Is the patient suffering from a shock syndrome that requires immediate fluid administration?

Shock patients (see Chapter 23) urgently require fluid therapy. The presence of altered mental status and cool extremities in association with tachycardia or severe bradycardia, mucous membrane pallor, prolonged or absent capillary refill time, reduced or absent peripheral pulses, and hypotension are among the most common physical examination findings in patients in shock. Such physical examination findings in association with a compatible clinical history are the basis for the decision to institute a resuscitation phase of fluid therapy. Some forms of shock may be associated with variations in these physical examination findings, and it is crucial to understand the different shock syndromes. (See Chapter 23 for more information on shock.)

The shock syndromes most likely to respond to marked volume expansion of the intravascular space are hypovolemic and distributive shock states. Obstructive forms of shock often respond favorably to moderate volume expansion. Fluid administration is contraindicated in patients with predominantly cardiogenic forms of shock.

Regardless of their underlying disease, severely dehydrated patients can be in shock and require a resuscitation phase of fluid therapy before initiating the rehydration phase. However, not all patients in shock are dehydrated and thus may or may not require a rehydration phase of therapy. The rapidity and volume of loss from both the intravascular and extravascular fluid compartments in conjunction with the extent of any compensatory response will determine whether the patient is in shock or is dehydrated.

Is the patient dehydrated?

The need for a rehydration phase is dependent on the underlying condition of the patient. For surgical patients, there are additional indications for fluid therapy, such as maintenance of venous access for emergencies and establishment of diuresis to maintain renal perfusion during anesthesia (see Chapter 17). For medical patients, the answer to this question depends on an assessment of the animal’s state of hydration. The hydration status of the animal is estimated by careful evaluation of the history, physical examination findings, and the results of a few simple laboratory tests.^7,11

In its most narrow sense, dehydration refers to loss of pure water. However, the term dehydration usually is used to include hypotonic, isotonic, and hypertonic fluid losses. The type of dehydration is classified by the tonicity of the fluid remaining in the body (e.g., a hypotonic loss would result in hypertonic dehydration). Isotonic and hypotonic losses are most common in small animal practice. Isotonic fluid loss can result in volume depletion and nonosmotic stimulation of antidiuretic hormone (ADH) release, thus preventing effective excretion of consumed water and resulting in hypotonic dehydration. Types of dehydration are depicted in Figure 3-1 and are discussed in detail in Chapter 3.

Fluid balance

Normal sources of fluid input are water consumed in food, water that is drunk, and water produced in the body as a result of metabolism. Nutrient oxidation produces approximately 0.1 g of water per kilocalorie of energy released.² Maintenance water and electrolyte needs parallel caloric expenditure,^20,22,23 and normal daily losses of water and electrolytes include respiratory, fecal, and urinary losses. Estimated daily caloric and water requirements for dogs and cats are shown in Tables 14-1 and 14-2²³ and in Figure 14-1.²⁰ Respiratory loss of fluid can be important in dogs because panting has been adapted for thermoregulation in this species. Pyrexic patients also can lose fluid by this route. Normally, cutaneous losses are unimportant in dogs and cats because eccrine sweat glands are limited to the foot pads and do not play an important role in thermoregulation in these species. Sympathetic stimulation as a result of heat stress in the cat may result in increased secretion of saliva, and a small volume of fluid may be lost by this route.

Table 14-1 Daily Water and Calorie Requirements for the Dog*

Table 14-2 Daily Water and Calorie Requirements for the Cat*

Figure 14-1 Daily water, calorie, and electrolyte requirements for dogs and cats.

(From Harrison JB, Sussman HH, Pickering DE. Fluid and electrolyte therapy in small animals. J Am Vet Med Assoc 1960;137:638.)

In disease states, decreased fluid intake results from anorexia, and increased fluid loss may occur by urinary (e.g., polyuria) and gastrointestinal (e.g., vomiting, diarrhea) routes. Other less common routes of loss include skin (e.g., extensive burns), respiratory tract, and salivary secretions, as described before. Third-space loss of fluid occurs when effective circulating volume is decreased, but the fluid lost remains in the body. Examples include intestinal obstruction, peritonitis, pancreatitis, and effusions or hemorrhage into body cavities. Decreased fluid intake and increased loss often coexist (e.g., anorexia, vomiting, and polyuria in a uremic animal).

