Fluid therapy prior to and during anesthesia is an important component of the anesthetic plan. Livestock affected with medical conditions that require anesthesia and surgical intervention commonly present with dehydration and different electrolyte and acid–base imbalances. Proper use of fluids can be beneficial and lifesaving in these cases. As with other medications, inappropriate use of fluids during anesthesia can lead to detrimental or lethal outcomes regardless of surgical expertise. Fluid therapy can be provided during the perioperative period, which is defined as the period of time beginning when the animal is examined on the farm or clinic and completed when the animal is returned to its normal environment or discharged from the clinic. In many situations, fluid therapy provided prior to and following anesthesia is more important to the outcome of the case than the fluids administered during anesthesia and surgery.

11.1 General Considerations

The physical state of the animal, especially the level of hydration, is an important consideration prior to anesthesia. Ideally, dehydration along with electrolyte and acid–base imbalances is corrected 12–24 hours prior to anesthetic induction; however, anesthesia and surgery might be necessary prior to completing fluid resuscitation. Under these circumstances, monitoring of cardiac, renal, and pulmonary function is critically important. Even when rehydration can be accomplished prior to anesthesia and surgery, another factor that impacts fluid balance is restriction of oral feed and water. Ruminants are prone to regurgitation and rumen tympany during general anesthesia, thus adult ruminants are often fasted for at least 12–24 hours prior to the procedure and restricted from oral water intake for a minimum of 8–12 hours. In very large livestock, or when greater reductions in rumen volume are preferred during abdominal surgery, longer periods of fasting of up to several days may be preferred. This restriction of oral water intake combined with ongoing fluid losses associated with a disease process makes consideration of perioperative intravenous (IV) or other parenteral fluid therapy imperative for successful management of anesthesia and surgery. Fluid loading, which is the administration of high volumes of IV fluids over a short duration prior to induction of anesthesia and surgery, is one way to accomplish the goal of maintaining hydration while restricting oral intake of food and water to prevent regurgitation.

Dehydration as a result of excessive loss of body fluids or failure of fluid intake is the most common indication for fluid therapy in livestock. Examples of fluid intake failures include a lack of thirst as a result of neurologic depression or toxemia or inability to drink as would occur with esophageal obstruction. Diarrhea is the most common cause of excessive fluid loss, although vomiting and polyuria (renal disease) should also be considered, especially in small ruminants. Other indications for fluid therapy include hypovolemic shock, endotoxemia, electrolyte abnormalities, disturbances in acid–base balance, hypoglycemia, hypothermia, diuresis following toxin exposure, malnutrition, trauma, and failure in the transfer of passive immunity.

The overall goals of fluid therapy in sick livestock are to expand the circulatory volume to improve organ and tissue perfusion, correct life‐threatening electrolyte abnormalities, and correct acid–base imbalances. Most drugs used for induction and maintenance of anesthesia decrease cardiac contractility and induce vasodilation, which result in a relative hypovolemia. These effects lead to decreased cardiac output and decreased arterial blood pressure. Benefits of the perioperative fluid therapy also include improvement of cardiac function, smooth muscle vascular tone, and blood flow distribution.

11.2 Physiology of Body Fluids

To administer fluids and electrolytes properly, a general understanding of the fluid composition of the patient and how this fluid is altered or lost during anesthesia, surgery, and disease states is necessary. Total body water comprises between 57% and 67% of body weight for ruminants, while pigs have less total body water (40–50%) as a percentage of weight likely due to less gastrointestinal (GI) water [1]. Most estimations of total body water approximate 60% of body weight [2]. This amount can vary slightly with age, body composition, and breed. In general, neonatal ruminants have relatively more body water than adults, and total body water in neonates may approach 75–80% of body weight. The larger total body water percentage is primarily due to a large extracellular fluid volume, but by 6 months of age the total body water and extracellular fluid space percent are similar to adult values. The larger total body water and extracellular fluid percent of neonatal ruminants does not provide a reservoir of fluid for the sick neonate [3]. Overweight animals have decreased total body water content as compared to lean animals since adipose tissue contains very little water. As an example, estimations of total body water for fattened sheep are approximately 50% of body weight [2].

