Fluid Therapy in Ruminants

Marie-Eve Fecteau

General Principles

In addition to its obvious use for correction of dehydration, fluid therapy in ruminants can be instrumental in the treatment of a variety of disorders, including sepsis, electrolyte and acid-base disturbances, gastrointestinal disease, liver disease, and renal disease.

In adult ruminants, estimation of dehydration can be based on several factors, including eyeball recession, upper eyelid skin tent, heart rate, and mucous membrane moistness (Table 44.2). Once the degree of dehydration is determined, maintenance fluid requirements (2 to 3 mL/kg/h for adults and 4 to 6 mL/ kg/h for neonates), as well as ongoing losses, also need to be considered. The amount of water an adult dairy cow needs just to make up for lactation is often underestimated. The daily water requirement of a 600-kg cow eating a corn silage-based diet in a thermoneutral environment, and milking 45 kg of milk per day, is at least 140 L of water per day.¹ Although laboratory measurements are necessary to confirm and quantify electrolyte and acid-base derangements, those derangements can be predicted on the basis of the suspected disease process. For example, cattle with abomasal volvulus typically suffer from a hypochloremic, hypokalemic, metabolic alkalosis, whereas ruminants with acute grain overload are typically acidotic and often hypocalcemic.² Patient-side biochemistry analyzers can be very useful during the initial evaluation of the patient and for monitoring fluid therapy in field situations.

Oral fluids are practical and economical and can be useful in correcting mild to moderate dehydration and metabolic disturbances in ruminants. In the adult ruminant, the rumen should be viewed as a large reservoir capable of absorbing large volumes of water and electrolytes.³ The main force for water movement across the rumen wall is the gradient of osmolality between ruminal fluid (normally isoosmotic) and blood perfusing the ruminal epithelium.⁴ Studies in adult cattle have shown that net flow of water from rumen to plasma occurs whenever plasma osmolality exceeds rumen osmolality by at least 20 mOsm/kg,⁵ which means that to optimize free­water absorption, rumen osmolality should be hypoosmotic. The opposite situation is best represented during an episode of acute grain overload when the rumen quickly becomes hyperosmotic and water is drawn to the rumen from the extravascular space, leading to severe dehydration in those animals.⁶ Therefore the practitioner should take advantage of the ruminal capacity for water and electrolyte absorption by administrating hypoosmotic oral electrolyte solutions to mildly and moderately dehydrated ruminants that do not suffer from preexisting abdominal distention or ileus. Enteral fluids should contain Na⁺, K⁺, and Ca²⁺. Because most dehydrated adult ruminants are alkalotic, nonalkalinizing fluid formulations are preferred.²

Intravenous fluids are preferred in patients that are severely dehydrated (≥8%) and/or that suffer from severe metabolic disturbances or sepsis. An intravenous catheter is typically placed in a jugular vein, although the use of an auricular vein has also proved useful in calves and adult cattle.⁷ The animal should be restrained in a tie stall or box stall and given enough length of fluid administration set to allow him or her to lie down.

Commercially available crystalloid solutions such as LRS, Normosol-R, Plasma-Lyte 148/Plasma-Lyte A, or 0.9% NaCl may be used, although homemade solutions can also be prepared cheaply on the farm or in a hospital setting. For example, a 0.9% NaCl solution can be prepared by adding 180 g of table salt to 20 L of filtered water using large plastic carboys. Additional recipes for homemade electrolyte solutions for cattle are included in Table 44.3. If tap (unfiltered) water is used in the mixing of the fluids, the author recommends that the animal not be kept on those fluids for extended periods (in a variety of cases including acute grain overload (rumen acidosis) and severe enteritis with profuse diarrhea. Severe ketosis, pregnancy toxemia, and sepsis can also lead to the development of a metabolic acidosis in the affected ruminants. Fluid therapy for ruminants with severe ketosis and sepsis is discussed later in this chapter.

Animals with acute rumen acidosis are typically severely debilitated and need aggressive medical and surgical therapy. Acute ruminal acidosis is discussed in detail in Chapter 32. The rapid accumulation of lactic acid in the rumen osmotically draws water into the rumen, resulting in marked dehydration. In addition, the chemical damage to the rumen mucosa allows bacterial transposition into the peritoneal cavity and portal circulation, resulting in the development of endotoxemia.^13-14 Moreover, the fermentation of larger amounts of carbohydrates induces an increased concentration of organic acids that leads to a decrease in intraluminal pH. This decrease in pH favors the overgrowth of bacteria that are able to produce high quantities of D- and L-lactate, also contributing to the acidosis. Finally, third spacing of proteins in the rumen as well as in the abdominal cavity can result in profound hypoproteinemia in the affected animals, which further compounds their precari­ous hemodynamic state by its negative impact on the animal's oncotic pressure.

