Fluid therapy 101 (Proceedings)

Fluid therapy is one of the most important aspects of patient management in veterinary medicine. It is however, important to realize that fluid therapy is a supportive measure and the underlying disease process that lead to aberrations in water, electrolyte and acid-base status must be identified and treated. The fluid therapy prescription is based on the assessment of the individual patient and must include the amount, type, route and rate of administration required. Normal homeostatic mechanisms, particularly in patients with normal heart and renal function, provide the clinician with a considerable "buffer-zone" when administering fluids. This is fortunate, as it is very difficult to accurately assess the patients fluid deficit and even more difficult to determine the exact type of fluid that the patient requires.

Review of Body Fluid Compartments

In a healthy adult animal, approximately 60% of the total body weight is due to water. However, age, sex, and nutritional status may cause this to vary. In young animals water content may be as much as 70-80%, while older animals may have water content that is 50-55% of their body weight. Almost 66% of the total body water is intracellular fluid (ICF) and the remaining 33% is extracellular fluid (ECF). Extracellular fluids can be further divided into intravascular or plasma water (water in the vascular space) (25% of the ECF volume) and interstitial fluid (fluid that is present in the spaces between cells) (75% of the ECF volume). Transcellular fluid (including cerebral spinal fluid, synovial fluid, plural fluid, peritoneal fluid, aqueous humor, and gastrointestinal secretions) is found in very small amounts and is also considered an ECF fluid. The distribution of fluid within the body s very important as the concentration of solutes in the ECF and ICF fluid compartments are strikingly different. ECF contains high concentrations of sodium, calcium, chloride and bicarbonate with only small amounts of potassium, phosphate, magnesium, and protein. In ICF the distribution is quite different with potassium, phosphate, and magnesium being the principle electrolytes.

Fluid Balance

A healthy animal with access to water will maintain a neutral fluid balance (i.e., water in = water out). The primary source of water for the body is that which is drunk or ingested in food. However a small amount of water (0.1g/kcal energy) is also produced via metabolism. When a patient develops dehydration, it is due to either decreased water intake, increase water loss, or a loss of fluid to an abnormal body compartment making it functionally unavailable to the animal. (AKA third space fluid). A decrease in the amount of water consumed may occur when the patient is anorexic due to an underlying disease condition or has restricted access to water. Primary adipsia is quite rare in veterinary medicine, but should be considered in a chronically dehydrated otherwise healthy, animal with free access to water.

Fluid may be lost from the body through urine, feces, sweat, saliva, respiration and draining wounds. Excessive fluid loss form the urinary tract may occur in many disease states with a few of the more common including kidney disease, diabetes mellitus and Cushing's syndrome. Another significant source of fluid loss from the body is the gastrointestinal tract. Typically only a small amount of fluid is excreted in feces, but animals with diarrhea may lose a significant amount of fluid and electrolytes. Likewise, vomiting patients may lose a significant amount of water and electrolytes. As most vomitus includes proximal duodenal contents, patients lose water, H+ , Cl- , Na+ , K+ , and HCO3- , resulting in dehydration alog with a metabolic acidosis. However, patients with pure gastric vomiting lose water, H+ , Cl- , Na+ , and K+ and tend to have a metabolic alkalosis. Third space fluid loss refers to fluid that is functionally lost from the animal, but is still contained within the body. i.e., ascites, pleural effusion, fluid-filled obstructed bowel, hematoma.

Indications for Fluid Therapy

The most common indications for fluid therapy in veterinary medicine include 1) correction or prevention of dehydration (extravascular fluid deficit), 2) correction of hypovolemia (shock, intravascular fluid deficit), 3) maintenance of tissue perfusion (especially during anesthesia), and 4) to provide colloidal oncotic support. Additionally, fluids may be used when a fluid diuresis is desired, such as with kidney disease or to hasten the elimination of toxins that are excreted by the kidneys. Fluid therapy may also be employed when there is a need to deliver specialized fluids such as blood or blood components, and total parenteral nutrition.

Prescribing Fluid Therapy

When developing a fluid therapy prescription, the clinician must make 4 major decisions;

      1. How much fluid does the patient require?

      2. By what route should the fluid be administered?

      3. Which type of fluid should be given?

     4. What is the rate of administration required?

1. Determining the Amount of Fluids Required

When determining the amount of fluid to deliver to a patient, the clinician must consider components of the fluid prescription: 1) volume required to correct dehydration, 2) volume required for normal maintenance, and 3) volume required for ongoing losses.

