small Animal Practice. 4th ed. St. Louis, MO: Elsevier; 2012:331–350.
  9. Gaudette S, Hughes D, Boller M. The endothelial glycocalyx: structure and function in health and critical illness. J Vet Emerg Crit Care (San Antonio). 2020;30(2):117-134. doi:10.1111/vec.12925
  10. Hansen B, DeFrancesco T. Relationship between hydration estimate and body weight change after fluid therapy in critically ill dogs and cats.J Vet Emerg Crit Care. 2002;12:235.
  11. Moore LE, Garvey MS. The effect of hetastarch on serum colloid osmotic pressure in hypoalbuminemic dogs. J Vet Intern Med 1996;10(5):300-3
  12. Rose RJ. Some physiological and biochemical effects of the intravenous administration of five different electrolyte solutions in the dog.J Vet Pharmacol Ther. 1979;2:279.
  13. Silverstein DC, Aldrich J, et al. Assessment of changes in blood volume in response to resuscitative fluid administration in dogs. Journal of Veterinary Emergency and Critical Care 2005;16(3):185–192.
  14. Trow AV, Rozanski EA, Delaforcade AM, et al. Evaluation of use of human albumin in critically ill dogs:73 cases (2003-2006). J Am Vet Med Assoc 2008;233(4):60

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