Total body water is distributed within two major compartments, the extracellular fluid compartment and the intracellular fluid compartment. Approximately two‐thirds of total body water is intracellular fluid (40% of body weight) and one‐third is extracellular fluid (20% of body weight). The extracellular fluid compartment can be further subdivided into interstitial fluid (~15% of body weight), the intravascular fluid or plasma volume (~5% of body weight), and transcellular fluid (very small % of body weight). The interstitial fluid compartment consists of cerebrospinal fluid, connective tissue, and, most importantly, the contents of the reticulorumen and the remaining GI tract. The reticulorumen is an important reservoir of fluid for adult ruminants during periods of water restriction, and the GI tract can also be a site for water deposition during disease processes such as grain overload or endotoxemia. Dehydration causes a decrease in extracellular fluid volume secondary to a decrease in total body water and is characterized by an increase in packed cell volume and total protein. Hypovolemia is generally defined as a decrease in fluid in the intravascular volume and can be treated rapidly by filling the vascular compartment to improve tissue perfusion. With dehydration, all fluid compartments are affected. Initially, dehydration results in reduction of the intravascular fluid compartment followed by contraction of the interstitial and intracellular fluid compartments. For dehydration, fluid deficits should be replaced more gradually to allow for fluid equilibriums to be reestablished between the various compartments of the extracellular fluid space.

Although the intracellular and extracellular fluid compartments differ in electrolyte composition, they are in osmotic equilibrium and water can freely diffuse between them. The movement of water and electrolytes between compartments is governed by hydrostatic and oncotic forces. Sodium is the most important cation within the extracellular fluid compartment, accounting for about 95% of the total cation pool. Potassium is the major intracellular cation. The concentrations of sodium and potassium are maintained within and outside of cells by the Na⁺/K⁺‐ATPase pump. Chloride and bicarbonate are the major anions within the extracellular fluid space, while phosphates, proteins, and other anions maintain electroneutrality with the potassium cation in the intracellular fluid compartment. Evaluation of blood electrolyte concentrations and acid–base status assess the extracellular fluid compartment. When fluids are administered to dehydrated animals, fluid losses are replaced within the extracellular fluid compartment, and thus the fluids being administered should contain concentrations of ions similar to what is found in the extracellular fluid compartment. Most disease processes in ruminants result in loss of fluids and electrolytes concurrently, and this is referred to as isotonic or iso‐osmolar dehydration. In these cases, providing both fluid and electrolytes (mainly sodium) is important. Hypertonic dehydration or a relative water deficit occurs when water losses exceed losses of electrolytes, and water deprivation is an example. In contrast, hypotonic dehydration or relative water excess occurs when electrolyte losses exceed water losses. Hypotonic dehydration can occur in livestock with diarrhea, in which loss of electrolytes and water occurs concurrently (isotonic dehydration). Another example of hypotonic dehydration occurs in ruminants with obstructive urolithiasis, in which sodium depletion exceeds water loss as sodium moves into the peritoneal cavity [4].

11.3 Patient Assessment

A physical examination is very important prior to anesthesia to ensure that the correct fluid type is administered at an appropriate rate, as well as for identifying underlying disease processes that could complicate anesthesia. Intercurrent disease processes, hypothermia, and perinatal asphyxia can lead to difficulty in accommodating IV fluids during anesthesia and surgery. Fluid therapy can have adverse effects such as volume overload and pulmonary edema, so particular attention should be given to the cardiovascular, pulmonary, and renal systems. Respiratory disease is common in ruminants, especially weaning‐age calves, kids, and lambs. In addition, mild, chronic lung pathology can be difficult to identify on clinical examination, yet this can negatively impact anesthesia and fluid therapy. Patients with respiratory disease have difficulty with gas exchange and are at greater risk of mortality, especially when general anesthesia and dorsal or lateral recumbency are utilized and the weight of the abdominal organs is further reducing lung capacity. Careful cardiac and pulmonary auscultation is important to identify animals at risk for anesthesia and fluid overload. Cardiovascular diseases may result in an inability to cope with an acute fluid load. Oliguric renal failure results in an impaired ability to excrete excess fluid. If concern exists following physical examination, preanesthetic diagnostic evaluation becomes important. A packed cell volume and total solids should be performed prior to induction of anesthesia for all cases. For cases in which disease or organ dysfunction is suspected, a complete blood count, serum chemistry profile, blood gases, and radiographs should be performed. For suspected cardiovascular disease, echocardiography and electrocardiograms should be performed. Evidence of respiratory, cardiovascular, or renal disease should delay anesthesia until the problem is resolved or characterized as to the impact on anesthesia. Life‐threatening abnormalities (dehydration and hypovolemia, hyperkalemia, metabolic acidosis) should be corrected prior to anesthesia if at all possible.