History

Historical information about the route of fluid loss may suggest the affected fluid compartment or compartments, as well as the patient’s electrolyte and acid-base derangements. The time period over which fluid losses have occurred and an estimate of their magnitude should be determined. Information about food and water consumption, gastrointestinal losses (e.g., vomiting, diarrhea), urinary losses (i.e., polyuria), and traumatic losses (e.g., blood loss, extensive burns) should be obtained from the owner. Excessive insensible water losses (e.g., increased panting, pyrexia) and third-space losses may be determined from the history and physical examination. In addition, the clinician’s knowledge of the suspected disease can aid in predicting the composition of the fluid lost (e.g., vomiting caused by pyloric obstruction leads to loss of hydrogen, chloride, potassium, and sodium ions and development of metabolic alkalosis, whereas small bowel diarrhea typically leads to loss of bicarbonate, chloride, sodium, and potassium ions and development of metabolic acidosis) (Table 14-3).

Table 14-3 Potential Fluid, Electrolyte, and Acid-Base Disturbances in Various Diseases and Suggested Crystalloid Solutions

Can the patient consume an adequate volume of water to sustain normal fluid balance?

Hospitalized patients that have been volume resuscitated and rehydrated may not have recovered to the extent that appetite and ability to consume water have returned to normal. Such patients require administration of adequate amounts of fluid to meet their needs. The needs of partially or completely anorexic hospitalized dogs and cats are not well understood. Most predictions about maintenance fluid requirements are extrapolated from studies of normal nonanorexic animals. Absorption of nutrients into the bloodstream and their subsequent metabolism produces solutes that must be excreted in urine. Sensible fluid losses and urine production are decreased during fasting in normal animals because less solute requires excretion. Thus the maintenance fluid requirements of partially or completely anorexic patients are difficult to predict. Typically fluids are administered to veterinary patients in volumes predicated on the needs of animals that are not anorexic. Careful observation of urine production in such patients is warranted. If the patient is expected to have normal urinary concentrating ability and is urinating large volumes of dilute urine frequently, excessive fluid administration may be a contributing factor. Careful reduction of fluid administration and subsequent observation are warranted in such patients.

What type of fluid should be given?

A fluid is said to be balanced if its composition resembles that of extracellular fluid (ECF; e.g., lactated Ringer’s solution, Normosol-R [Abbott Laboratories, Abbott Park, Ill.], Plasma-Lyte 148 [Baxter Healthcare, Deerfield, Ill.]) and unbalanced if it does not (e.g., normal saline). Fluid preparations may be further classified as crystalloids or colloids. Crystalloids are solutions containing electrolyte and nonelectrolyte solutes capable of entering all body fluid compartments (e.g., 5% dextrose, 0.9% saline, lactated Ringer’s solution). Crystalloids exert their effects primarily on the interstitial and intracellular compartments. Colloids are large-molecular-weight substances that are restricted to the plasma compartment in patients with an uncompromised intact endothelium and include plasma, dextrans, hydroxyethyl starch (hetastarch), and hemoglobin-based oxygen-carrying (HBOC) fluids. Colloids exert their primary effect on the intravascular compartment.

Some types of colloids may be used in patients with shock and in those with severe hypoalbuminemia (i.e., albumin <1.5 g/dL). A major limitation to the use of plasma as a colloid is the rapid disappearance of albumin from the vascular space. Dextran 70 is a polymer of glucose that has an average molecular weight of 70,000. Its use in humans has been associated with coagulopathies. Hetastarch has an average molecular weight of 480,000. In humans, coagulopathies also have been associated with the use of hetastarch but typically only when standard dosage recommendations have been exceeded. The main advantages of colloids are that more of the administered solution remains in the plasma compartment and there generally is thought to be less risk of edema in patients with an intact endothelium. Colloids are discussed in detail in Chapter 27.

Crystalloid solutions are equally effective in expanding the plasma compartment, but 2.5 to 3.0 times as much crystalloid solution must be given (compared with a colloid solution) because the crystalloid is distributed to other sites (e.g., interstitial compartment, intracellular compartment).^26,27,39 Pulmonary capillaries normally are more permeable to protein, resulting in a higher interstitial concentration of protein and more resistance to leakage of fluid from capillaries.³⁰ Peripheral edema is more likely to occur after crystalloid administration because muscle and subcutaneous capillaries are less permeable to protein.

Crystalloid solutions also can be classified as replacement or maintenance solutions. The composition of replacement solutions (e.g., lactated Ringer’s, Normosol-R, Plasma-Lyte 148) resembles that of ECF (Figure 14-2). Maintenance solutions (e.g., Normosol-M, Plasma-Lyte 56) contain less sodium (40 to 60 mEq/L) and more potassium (15 to 30 mEq/L) than replacement fluids. A simple maintenance solution can be formulated by mixing one part 0.9% NaCl with two parts 5% dextrose and adding 20 mEq KCl per liter of final solution. The approximate composition of such a fluid would be 51 mEq/L sodium, 20 mEq/L potassium, 71 mEq/L chloride, and 33.5 g/L dextrose. It would provide 133 kcal/L and have an osmolality of 328 mOsm/kg. An alternative maintenance solution may be made by mixing one part lactated Ringer’s solution with two parts 5% dextrose and adding 20 mEq KCl per liter of final solution. This solution has the following approximate composition: 43 mEq/L sodium, 21 mEq/L potassium, 56 mEq/L chloride, 1 mEq/L calcium, 9 mEq/L lactate, and 33.5 g/L dextrose. It would provide 133 kcal/L and have an osmolality of 317 mOsm/kg.

