Fluid therapy: Calculating the rate and choosing the correct solution

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As discussed in "Fluid therapy in small animals: The technician's role," technicians are a vital part of making sure intravenous (IV) fluids are administered correctly in dehydrated patients. But before the fluids can be administered, the veterinarian must decide what fluids to provide and at what rate. 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

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, so fluids are used to provide hemodynamic support. If the patient's blood pressure goes below 60 mm Hg, some tissues and organs may see a decrease in perfusion. The body will protect certain important organs first, such as the lungs, heart, and brain. The kidney may see a decrease in perfusion, and acute renal failure can result from prolonged periods of extremely low blood pressure during anesthesia. This article focuses on dehydrated patients.

Determining a patient's degree of dehydration

The clinical signs of dehydration and their corresponding body dehydration percentages are presented below3:

No detectable clinical signs

5%-6% dehydrated: Subtle loss of skin elasticity

6%-8% dehydrated: Definite delay in return of skin to normal position (skin turgor), slight increase in capillary refill time, and eyes may be slightly sunken into orbits

10%-12% dehydrated: Extremely dry mucous membranes, complete loss of skin turgor, eyes sunken into orbits, dull eyes, possible signs of shock (tachycardia, cool extremities, and rapid and weak pulses), and possible alteration in consciousness

12%-15% dehydrated: Definite signs of shock; death is imminent if not corrected

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.4 Therefore, when dealing with cases like these, one must evaluate the full patient and not rely only on a few parameters to gauge hydration status.

Types of fluids

There are two categories of fluids: crystalloid and colloid solutions.

Crystalloid solutions. These fluids 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. These three groups are divided 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 will cause fluids to neither exit nor enter the cells. These solutions are given to patients for perfusion support and volume replacement. Commonly used isotonic crystalloids are Normosol-R, Plasma-Lyte-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. It causes a fluid shift that will draw fluids from the interstitial and intracellular spaces into the intravascular space. Hypertonic solutions are extremely useful in patients that need to receive a large amount of fluid quickly, but it is difficult to administer an isotonic crystalloid quickly enough. Ideal candidates for hypertonic solutions are large dogs that are in shock due to gastric dilatation-volvulus or patients that should not receive large volumes of fluid such as with head trauma. Examples of this type of crystalloid are 7% and 23% hypertonic saline solutions. Note that 23% hypertonic saline solution must be diluted to a 7.5% solution before administration. Hypertonic saline solution is normally diluted with a colloid solution in a 60-ml syringe. To achieve a 7.5% dilution, add 17 ml of the 23% hypertonic saline solution to 43 ml of the colloid solution. Over a 20-minute period, dogs should receive 4 to 7 ml/kg of the colloid solution and cats should receive 2 to 4 ml/kg. Hypertonic saline solution is contraindicated in patients that are hypernatremic, since these fluids contain high concentrations of sodium.

A hypotonic crystalloid solution has lower effective osmolality (a lower concentration of solutes that do not readily cross membranes) than intravascular fluid and, thus, draws fluid into the cells. These commonly are used to correct electrolyte imbalances (e.g. hypernatermia). Examples of hypotonic crystalloid solutions are 5% dextrose in water and 0.45% sodium chloride.

Colloid solutions. The second type of fluids is colloid solutions. They consist of larger molecules and are restricted to the plasma compartment of the body. Therefore, they are used in patients that may have an increased risk for third spacing and in patients that received crystalloid solutions but were not fully resuscitated (e.g. patients with severe hypoalbuminemia). Colloid solutions can be natural or synthetic. Natural colloids are whole blood and plasma. Examples of synthetic colloids are hetastarch, pentastarch, dextran, and hemoglobin glutamer-200 (Oxyglobin-Biopure). Hetastarch helps to retain the fluid in the intravascular compartment. Hemoglobin glutamer-200 has colloidal properties like hetastarch but is a bovine hemoglobin-based solution that is used to increase plasma and total hemoglobin concentrations in anemic animals. Whole blood and plasma are also colloids used in patients that are anemic, have a hypercoagulable disorder, or both. Whole blood and plasma should be used only when they are medically necessary, since there is a risk for anaphylaxis. Cats have been known to have difficulties with colloids. If colloids are administered too quickly, cats may become nauseated and occasionally vomit. Cats can easily experience fluid overload when receiving colloids, so they should be monitored closely while receiving these products.