Dehydration is most accurately assessed by changes in body weight before and after a disease event, but this information is not usually available to the clinician, therefore clinical assessment is utilized to assess degree of dehydration. The packed cell volume and total plasma protein can be used as tools to assess hydration status, and the packed cell volume also provides an assessment of oxygen‐carrying capacity. However, measurements of packed cell volume and total solids cannot be used solely to replace an estimation of hydration status from physical examination. For example, the reference ranges for packed cell volume in healthy ruminants can be quite broad, which makes this measurement too variable to be useful for sole estimation of hydration status [4]. Total plasma protein concentration is dependent on colostral intake in neonates and can be elevated in patients with chronic inflammation. In addition, sheep, goats, and camelids with anemia and hypoproteinemia in conjunction with dehydration, as occurs with intestinal parasitism, can have a normal packed cell volume and total plasma protein concentration. Packed cell volume and total plasma protein concentrations are most useful in monitoring the progress of fluid therapy to prevent overhydration.

Percent dehydration can be easily estimated by assessing eyeball recession into the sockets (extent of enophthalmos), skin turgor, and the moisture of the mucous membranes (Table 11.1). In general, the presence of a gap between the eye and the orbit, decreased skin turgor, and dry/tacky mucous membranes are good indicators of dehydration in ruminants [5]. Results from one study demonstrated that the number of seconds of a cervical skin tent and the extent of enophthalmos are well correlated with the percent of dehydration in calves [5]. The extent of enophthalmos can be used to assess hydration in young small ruminants as well. The percent dehydration can be estimated by measuring the eyeball recession (in mm) and multiplying by 2. For example, a patient with eyeball recession of 4 mm is calculated to be 8% dehydrated. The duration of skin tenting can also be used to estimate hydration status. The percent dehydration is estimated by measuring the skin tent (in seconds), multiplying by 2, and then subtracting 4. For example, a calf with a skin tent of 6 seconds is estimated to be 8% dehydrated. Some caveats to this clinical assessment of dehydration exist, and it is important for the clinician to remember that emaciated ruminants usually have poor skin elasticity and enophthalmos, which can mimic dehydration [6]. Additionally, Bos indicus cattle with excessive skin on the neck have a prolonged skin tent regardless of hydration status.

In the absence of laboratory testing, acid–base and electrolyte abnormalities are more difficult to assess. Serum biochemistry and blood gas analyses can provide information on serum electrolyte abnormalities (sodium, potassium, chloride, bicarbonate, calcium, magnesium, phosphorus), acid–base disorders, or glucose abnormalities that require correction. A negative base excess (BE) in addition to low serum bicarbonate, low total carbon dioxide (TCO₂), and a high anion gap indicate metabolic acidosis. In contrast, a positive base excess in addition to increased TCO₂, increased serum bicarbonate, and hypochloremia indicate alkalosis [7]. However, the lack of laboratory equipment in the field usually makes the determination of acid base and electrolyte abnormalities challenging for the clinician. Under these circumstances, history, clinical signs, and presumptive diagnosis are essential tools to determine the metabolic status and electrolyte imbalances of the patient. In general, surgical conditions affecting ruminants present with consistent electrolyte and acid–base abnormalities that allow treatment without laboratory support.

