Veterinary fluid therapy update: Calculating the rate and choosing the correct solution

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Editor's note: Ken Yagi would like to thank Brandy Terry, CVT, VTS (ECC), for the work she did on the original article, before updates were made.

Technicians are a vital part of making sure intravenous (IV) fluids are administered correctly in fluid-deficient patients. Knowledge of the types, amount and strategy in administration of fluids best helps a veterinary technician aid in the formulation of a treatment plan. This article provides an overview of how fluid is normally distributed in the body, what types of fluids can be given to correct any fluid imbalances and how to calculate the volume of fluid needed for each dehydrated patient.

Normal body fluid distribution

An adult animal's body weight is composed of about 60% water, which is distributed throughout the intracellular and extracellular compartments. The intracellular compartment consists of the largest volume of fluid, about two-thirds of total body water (approximately 40% of body weight).1 The extracellular space, which constitutes about one-third of the total body water, contains the fluid that is not in the cells. It is divided into three subcompartments: interstitial, intravascular and transcellular.1

The interstitial compartment contains three-quarters of all the fluid in the extracellular space. The intravascular compartment contains the fluid, mostly plasma, that is within the blood vessels. The fluid in the transcellular compartment is produced by specialized cells responsible for cerebrospinal fluid, gastrointestinal fluid, bile, glandular secretions, respiratory sections and synovial fluids.1

The intracellular and extracellular compartments are separated by specialized membranes that are semipermeable to allow water to equilibrate across the membrane according to the osmotic-pressure gradient. Dehydrated patients have a water loss in the extravascular space, and when fluids are intravenously administered, they are redistributed into the other compartments until all the solutes are in equilibrium again, thus correcting the water loss in the extravascular space.2 Sources of acute hemorrhaging can lead to hypovolemia and a loss of water primarily from the intravascular space in the short term, which can be corrected through IV fluid administration.

Fluids are administered to patients not only to replace fluid loss but also to correct electrolyte abnormalities, promote kidney diuresis and maintain the tissue or organ perfusion rate while a patient is undergoing anesthesia. For example, fluids can be added to replace fluid losses (e.g. vomiting, blood loss, water loss from the respiratory system) that occur before and during surgery. In addition, many sedatives and anesthetics will adversely affect the circulatory system, requiring fluids for hemodynamic support. If the patient's mean arterial pressure decreases below 60 mm Hg, some tissues and organs may experience inadequate oxygen delivery from reduced perfusion and subsequent hypoxia. The body will protect the vital organs such as the lungs, heart and brain in a state of low perfusion by causing peripheral vasoconstriction and shifting the blood volume to the body core. The kidney may see a decrease in perfusion, and acute renal failure can result from prolonged periods of extremely low blood pressure during anesthesia. Basically, patients can experience dehydration, hypovolemia or hypotension, or any combination of each depending on the situation.

This article focuses largely on dehydration, which results from less acute changes in fluid balance that gradually shifts water away from intracellular compartments through extracellular routes.

Determining a patient's degree of dehydration

The clinical signs of dehydration and their corresponding body dehydration percentages are presented below:3,4

• Less than 5% dehydration: No detectable clinical signs

• 5 to 8% dehydration: Subtle loss of skin elasticity and skin turgor, tacky or sticky mucous membranes, normal eyes

• 8 to 10% dehydration: Skin tenting, dry mucous membranes, sunken eyes, dull corneas, tachycardia

• More than 10% dehydration: Considerable loss of skin turgor, severely sunken eyes, tachycardia, weak or thread pulses, hypotension, altered level of consciousness

Certain circumstances make it difficult to determine how dehydrated a patient is. For example, emaciated animals that have metabolized the fat from around their eyes and in their skin will have sunken eyes and decreased skin turgor caused by the loss of fat and elastin in the subcutaneous area. Also, dogs that profusely pant will have dry mucous membranes, making it more difficult to assess hydration status. A patient that has fluid leaking into spaces within the body cavity (third spacing) will have a rapid change in fluid from the intravascular compartments before the interstitial loss is seen.5 Therefore, one must evaluate the patient with individual clinical situations in mind instead of relying on a few parameters to gauge hydration status.

Types of fluids

There are two categories of fluids: crystalloid and colloid solutions. Crystalloids contain electrolyte and nonelectrolyte solutes, which can move freely around the body's fluid compartments. Crystalloid fluids are divided into three groups: isotonic, hypertonic and hypotonic, based on their tonicity, which is the ability to shift water across the semipermeable membranes in the intracellular and extracellular skin compartments.

An isotonic crystalloid fluid is a balanced electrolyte solution equivalent to the osmolality of the patient's red blood cells and plasma. It results in the net effect of fluids neither exiting nor entering the cells. These solutions are given to patients for perfusion support and intravascular volume replacement. Commonly used isotonic crystalloids are Normosol-R, PlasmaLyte-A, Lactated Ringer's Solution and 0.9% normal saline solution.