Calculating the fluid replacement volume and fluid rate

The veterinarian will calculate the fluid replacement volume based on three values: the percent dehydration, ongoing losses, and the maintenance requirement. To calculate the percent dehydration, or hydration deficit, the following formula is used:

Body weight in kg x percent dehydration (as a decimal) = the fluid deficit in ml

or

Body weight in lb x percent dehydration (as a decimal) x 500 = fluid deficit in ml

The two categories of ongoing fluid loss include sensible and insensible losses. Sensible losses are those that can be quantified (e.g. urination). 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 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 (a chuck pad). With the free-catch method, you can directly measure the volume voided in milliliters (ml) using a graduated cylinder or a bowl and syringes. When using a chuck pad be sure to weigh a clean, unused pad and then weigh the soiled chuck. The weight difference is the amount of urine collected. Each 2.2 lb (1 kg) more than the normal weight of the clean chuck will equal about 1,000 ml of urine.5

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.

Maintenance fluids are defined as the required volume needed per day to keep the patient in balance, with no change in total body water.1 Most veterinarians use the rule of thumb of 40 to 60 ml/kg/day.6 To determine the volume of fluid therapy required and a fluid rate, the veterinarian must use the calculated maintenance requirement, the estimation of ongoing losses, and the calculation of hydration deficit. To calculate the patient's fluid deficit, the veterinarian will multiply the patient's body weight (lb) by the percent dehydration as a decimal and then multiply it by 500.

The result of this calculation is the amount of fluid a patient needs to become rehydrated if there are no ongoing losses. If the patient has ongoing losses (e.g. vomiting, excessive panting), the estimated losses are added to the fluid deficit. The individual patient's fluid rate will be based on the calculated maintenance rate, estimated ongoing losses, and the calculated hydration deficit. The total of the hydration deficit and ongoing losses represents the fluid volume to be replaced.

If a patient is in shock, the veterinarian probably will administer a fluid bolus. The standard shock rate of crystalloid solutions is 80 to 90 ml/kg for a dog and 40 to 60 ml/kg for a cat, and it is normally given in increments (e.g. one-third, one-half) of the calculated amount over a period of 10 to 30 minutes.7 The patient should be reassessed as soon as the bolus is complete to ensure the heart rate and blood pressure are improving. The crystalloid solution bolus is repeated as needed to achieve a normal heart rate and blood pressure. If the patient does not respond to the crystalloid fluids, then a colloid solution bolus is indicated. The standard shock rate of colloid solution bolus is 10 to 20 ml/kg for dogs and 5 to 10 ml/kg for cats (given slower in cats).

REFERENCES

1. DiBartola SP. Applied physiology of body fluid in dogs and cats. In: Fluid, electrolyte, and acid-base disorders in small animal practice. 3rd ed. St. Louis, Mo: Saunders Elsevier, 2006;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. Muir WW, DiBartola SP. Fluid therapy. In: Kirk RW, ed. Current veterinary therapy VIII. Philadelphia, Pa.: WB Saunders Co, 1983;33.

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

5. 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.

6. Wanamaker BP, Massey K. Therapeutic nutritional, fluid, and electrolyte replacements. In: Applied pharmacology for veterinary technicians. 4th ed. St. Louis: Mo.: Saunders Elsevier, 2008;298-324.

7. Waddell L. Fluid therapy in the veterinary patient, in Proceedings. Western Vet Conf, 2009.