11.4 Fluid and Electrolyte Therapy in the Perioperative Period

Fluid and electrolyte replacement therapy in livestock is required when fluid intake by the animal is not enough to meet their metabolic needs. Rehydration, replacement of lost electrolytes, and restoration of acid–base balance are the goals for fluid therapy. Provision of IV fluids can restore the circulatory capacity and mental status sufficiently that nutrition and replacement of ongoing losses can be provided through oral fluids. The aggressiveness of treatment is dictated by the severity of the condition as well as economic considerations. When developing the fluid and electrolyte replacement therapy plan, three basic assessments should be made: (i) quantity and rate of fluid administration, (ii) fluid type selection for such condition, and (iii) method of fluid administration.

11.4.1 Quantity and Rate of Fluid Administration

Patients that are to be anesthetized should ideally have IV access for administration of fluids and drugs. Fluid therapy is not always indicated for patients undergoing short duration of anesthesia and surgery [8]. When anesthesia and surgical time exceed 1 hour, fluid therapy is indicated to replace insensible water loss, counteract the hypotensive effects of anesthetics, and maintain tissue perfusion. Recipe‐based and goal‐directed fluid therapy is used to maintain vascular volume and tissue perfusion [9]. A recipe‐based or liberal approach is the most often utilized method for fluid administration during anesthesia for farm animals. Fluid administration rates of 3–10 ml/kg/hour of crystalloid solutions (e.g. normal saline, lactated Ringer’s solution), but not to exceed 20 ml/kg, are used when the surgical procedure does not result in significant blood loss [9]. In the majority of situations, the rate of fluid administration should not exceed 10 ml/kg/hour. If minor blood loss occurs (<20 ml/kg), patients should receive 3 ml of a balanced electrolyte solution for every 1 ml of lost blood. In surgical procedures in which significant blood loss occurs (>20 ml/kg), a colloid solution or whole blood should be administered for each volume of blood lost. Attempts should be made to maintain the packed cell volume greater than 20%, since a low packed cell volume in combination with hypotension can result in poor tissue perfusion and oxygenation. In cases in which the total plasma protein becomes less than 3 g/dl, plasma administration should be considered. Goal‐directed fluid therapy involves administration of fluids during anesthesia and surgery based on measurements of cardiac performance such as systolic arterial blood pressure. In this situation, combinations of crystalloids and colloids are administered to maintain mean arterial blood pressure in the normal range during surgery (90–120 mmHg) [9]. Colloids such as albumin, hetastarch, and dextrans are not frequently used in anesthesia or fluid therapy for farm animal patients.

For anesthesia and surgical cases that present with dehydration, the fluid therapy plan is calculated to replace deficits while supplying maintenance fluid needs and accounting for ongoing loss of fluids associated with the disease process (Table 11.2). The first priority for treating dehydration is to restore the extracellular fluid volume back to normal. Multiply the estimated percentage of dehydration by body weight in kilograms, which will provide the quantity of fluid in liters recommended to restore an animal to a normal hydration. IV fluid therapy is recommended in cases with clinical signs indicative of shock including tachycardia, increased capillary refill time (CRT >2 seconds), decreased mentation (depression, lethargy, absence or weakness of suckle reflex in calves), cold ears and extremities, and inability to stand. This is true for most ruminants affected with strangulating abomasal or intestinal disease, urinary tract obstruction or rupture, or severe endotoxemia. One report suggested that when dehydration is 8% or greater, IV fluids are necessary in ruminants because oral fluid therapy would not be effective [10]. Since the methods for precisely measuring dehydration are not available, it is important for the clinician to remember that replacing the exact fluid deficit is not of chief concern. Rather, the clinician should replace a fluid deficit to restore tissue perfusion and improve response to anesthesia and surgery.

Fluids given intravenously should be warmed prior to administration because cold fluids result in energy dissipation by the patient [11]. If rehydration can be accomplished prior to anesthesia, a general rule of thumb is to replace half of the fluid deficit over 4–6 hours with the balance given over 12–24 hours. More often, the fluid deficit (resulting in dehydration) is replaced more rapidly (6 hours); however, care should be taken in hypothermic neonates or in cases of sepsis/endotoxemia with increased vascular permeability and disseminated intravascular coagulopathy (DIC), as generalized, pulmonary and cerebral edema may occur [12]

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4. Preanesthetic Considerations

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