Figure 14-2 Comparison of electrolyte composition of plasma with that of commonly used crystalloid solutions.

(From Muir WW, DiBartola SP. Fluid therapy. In: Kirk RW, editor. Current veterinary therapy VIII. Philadelphia: WB Saunders, 1983: 30.)

Another commonly used crystalloid is 5% dextrose. Administering 5% dextrose is equivalent to giving water because the glucose is oxidized to CO₂ and water. In fact, the main reason for giving 5% dextrose is to correct a pure water deficit. Except in very small animals, administration of 5% dextrose cannot be relied on to maintain daily caloric needs because 5% dextrose contains only 200 kcal/L. Consider a normal, active 10-kg dog. Its maintenance energy requirement (MER) is approximately 740 kcal:

To provide this number of kilocalories from 5% dextrose (200 kcal/L), almost 4 L of fluid must be administered per day. Such a volume is almost seven times more than the daily maintenance requirement for fluid in this dog. Administration of 4 L of 5% dextrose over a 24-hour period would initiate a diuresis that would impair use of the administered dextrose and cause increased urinary losses of electrolytes.

The veterinary practitioner can manage most animals requiring fluid therapy with a limited number of crystalloid and additive solutions. The most useful crystalloid solutions for routine use are a balanced replacement solution (e.g., lactated Ringer’s solution, Normosol-R, Plasma-Lyte 148), 0.9% saline, and 5% dextrose in water. The solute composition of these fluids is compared with that of ECF in Figure 14-2, and the electrolyte composition of several commercially available solutions is summarized in Table 14-6.

Table 14-6 Electrolyte Composition of Commercially Available Fluids

Supplementation of crystalloid solutions with KCl may be necessary when body fluid losses include large amounts of potassium. An empirical scale has been devised to estimate the amount of potassium to add to parenterally administered fluids (Table 14-7).¹⁷ This protocol has not been evaluated experimentally in dogs or cats but has been used successfully in clinical veterinary patients during the past 30 years. Potassium supplementation is discussed in Chapter 5.

Table 14-7 Sliding Scale for Potassium Supplementation

* So as not to exceed 0.5 mEq/kg/hr.

From Muir WW, DiBartola SP. Fluid therapy. In: Kirk RW, editor. Current veterinary therapy VIII. Philadelphia: WB Saunders, 1983: 38.

Other additive solutions include 50% dextrose, calcium chloride, calcium gluconate, potassium phosphate, 8.4% sodium bicarbonate, and water-soluble B vitamins. Thiamine supplementation may be particularly important in cats because their requirement for this vitamin may be higher than that of dogs. Phosphate rarely is used as an additive but often is required in patients with diabetic ketoacidosis during insulin therapy.⁴⁰ Phosphate supplementation is discussed in Chapter 7. Theoretically, sodium bicarbonate should not be added to solutions containing calcium (e.g., lactated Ringer’s solution, Plasmalyte-R) because of the risk of forming insoluble calcium carbonate crystals. Despite this concern, no adverse consequences have been observed when small amounts of sodium bicarbonate have been added to lactated Ringer’s solution.^22,23

When additives are used, the clinician must keep in mind that the final osmolality of the fluid may be higher than anticipated. The final osmolality may be approximated by adding the number of milliequivalents per liter of electrolyte and millimoles per liter of nonelectrolyte solutes found in the solution. The final osmolality of the solution also may differ depending on how the solution was formulated. For example, if 500 mL of lactated Ringer’s solution is mixed with 500 mL of 5% dextrose to create a replacement solution with 2.5% dextrose, the resulting solution has an approximate osmolality of 275 mOsm/kg (virtually the same as that of lactated Ringer’s solution). Conversely, if 50 mL of 50% dextrose is added to 1 L of lactated Ringer’s solution, the resulting solution contains 2.5% dextrose but has an approximate osmolality of 391 mOsm/kg, which is substantially higher.