Determining Fluid Deficit: The degree of dehydration is best assessed from the physical exam. For guidelines on assessing the degree of dehydration from the physical exam, please refer to Table 1. When assessing hydration, it is important to recognize that certain variables may affect the appearance of hydration. For example, obese animals may be more dehydrated than they appear, as the subcutaneous fat will dampen any skin tent. Conversely, emaciated animals may have a prolonged skin tent, even if normally hydrated, due to lack of subcutaneous fat. Perhaps the most accurate method of assessing dehydration is by body weight. Unfortunately, the accurate pre-dehydration weight of a patient is seldom known. Likewise, packed cell volume (PCV) and total solids (TS) measurement would be valuable if pre-dehydration values were known.

Table 1: Guideline for Estimating Dehydration

Once the percent dehydration has been determined in the individual patient, the volume of fluid required to correct the deficit may be determined from the following equation:

% dehydration x BW (kg) x 1000 = hydration deficit in mls

   Determining Maintenance Fluid Requirements: Maintenance fluid in dogs and cats are typically 40-60ml/kg/day. This is an estimate based on expected sensible losses (urine) of 27-40 ml/kg/day in a patient with normal renal function and expected insensible losses (feces, cutaneous, respiration) of 13-20 ml/kg/day.

 Determining Ongoing Losses: Ongoing losses may be divided into sensible (urine) and insensible (everything else). Patients with kidney disease, diabetes, Cushing's syndrome, or on certain medications, will require additional fluids to counter their increased losses. Ideally the amount of urine produce in a specified time is recorded, and any volume in excess of the allotted maintenance fluids should be delivered to the animal. If urine collection is not feasible, the patient should be weighed frequently to ensure that their fluid balanced is maintained. Insensible losses such as vomitus, fecal, cutaneous and respiratory water are difficult to assess and typically rely on serial physical exams and measurement of body weight, PCV and TS.

2. Determining the route of fluid administration

The route of fluid administration will depend on the patient's clinical condition, the degree of dehydration and the rapidity with which the patient needs to be rehydrated. Rehydration using oral fluids is the most physiologic route and probably the safest, however, many patients require emergency fluid therapy or are vomiting which precludes using the oral route. The advantages and disadvantages of the various routes of fluid administration are outlined in the table below. The focus of this lecture will be on the use of intravenous fluids as this is the most commonly used route in veterinary medicine.

Advantages and disadvantages of the various routes of fluid administration.

3. Determining the type of fluid to administer

There are five broad categories of fluids used in veterinary practice: 1) crystalloids, 2) colloids, 3) blood products, 4) hemoglobin-based oxygen carrying solutions, and 5) parenteral nutrition.

Crystalloids: The foundation of fluid therapy is based on the crystalloids, which consist primarily of water and sodium with the addition of other electrolytes and/or buffers. Within the crystalloid group are 4 different sub-groups of fluids: 1) replacement solutions, 2) maintenance solutions, 3) hypertonic solutions, and 4) dextrose in water. (see Table 3 following these notes)

Replacement crystalloid solutions contain dissolved solutes that approximate the solute concentration of extracellular fluid, especially plasma. These solutions are indicated for the rapid replacement of intravascular volume and electrolytes as seen with shock and hemorrhage or severe volume depletion secondary to the losses associated with vomiting, diarrhea, third body spacing, or excessive diuresis. Crystalloids are characterized by rapid redistribution to all body compartments to the extent that only 20-25% of the infused volume of fluid remains within the intravascular space 1 hour after infusion. The commonly available replacement solutions include normal saline (0.9% NaCl), Ringer's solution (lactate or acetate), Normosol R, and Plasmalyte A. Although similar, each of these fluids has a slightly different composition that would promote its use in specific situations.

Maintenance solutions differ from replacement fluids in that they contain comparatively less sodium, more potassium and more free water. Maintenance solutions are designed to fulfill the electrolyte requirements of patients with normal daily electrolyte losses that are unable to maintain adequate fluid or electrolyte intake. Since these solutions are usually hypotonic, less than 10% of the infused volume remains within the vascular space 1 hour after infusion. Commercially available maintenance fluid solutions include half strength saline (0.45% NaCl), half strength saline + dextrose, and Plasmalyte 56.