A hypertonic crystalloid solution has a higher osmolality than the blood cells and plasma (a higher concentration of solutes that do not readily cross membranes) and, thus, promotes fluid movement into the intravascular space. Hypertonic solutions are useful in patients that need to regain large amounts of fluid in the intravascular space quickly and is aimed at retaining more free water in the intravascular space by raising its osmolarity. Ideal candidates for hypertonic solutions are those that require small volume resuscitation, such as those with septic shock, hemorrhagic shock and head trauma.

Specifically, in patients with traumatic brain injury, hypertonic saline may be useful in reducing intracranial pressure.6 Hypertonic saline is commonly administered as 7% or 7.5% solution at 3 to 5 mL/kg over 10 minutes. Its hyperosmolar effect is transient and is redistributed within 30 minutes. Because of this, hypertonic saline is commonly combined with synthetic colloids. A mixture of 7.5% hypertonic saline and colloid can be prepared by combining 23% hypertonic saline solution and 6% hetastarch at a 1:2 ratio (20 mL of 23% NaCl solution with 40 mL 6% hetastarch in a 60 mL syringe, for example). The mixture is associated with faster hemodynamic improvement with reduced crystalloid requirement and is delivered at 3 to 5 mL/kg over 10 to 20 minutes. Hypertonic saline solution is contraindicated in patients that are hypernatremic, since these fluids contain high concentrations of sodium. Hypertonic solutions can exacerbate dehydration and should be avoided in patients that are already dehydrated.

A hypotonic crystalloid solution has lower effective osmolality (a lower concentration of solutes that do not readily cross membranes) than intravascular fluid and, thus, promotes fluid movement into the cells. These are commonly used to correct electrolyte imbalances like hypernatermia and with heart failure and renal failure in patients with less tolerance for sodium load, for example. Hypotonic fluids are not used to correct hypovolemia since it encourages free water movement out of the intravascular space and can lead to overhydration of tissues causing edema. Examples of hypotonic crystalloid solutions are 5% dextrose in water and 0.45% sodium chloride.

Colloid solutions contain larger molecules that create colloid osmotic pressure that pull water toward the compartment they reside in and do not readily cross vascular barriers. The property allows their effect to be sustained for longer than crystalloids. Colloid solutions are also used to pull fluid into the vascular space and require a smaller volume compared to crystalloids to achieve the same effect (given there is enough interstitial and intracellular fluid to pull from).

The natural form of colloids is albumin, which can be depleted due to protein depleting pathologies (e.g. GI disease, kidney disease, wounds, liver disease), and synthetic colloids is one method of supplementing colloid osmotic pressure. Examples of synthetic colloids are hetastarch, pentastarch and tetrastarch, classified by their structure and vary in molecular weight. Doses commonly used range from 10 to 20 mL/kg for dogs and 5 to 10 mL/kg for cats.

Synthetic colloids have two notable adverse effects: coagulation impairment and acute kidney injury. Administration of colloids can lead to impairment of hemostasis by causing hemodilution and interfering with platelet, coagulation protein and the fibrinolytic system as well as suppressing the activity of coagulation factors and platelets. Hetastarch at doses higher than 20 mL/kg/day and tetrastarch at higher than 40 mL/kg/day is thought to have potential of causing coagulation issues.7 Additionally, synthetic colloids use in humans has evidence linking it to acute kidney injury in patients with sepsis. Veterinary evidence is less conclusive with contradicting evidence,8-12 keeping the veterinary community cautious in its use.

Calculating the fluid replacement volume and fluid rate

When administering fluids and considering the volume and rate, two different situations arise that require two different strategies. In the case of patients experiencing severe hypovolemia (relative or absolute) and resulting loss of perfusion and oxygen delivery, the urgency in fluid replacement is critical to survival. In these cases, fluids are administered as boluses at a “shock dose.”

Traditional shock doses cited are 90 mL/kg for dogs and 60 mL/kg for cats, though these numbers represent the total estimated blood volume of a patient and can lead to volume overload, especially if the patient has reasons to be less tolerant of fluid administration (heart disease, for example). An updated approach calls for a more conservative dosage of 10 to 20 mL/kg over 15 to 20 minutes and reassessing the patient's perfusion parameters (heart rate, pulse quality, mucous membrane color, capillary refill time, extremity temperature and mentation). Fluid boluses are re-dosed if enough improvement is not seen.

In a less emergent situation or once the patient has stabilized, fluid replacement volumes are calculated and a plan is made based on three values: dehydration deficit, maintenance requirement and ongoing losses.

Dehydration deficit is the amount of water the patient has already lost and needs to replace. To calculate the dehydration deficit, the following formula is used, in which the percent dehydration is the estimated dehydration based on physical assessment mentioned earlier:

Body weight in kg x percent dehydration (as a decimal) x 1000 mL/L = the fluid deficit in mL

Any boluses that were given initially should be considered a part of the replacement volume and accounted for in this value. The calculated deficit is then replaced over 2 to 24 hours.