The choice of fluid to administer is dependent on the nature of the disease process and the composition of the fluid lost. Underlying acid-base and electrolyte disturbances should be taken into consideration when choosing the type of fluid to administer. The clinician should attempt to replace losses with a fluid that is similar in volume and electrolyte composition to that which has been lost from the body (see Table 14-3). If clinical assessment of the patient suggests a fluid-responsive type of shock, the resuscitation phase of fluid therapy should be instituted. If the patient has abnormally low oncotic pressure or an underlying disease condition for which a low-volume resuscitation strategy may prove advantageous, synthetic colloids should be considered as the primary fluid choice for resuscitation (see Chapters 23 and 27). If neither of these considerations applies, resuscitation with a balanced crystalloid solution is indicated. If there are no clinical signs of hypovolemia, the hydration deficit and maintenance needs may be combined and administered during the next 24 hours.

Persistent vomiting caused by pyloric obstruction would be expected to result in losses of hydrochloric acid, potassium, sodium, and water, potentially producing hypokalemia, hypochloremia, and metabolic alkalosis. The initial fluid of choice in this setting is 0.9% NaCl with 20 to 30 mEq KCl per liter. Except in the case of vomiting of stomach contents, lactated Ringer’s is a good first choice for fluid therapy while awaiting laboratory results. Normal saline (0.9% NaCl) is less ideal because it is not a balanced solution. It contains chloride in greater concentration than body fluids (154 mEq/L versus 110 mEq/L in dogs and 120 mEq/L in cats), and as a result of displacement of bicarbonate with chloride in ECF and initiation of natriuresis, it has a mild acidifying effect.³² Examples of fluid therapy in specific diseases are listed in Table 14-3.

In one study, five different solutions were administered to unanesthetized dogs during a 1-hour period: 0.9% NaCl, 0.9% NaCl with 5% dextrose, lactated Ringer’s solution, Normosol-R, and Normosol-R with 2% dextran.³² The approximate composition of these fluids is presented in Table 14-8. The fluids were warmed to body temperature, and no decreases in rectal temperature were observed. Laboratory variables were measured after 1 hour of infusion. Fluids were administered at 76 mL/kg/hr except for Normosol-R with dextran, which was administered at a rate of 31.5 mL/kg/hr.

Table 14-8 Composition of Fluids Administered to Awake Dogs

Most of the fluids increased heart rate, diastolic arterial pressure, and central venous pressure (CVP), and all of them decreased hematocrit, hemoglobin, and total protein concentrations by 21% to 25%. All solutions except for Normosol-R and Normosol-R with 2% dextran caused an increase in serum chloride concentration, and the saline solutions decreased pH and bicarbonate concentration. All solutions except Normosol-R caused a decrease in serum potassium concentration. The causes of the decreased serum potassium concentrations in these dogs presumably included dilution and increased distal tubular flow rate with enhanced urinary excretion of potassium. The presence of 5% dextrose in two of the solutions resulted in significantly lower serum potassium concentrations, suggesting movement of potassium into cells with glucose.

Serum sodium concentrations were similar despite differences in the sodium concentrations of the various fluids, demonstrating effective natriuresis in normal dogs receiving sodium-containing crystalloid solutions. Serum chloride concentration increased with administration of the saline solutions containing 154 mEq/L chloride, and mild metabolic acidosis developed. Serum chloride concentration also increased slightly with administration of lactated Ringer’s solution (112 mEq/L chloride), but there was no change in acid-base balance. The increased serum chloride concentration and alterations in acid-base balance could have resulted from decreased reabsorption of bicarbonate with sodium in the kidney during natriuresis and decreased strong ion difference.^37,38 Expansion acidosis is an unlikely explanation because all fluids administered presumably expanded the ECF volume.

Anions such as acetate, gluconate, and lactate are added to crystalloid solutions as a source of base because their oxidative metabolism in the body yields bicarbonate. The alkalinizing effect of the metabolism of these anions and that of citrate is as follows:

Acetate

Citrate

Gluconate

Lactate

Most lactate is produced in muscle and gut and metabolized to either glucose (via cytosolic gluconeogenesis) or CO₂ and water (via mitochondrial oxidation) in the liver. Normally, gluconeogenesis predominates. Acetate is metabolized primarily in muscle. The alkalinizing effect of these anions is delayed because of the requirement for metabolism. In one study, equivalent doses of acetate, bicarbonate, and lactate had similar alkalinizing effects in anesthetized dogs 45 minutes after infusion.²¹ The effect of bicarbonate occurred earliest because metabolism was not necessary.

Lactate originally was introduced for the treatment of acidosis because of technical difficulties in preparation of bicarbonate solutions suitable for intravenous use.^5,36 These technical difficulties have been overcome, but crystalloid solutions containing lactate as a source of base (e.g., lactated Ringer’s solution) still are widely used for fluid therapy in clinical practice. Most patients treated with lactate-containing replacement solutions respond well, probably as a result of ECF volume expansion and improved tissue perfusion.