Colloids: Colloids are large molecular weight molecules that are restricted to the vascular space and thus draw water into the vascular space from the interstitium. A given volume of colloid expands the vascular volume 2 to 4 times as much as the same volume of crystalloid and the effect is more sustained. When colloids are used to restore circulating volume, crystalloids must also be administered to replace the extravascular deficit, but the dosage may often be reduced by 40-60%. In addition to being useful in the initial management of hypovolemia, colloids are also useful in inflammatory conditions where vasculitis may allow other fluids to leak form the vascular space. Colloids are often used to help maintain oncotic pressure in animals with hypoproteinemia. Plasma, human albumin, hetastarch, dextrans and other synthetic compounds are all considered to be colloids. Contra-indications for the use of colloidal fluids include coagulopathies, volume intolerance (heart disease, pulmonary disease, oliguric renal failure), and immune reactions (if using plasma or human albumin).

Blood Products: Blood products include fresh whole blood, packed red blood cells, plasma and plasma components such as platelets and coagulation factors. Blood products are indicated to replace red blood cells, plasma proteins, platelets, and/or coagulation factors. The type of product needed is based on the component of patient blood that needs to be replaced. Whole Blood is indicated when the oxygen content of the blood falls to critically low levels. These low levels will correspond to an approximate PCV of 25 (acute loss) to 15 (chronic loss). While cross-matching is strongly recommended in both dogs and cats, at least confirming the patients blood type blood is essential prior to administering blood products to cats as they have pre-formed antibodies.

Hemoglobin-based oxygen carrying solutions: Oxyglobin® is a polymerized bovine hemoglobin in a modified lactated ringer's, solution. It is used primarily as a blood substitute in the volume resuscitation of severe hypovolemic states and in the management of anemia when blood products are not available. Oxyglobin® has significant colloidal activity with a colloid oncotic pressure of approximately 40. Oxyglobin® may also improve microvascualr perfusion as it is able to deliver oxygen to the tissues more effectively since it is not limited by red blood cell flow. Disadvantages to using Oxyglobin® are that it significantly interferes with the determination of any chemistry parameter that relies on a color as a component of the assay. It's strong colloidal pull is often a disadvantage, especially when dealing with a patient who is already volume sensitive.

Parenteral nutrition: Total Parenteral Nutrition (TPN) is used to facilitate the delivery of nutrients, water, and electrolytes to animals in need of nutritional support. TPN is indicated when enteral nutrition is impossible, hazardous, or incapable of meeting the patient's needs. Clinical examples include severe pancreatitis, malabsorption, hepatic lipidosis of cats, sepsis, and coma. By necessity, TPN solutions are markedly hyperosmolar and must be delivered through a central vein to prevent phlebitis.

Fluid Additives

The most commonly used fluid additive is potassium chloride. The potassium requirement is approximately 1+ mEq/Kg/day. Potassium depletion occurs with anorexia or prolonged fluid therapy with potassium free solutions. The assessment of potassium debt is difficult since this ion is primarily intracellular, but it is generally recommended that potassium be supplemented to the patients' fluids following the guidelines in the table below. Once supplemented, serum potassium be re-evaluated routinely and therapy adjusted if necessary.

Guidelines for potassium supplementation (when fluids given at maintenance rate)

4. Determining the rate of administration

There are no set "rules" governing the rate of fluid administration, but the following guidelines may be helpful. Animals in shock and/or those with acute fluid loss should have their deficit replaced over 2-6 hours. One approach is to replace half of the deficit rapidly and they give the other half over the 2-6 hours. More chronic fluid deficits may be replaced over 10-12 hours, but patients with chronic kidney disease should be re-hydrated as quickly as their clinical condition will allow renal perfusion to be restored and minimize further kidney damage. The rate of administration may need to be adjusted based on the patient's response. No single monitoring parameter will provide all of the information required to guide fluid therapy, but monitoring should be based on serial evaluations of body weight, skin turgor, pulse rate and quality, respiratory rate and effort, mentation, mucus membrane color, and capillary refill time. If an indwelling urinary catheter is in place, serial evaluation of fluid input and urine output can provide important information concerning the adequacy of the fluid therapy being administered. Laboratory values such as PCV, TP, albumin, BUN, creatinine and lactate may also be useful in adjusting the fluid prescription.

Discontinuation of Fluid Therapy

Discontinuing fluid therapy should not be done abruptly. When a patient is receiving fluid therapy, the solute gradient in the kidneys may be altered due to the diuresis; this is known as medullary washout. If the patient is then abruptly discontinued from fluid therapy, the medullary washout may prevent adequate urine concentrating ability thus predisposing the patient to dehydration. To prevent this from occurring, patient should be weaned off of intravenous fluid therapy over approximately 24 hours. If this is not possible or practical, every effort should be made to ensure that the animal is drinking well and maintaining hydration.