Maintenance fluids are intended to replace the fluid volume lost per day to normal physiologic processes such as metabolism, evaporative loss and moisture lost through normal stool.6 There are several formulas commonly used for maintenance fluid requirement, ranging from 40 to 75 mL/kg/day or by using the formula 70 x [Body Weight (in kg)]0.75 per day, with some variance in opinion of what the constant (70, in this case) should be. Regardless of which formula is used, the key element is diligent patient monitoring and assessment to ensure the plan set is benefiting the patient and adjusting as needed.

Ongoing losses refer to other quantifiable fluid losses, especially pathologic losses.4 To be able to accurately measure urine production, a urinary catheter must be inserted and a collection system must be set up and emptied and measured regularly (e.g. every two to four hours). If inserting a urinary catheter is not an option, collect the urine via free catch or on a medical absorbent pad. With the free-catch method, the volume voided can be directly measured in milliliters (mL) using a graduated cylinder or a bowl and syringes. When using an absorbent pad, be sure to weigh a clean, unused pad and then weigh the soiled pad. The weight difference is the amount of urine collected. Each 2.2 lb (1 kg) more than the normal weight of the absorbent pad will equal about 1,000 mL of urine.13 Insensible losses are those that cannot be quantified (e.g. cutaneous losses with fevers, respiratory tract losses such as in a panting dog, fluids lost in feces). The veterinarian will estimate the insensible losses and incorporate that into the total fluid rate, should they feel it be significant. The volume of ongoing loss per hour is estimated to be added to the fluid replacement plan.

As the last step, the dehydration deficit, maintenance fluids and ongoing loss replacement are totaled to derive the hourly fluid rate the patient should receive. The effect of the fluids on the patient is regularly evaluated to adjust as needed.

The veterinary technician and nurse's role

Veterinary technicians and nurses have a significant role in patient assessment and formulation of a fluid plan as it is a vital component of patient care in a veterinary practice. The knowledge of assessment parameters, types of fluids and administration strategies are key concepts in functioning effectively as a part of the team.

References

1. DiBartola SP. Applied physiology of body fluid in dogs and cats. In: Fluid, electrolyte, and acid-base disorders in small animal practice. 4th ed. St. Louis, Mo: Saunders Elsevier, 2012;3-26.

2. Donohoe C. The technician's role in fluid therapy-from catheters to colloids, Part 2, in Proceedings. North Am Vet Conf, 2007. Available from the International Veterinary Information Service (www.ivis.org).

3. Davis H, Jensen T, Johnson A, et al. 2012 AAHA/AAFP Fluid Therapy Guidelines for Dogs and Cats. J Am Anim Hosp Assoc 2013; 49:149-159.

4. Hundley D, Brooks A, Thomovsky, E, et al. Crystalloids: A Quick Reference for Challenges in Daily Practice. Topics in Compan An Med 2016;31:46-53.

5. Fluid compartment deficits. In: Merck veterinary manual. Available at: www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/160403.htm. Accessed Dec. 13, 2009.

6. Mortazavi MM, Romeo AK, Deep A, et al. Hypertonic saline for treating raised intracranial pressure: literature review with meta-analyses. J Neurosurg 2012;116:210.

7. Balakrishnan A, Silverstein D. Shock Fluids and Fluid Challenge. In: Small Animal Critical Care Medicine. 2nd ed. St. Louis, MO: Saunders Elsevier, 2015;321-327.

8. Boyd CJ, Claus MA, Raisis AL. Evaluation of biomarkers of kidney injury following 4% succinylated gelatin and 6% hydroxyethyl starch 130/0.4 administration in a canine hemorrhagic shock model. J Vet Emerg Crit Care 2019;29:132-142.

9. Sigrist NE, Kalin N, Dreyfus A. Effects of Hydroxyethyl Starch 130/0.4 on Serum Creatinine Concentration and Development of Acute Kidney Injury in Nonazotemic Cats. J Vet Int Med 2017;31:1749-1756.

10. Diniz MS, Teixeira-Neto FJ, Celeita-Rodriguez N, et al. Effects of 6% Tetrastarch and Lactated Ringer's Solution on Extravascular Lung Water and Markers of Acute Renal Injury in Hemorrhaged, Isoflurane?Anesthetized Healthy Dogs. J Vet Int Med 2018;32:712-721.

11. Sigrist NE, Kalin N, Dreyfus A. Changes in Serum Creatinine Concentration and Acute Kidney Injury (AKI) Grade in Dogs Treated with Hydroxyethyl Starch 130/0.4 From 2013 to 2015. J Vet Int Med 2017;31:434-441.

12. Yozova ID, Howard J, Adamik KN. Effect of tetrastarch (hydroxyethyl starch 130/0.4) on plasma creatinine concentration in cats: a retrospective analysis (2010–2015). J Feline Med Surg 2017;19:1073-1079.

13. DiBartola SP. Introduction to fluid therapy. In: Fluid, electrolyte, and acid-base disorders in small animal practice. 3rd ed. St. Louis, Mo: Saunders Elsevier, 2006;325-344.