Whether it is converted to glucose or oxidized to CO₂ and water, the metabolism of lactate consumes hydrogen ions and has an alkalinizing effect:

Gluconeogenesis

Oxidative metabolism

There has been some concern that lactate in lactated Ringer’s solution may be harmful to patients with poor tissue perfusion and severe metabolic acidosis (pH, <7.1 to 7.2). Administration of lactate as a salt cannot contribute directly to metabolic acidosis. Rather, the ability of the liver to metabolize lactate and the potentially detrimental effect of lactate on myocardial contractility have been debated. During severe hypoxia, increased lactate production in gut and muscle and decreased hepatic extraction of lactate led to progressive lactic acidosis. In moderate metabolic acidosis, administration of lactated Ringer’s solution probably is beneficial because any tendency toward lactate accumulation is likely to be offset by improved hepatic perfusion and oxygen delivery as a result of ECF volume expansion.

Newer commercially available balanced crystalloid solutions contain approximately twice the amount of bicarbonate precursors when compared with lactated Ringer’s solution. As a result, these solutions generally are thought to be more efficient than lactated Ringer’s solution in treatment of metabolic acidosis, provided that metabolic conversion of the precursors to bicarbonate occurs quickly. There is some concern that such fluids may contribute to the development of metabolic alkalosis, but this does not occur in animals with relatively normal renal function because the kidneys can efficiently excrete the excess bicarbonate.

Crystalloid solutions with preservatives must be avoided in cats. Benzoic acid derivatives (e.g., benzyl alcohol, methylparaben, propylparaben, ethylparaben) have been added to some solutions for their antimicrobial effect. Clinical signs in cats receiving fluids with such preservatives have included behavioral changes, hypersalivation, ataxia, muscle fasciculations, seizures, dilated nonresponsive pupils, coma, and death.^3,9,33 Young cats may be at increased risk for these complications.

By what route should fluids be given?

The route of fluid therapy depends on the nature of the clinical disorder, its severity, and its duration.

How rapidly may fluids be given?

Poiseuille’s law governs the flow of fluids through a catheter:

where P₁ − P₂ represents the pressure differential on the fluid, η is the viscosity of the fluid, r is the radius of the catheter, and L is the length of the catheter. Thus the diameter of the catheter is of primary importance in establishing a rapid rate of flow. The choice of catheter length sometimes is affected by factors other than flow rate (e.g., use of jugular catheters to monitor CVP). In a study of gravity flow of lactated Ringer’s solution, in vivo flow rates averaged 7% less than in vitro flow rates, presumably because of tissue pressure.¹⁵ Fluid flow rate increased by 50% when the pressure differential was increased by raising the fluid bag from 0.91 to 1.75 m. Flow rate increased linearly with increasing catheter radius rather than geometrically as predicted by Poiseuille’s law.

The rate of fluid administration is dictated by the magnitude and rapidity of the fluid loss. The patient with fluid-responsive shock syndrome requires aggressive fluid administration. Fluid administration rates may vary, depending on the type of fluid or combination of types that has been chosen. One approach is to calculate a “shock fluid dose” and administer it as rapidly as possible in divided aliquots until a stable and sustainable cardiovascular endpoint has been achieved (see Chapter 23). Clinical evaluation of the patient should occur after administration of each aliquot using a “titrate to effect” approach. The shock dosage of synthetic colloids is 20 mL/kg for dogs and 10 to 15 mL/kg for cats. The shock dosage of isotonic crystalloids is 80 to 90 mL/kg for dogs and 40 to 60 mL/kg for cats. In experimental studies, crystalloid fluids administered at 90 mL/kg/hr did not cause pulmonary edema in normal dogs and cats.^4,8

Anesthetized cats receiving lactated Ringer’s solution at a rate of 225 mL/kg for 1 hour developed serous nasal discharge, chemosis, ascites, diarrhea, and fluid exudation from catheter sites. At necropsy, these cats had ascites, pancreatic edema, and accumulation of free fluid in the trachea. Body temperature decreased and CVP and left atrial pressure increased in cats receiving 225 mL/kg/hr, whereas hematocrit, total protein concentration, and colloidal osmotic pressure decreased in cats receiving both 90 and 225 mL/kg/hr.⁴

Lactated Ringer’s solution was administered to unanesthetized, dehydrated dogs at rates of 90, 225, and 360 mL/kg for 1 hour.⁸ At rates of 90 and 225 mL/kg/hr, some dogs had serous nasal discharge, mild coughing, and slight chemosis. At 360 mL/kg/hr, marked serous nasal discharge, restlessness, coughing, dyspnea, pulmonary crackles, ascites, polyuria, chemosis, protrusion of eyes, and diarrhea were observed. These signs resolved when fluid administration was discontinued. Hematocrit, TPP, and serum potassium concentration decreased during fluid administration. In this study, body temperature decreased despite the fact that fluids were warmed to 37° C. Serum sodium concentration remained unchanged, but pulse rate, respiratory rate, and systemic arterial pressure increased slightly. Pulmonary capillary wedge pressure (PWP) and CVP increased, and these measurements correlated well with one another. It was concluded that lactated Ringer’s solution at 90 mL/kg/hr was tolerated safely. CVP should be monitored if fluids must be administered at rates in excess of 90 mL/kg/hr.

Contemporary losses must also be considered when adjusting the rate of fluid administration. Severe ongoing losses (e.g., vomiting and diarrhea in a patient with acute gastroenteritis) may necessitate rapid administration to keep pace with contemporary fluid loss. When fluids are given rapidly, it is necessary to monitor cardiovascular and renal function.

It usually is not necessary or desirable to replace the hydration deficit rapidly in chronic disease states. Instead, the hydration deficit may be calculated, the daily maintenance requirement of fluid added to this amount, and the total volume administered over 24 hours.³⁵ Ongoing or contemporary losses also must be considered and taken into consideration when estimating the patient’s fluid requirements for a 24-hour period. This approach allows adequate time for equilibration of fluid and electrolytes with the intracellular compartment and avoids potential complications (e.g., edema or effusion related to increased hydrostatic pressure, diuresis, and loss of administered electrolytes in urine). It is the method most commonly used for medical patients at the Ohio State University Veterinary Teaching Hospital.

Whenever possible, intravascular volume deficits should be replaced before anesthesia and surgery. Ideally, such patients also should be rehydrated depending on the urgency of their underlying condition. During induction and maintenance of anesthesia, prevention of hypovolemia, and maintenance of renal perfusion are essential. Induction of diuresis in this setting may be an important factor in prevention of intraoperative acute renal failure. A basal fluid administration rate of 5 to 10 mL/kg/hr is recommended during anesthesia and surgery. During major surgery (e.g., exploratory laparotomy, thoracotomy), fluid administration at twice this basal rate is recommended. Fluid therapy during anesthesia and surgery is discussed in more detail in Chapter 17.

Most administration sets designed for adult human patients deliver 10 to 20 drops/mL, whereas pediatric administration sets deliver 60 drops/mL.³⁵ This information is used to calculate the drip rate:

Adult administration set:

or

Pediatric administration set:

or

Fluid orders should be written so that the volume to be administered is recorded as mL/day, mL/hr, and drops/min. This allows personnel to detect errors in calculations. The clinician should not assume that the animal has received the volume of fluid ordered, and the volume actually received should be noted in the record by nursing personnel. All additives should be clearly listed on the bottle, and adhesive labels for this purpose are available (Figure 14-3). Infusion pumps are available for clinical use (e.g., Heska, Baxter) and provide a highly accurate record of the volume infused (Figure 14-4). These pumps also have alarm systems that can alert personnel when flow is obstructed. The availability of affordable electronic fluid pumps has resulted in widespread incorporation of such equipment into veterinary practice. Although use of infusion pumps makes fluid administration safer and more accurate, the equipment must be used appropriately, maintained in good working order, and tested regularly for accuracy. Mistakes in fluid administration still can occur as a consequence of human error or equipment failure. For practices that do not routinely use electronic fluid pumps, several management practices may assist in accurately and safely delivering fluid therapy. A strip of adhesive tape can be attached to the bottle and marked appropriately to provide a quick visual estimate of the volume of fluid received (Figure 14-5). In the Buretrol system (Baxter, Deerfield, Ill.), a reservoir allows a predetermined volume of fluid to be delivered over a given period (Figure 14-6). This approach prevents infusion of excessive volumes of fluid to small animals. The technical aspects of fluid therapy are discussed in detail in Chapter 15.

Figure 14-3 Adhesive label for fluid additives.

(From Chew DJ. Parenteral fluid therapy. In: Sherding RG, editor. The cat: diseases and clinical management. New York: Churchill Livingstone, 1989: 50.)

Figure 14-4 A and B, Fluid infusion pumps. A, Baxter Flo-Gard 6200 Volumetric Infusion Pump (Baxter Health Care, Deerfield Ill.). B, Medex Medfusion 2010 Syringe Infusion Pump (Medex, Carlsbad Calif.).

Figure 14-5 Use of labeled adhesive tape to monitor rate of fluid administration.

(From Chew DJ. Parenteral fluid therapy. In: Sherding RG, editor. The cat: diseases and clinical management. New York: Churchill Livingstone, 1989: 54.)

Figure 14-6 Buretrol device.

(From Chew DJ. Parenteral fluid therapy. In: Sherding RG, editor. The cat: diseases and clinical management. New York: Churchill Livingstone, 1989: 53.)

How much fluid should be given?

The purpose of fluid therapy is to increase tissue perfusion, repair fluid deficits, supply daily fluid needs, and replace ongoing losses. It has been emphasized: “the aim of therapy is not to administer fluids but to induce positive fluid balance.”³¹

Components of fluid therapy

The volume requirements of patients with fluid-responsive shock syndromes can vary widely. Ultimately, the goal of reestablishing widespread effective tissue perfusion should dictate the volume of fluid administered. In general, the same cardiovascular parameters used to characterize the patient’s shock syndrome should return to normal or to the extent they are able to do so given the limitations of the patient’s underlying disease condition. For example, a severely dehydrated dog with tachycardia, pale mucous membranes, prolonged capillary refill time, and hypotension should receive a volume of fluid sufficient to return these cardiovascular parameters to normal, and these parameters should not deviate from normal when the rate of fluid administration is decreased and rehydration of the patient begun. In patients not experiencing ongoing loss of fluids from the intravascular compartment or other more complex cardiovascular derangements, response to fluid resuscitation should be rapid and complete.

The initial assessment of hydration determines the volume of fluid needed to replace the hydration deficit (replacement requirement).^12,20 The hydration deficit is calculated as the percentage dehydration (estimated by physical examination) times the patient’s body weight in kilograms. The resultant value is the fluid deficit in liters. During the rehydration phase of therapy, this volume is administered for 24 hours in conjunction with maintenance fluid requirements and replacement of ongoing or contemporary losses that are occurring.

Coincident with or after replacement of the animal’s hydration deficit, the maintenance fluid requirement must be administered.^12,20 The maintenance fluid requirement is the volume needed per day to keep the animal in balance (i.e., no net change in body water). Daily fluid requirements (milliliters per kilogram per day) parallel energy requirements (kilocalories per kilogram per day).^20,22,23

The basal energy requirement (BER) is that of a resting animal in a thermoneutral environment 12 to 18 hours after eating.²⁴ In dogs, BER is not a linear function of body weight but rather is related to body surface area by the following equation¹:

where W is body weight in kilograms. This relationship is plotted in Figure 14-7 so that BER may be determined from body weight.

Figure 14-7 Basal energy rate as a function of body weight in pounds.

The maintenance energy requirement (MER) is that of a moderately active adult animal in a nonthermoneutral environment. The MER in sedentary animals is approximately 1.5 to 2.0 BER.

In domestic cats, the relationship of basal heat production to body weight is almost linear because of the small size and relatively narrow normal range of body weight in this species.²⁵ Based on available data, BER in cats may be estimated as 50 to 60 kcal/kg/day. However, the question remains whether daily energy requirements approximate daily fluid requirements. Daily fluid requirements of anorexic dogs and cats in a hospital environment and the relationship of these fluid requirements to the daily urinary solute load are areas deserving future clinical study.

At the Ohio State University Veterinary Teaching Hospital, the maintenance fluid requirement for dogs and cats is determined from reference charts that use the above formulas to calculate accurate daily fluid requirements based on caloric needs. Although estimates of 40 to 60 mL/kg/day frequently are used to calculate maintenance fluid requirements, it is important to recognize that such estimates are only accurate for some veterinary patients. Cats, very small dogs, and very large dogs are not well served by the use of such estimates, and these patients likely will benefit from more accurate assessment of their fluid requirements. Approximately two thirds of the maintenance requirement represents sensible (i.e., easy to measure) losses of fluid (urine output), and one third represents insensible (i.e., difficult to measure) losses (primarily fecal and respiratory water loss). Thus daily maintenance for a 10-kg dog may be 600 mL, with 400 mL representing sensible loss and 200 mL representing insensible loss.

Some clinicians multiply maintenance fluid requirements by some factor between 1 and 3 to estimate a patient’s 24-hour fluid needs. Assuming 60 mL/kg/day to represent the maintenance rate of fluid administration, the information in Table 14-9 can be used to quickly determine the implied hydration deficit and actual rate of fluid administration using this approach.

Table 14-9 Maintenance and Dehydration Fluid Volume Requirements*

Maintenance (M) + Dehydration (%)	mL/kg/day	Factor ×Maintenance
M + 1	70	1.17
M + 2	80	1.33
M + 3	90	1.50
M + 4	100	1.67
M + 5	110	1.83
M + 6	120	2.00
M + 7	130	2.17
M + 8	140	2.33
M + 9	150	2.50
M + 10	160	2.67

* Maintenance defined as 60 mL/kg/day.

From Chew DJ, Kohn CW, DiBartola SP. Disorders of fluid balance and fluid therapy. In: Fenner WR, editor. Quick reference to veterinary medicine, 2nd ed. Philadelphia: JB Lippincott, 1991: 570.

In addition to the hydration deficit (replacement requirement) and maintenance requirement, contemporary (ongoing) losses must be considered. These are not always easily determined or quantitated in small animals but can be very important in fluid therapy. An attempt should be made to estimate ongoing losses, which may include losses related to vomiting, diarrhea, polyuria, large wounds or burns, drains, peritoneal or pleural losses, panting, fever, and blood loss. During surgical procedures, careful attention should be given to the amount of blood lost, drying of exposed tissues, and effusions removed by suction. Blood lost at surgery should be estimated, and 3 mL of crystalloid solution should be administered for each milliliter of blood lost. Each 4 × 4-inch gauze sponge, when saturated with blood, represents a blood loss of 15 mL.²⁸ Contemporary losses must be estimated and carefully replaced along with the maintenance volume of fluid. Box 14-1 summarizes the components of fluid therapy and their calculation.

Box 14-1 Calculation of Replacement Requirement (Hydration Deficit)

1. Hydration deficit (replacement requirement)

a. Body weight (lb) × % dehydration as a decimal × 500* = deficit in milliliters

b. Body weight (kg) × % dehydration as a decimal = deficit in liters

2. Maintenance requirement (40-60 mL/kg/day)

a. Sensible losses (urine output): 27-40 mL/kg/day

b. Insensible losses (fecal, cutaneous, respiratory): 13-20 mL/kg/day

3. Contemporary (ongoing) losses (e.g., vomiting, diarrhea, polyuria)

From Muir WW, DiBartola SP. Fluid therapy. In: Kirk RW, editor. Current veterinary therapy VIII. Philadelphia: WB Saunders, 1983: 35.

^* 500 mL = 1 lb.

Monitoring fluid therapy

It is important to remember that the hydration deficit as estimated by history and physical examination is only an estimate, and fluid therapy must be tailored to physical (e.g., body weight) and laboratory (e.g., PCV, TPP) findings during the first few days of fluid therapy.

Central venous pressure

Measurement of CVP with a jugular catheter positioned at the level of the right atrium allows the cardiovascular response to fluid administration to be monitored. Normal CVP is 0 to 3 cm H₂O. CVP increases from below normal into the normal range when fluids are administered to a dehydrated animal. A progressive increase in CVP above normal during fluid therapy is an indication to decrease the rate of fluid administration or to stop fluid therapy temporarily. A sudden and sustained increase in CVP may indicate failure of the cardiovascular system to handle the fluid load effectively and could result in pulmonary edema caused by left-sided heart failure. In addition to the volume of fluid administered, other factors that may affect CVP include heart rate, vascular capacity, and cardiac contractility. A reduction in any of these three parameters could cause an increase in CVP.

The Frank-Starling curve (Figure 14-8) relates stroke volume (SV) to left ventricular end-diastolic pressure (LVEDP). If there is no obstruction across the mitral valve, left atrial pressure (LAP) should equal LVEDP, a measure of cardiac function. PWP measured with a Swan-Ganz catheter is an estimate of LAP. Generally, there is a direct relationship between right atrial pressure (RAP) and LAP. Thus measuring CVP gives an indirect indication of LVEDP. Without cardiac dysfunction, CVP correlates well with PWP and LAP. In one study, pulmonary artery diastolic pressure and PWP increased before CVP in dogs receiving an infusion of lactated Ringer’s solution.⁸

Figure 14-8 Frank-Starling law of the heart relating stroke volume (SV) to left ventricular end-diastolic pressure (LVEDP).

(From Rose BD. Clinical physiology of acid-base and electrolyte disorders, 3rd ed. New York: McGraw-Hill, 1989: 369, with permission of the McGraw-Hill Companies.)

When should fluid therapy be discontinued?

Ideally, fluid therapy is discontinued when hydration is restored and when the animal can maintain fluid balance on its own by oral intake of food and water. As the animal recovers, fluid therapy usually is tapered by decreasing the volume of fluid administered by 25% to 50% per day. If an animal remains anorexic for more than 3 to 5 days, enteral or parenteral nutritional therapy must be considered. Parenteral nutrition is discussed in Chapter 25.