Fluid therapy in calves with diarrhea is discussed at length in Chapter 20. Adult ruminants with severe enteritis and diarrhea, such as those with salmonellosis, often require intravenous fluid therapy if they are completely anorectic, depressed, and significantly dehydrated. Oral fluids and electrolytes may be helpful in ruminants that are mildly to moderately dehydrated, but the effectiveness of those fluids may be compromised by malabsorption and ileus resulting from the enteritis. Similar to ruminants with acute rumen acidosis, cattle with acute diarrhea will suffer from varying degrees of metabolic acidosis, which is a result of the loss of bicarbonates in the diarrhea and from the enteric bacterial production of D-lactate. The poor perfusion of peripheral tissues that is sometimes seen in severe cases of hypovolemia may lead to the development of a lactic acidosis, which may also contribute to the overall acidosis in these patients. Also, similar to ruminants with acute grain overload, ruminants with severe enteritis and diarrhea can develop hypoproteinemia from gastrointestinal losses and third spacing within the abdominal cavity. Finally, although a lesser concern in adult ruminants, sepsis caused by severe enteritis may pose an additional problem that should be addressed by the use of intravenous fluids. Intravenous fluid therapy for sepsis is discussed further later in this chapter.

The goals of fluid therapy in ruminants with severe acute rumen acidosis or acute enteritis with profuse diarrhea should be (1) volume repletion, (2) correction of acid-base and electrolyte disturbances, (3) addressing the endotoxemia or sepsis, and (4) providing colloid support. Fluids should be provided in the intravenous form, especially in the case of rumen acidosis, to avoid exacerbating the abdominal distention and because most animals suffer from severe ileus. The choice of crystalloid solution should be based on the documented or suspected metabolic disturbances.

Fluid Therapy for Negative Energy Balance Disorders (Hepatic Lipidosis, Ketosis, Pregnancy Toxemia)

The vast majority of cattle and small ruminants with type 1 (primary) ketosis respond quickly to simple treatments that usually involve the administration of glucose precursors (such as propylene glycol) and/or a single administration of intra­venous dextrose. However, cattle and small ruminants with severe ketosis, hepatic lipidosis, or advanced pregnancy toxemia may need more aggressive treatment, which usually involves the use of a CRI of intravenous fluids containing dextrose. Ruminants with liver dysfunction or failure have a decreased ability to perform normal metabolic functions such as gluco­neogenesis, to synthetize proteins, and to clear a variety of metabolites, including lactate.

The main goal of treatment of hepatic lipidosis, severe ketosis, and pregnancy toxemia is to limit the mobilization of fat by increasing the availability of glucose or glucose precursors and promoting uptake of glucose by the cells. It is important to note that providing dextrose is critical in such cases, even in the face of normoglycemia, because it will minimize glu­coneogenesis demands on the liver and prevent further produc­tion of ketone bodies.

Fluids should also contain potassium chloride (20 to 40 mEq/L), since most ruminants with severe liver disease are partially or completely anorectic and may also experience a shift of potassium intracellularly in response to dextrose administration.¹² Special caution should be taken to monitor plasma potassium concentrations closely in cows receiving mixed glucocorticoid-mineralocorticoid drugs to aid in the treatment of hepatic lipidosis, since their chronic use has been associated with the development of severe hypokalemia.²⁰ As discussed earlier, cows with marked hypokalemia should receive oral and intravenous supplementation with potassium chloride. Calcium supplementation (25 mL/L of a 23% solution) is also important, since most cows with hepatic lipidosis are in the peripartum period and may also suffer from subclinical hypo­calcemia; in addition, clinical signs of pregnancy toxemia may resemble those of preeclampsia in small ruminants. Plasma or serum phosphorus concentrations should also be monitored in ruminants receiving repeated infusions of intravenous dextrose because this can cause significant decreases in plasma phosphorus 2122

concentrations.^21,22

Fluid Therapy for Obstructive Urolithiasis

Most ruminants with obstructive urolithiasis are presented with azotemia, which is usually postrenal in origin, although prerenal azotemia from dehydration and renal azotemia from renal injury are also possible. Metabolic disturbances typically include metabolic alkalosis, hyperkalemia, hyponatremia, and hypochloremia.²³ Hypophosphatemia is another common biochemical finding in goats presenting with acute urolithiasis.²³ If a rupture in the urinary tract is present on admission, the degree of hyperkalemia may be severe. Ruminants with urinary tract obstruction, particularly those with severe electrolyte disturbances, often need to be stabilized before surgery.

Although it may not be advised to administer large volumes of intravenous fluids prior to relieving the obstruction, most patients' hemodynamic state will benefit from a fluid bolus prior to surgery. In very unstable patients with severe electrolyte disturbances (K⁺ >6 mmol/L), draining the bladder (or the uroabdomen) prior to surgery by performing a cystocentesis or by placing a self-retaining suprapubic cystic catheter (Bonnano catheter [BD, Franklin Lakes, N.J.]) can be beneficial. The suprapubic catheters can stay in place for up to 24 hours in most small ruminants and allow for urine egress from the bladder while the patient is receiving intravenous fluid therapy. It should be emphasized that the ultimate goals of preoperative fluid therapy are to improve the patient's hemodynamic state and to address its most severe electrolyte abnormalities (usually hyperkalemia) so that the patient can more safely undergo general anesthesia. It should be noted, however, that surgery should not be delayed until the animal's azotemia is completely resolved, as azotemia may persist for several days following surgery in some animals.

Much has been written about the best fluid composition to use in hyperkalemic animals. The most common recommenda­tion is to administer fluids that do not contain potassium, such as physiologic saline, D₅W, or isotonic sodium bicarbonate solution. However, others may argue that balanced electrolyte solutions such as LRS also represent a valid option, even in the face of hyperkalemia.²⁴ Insulin may also be used to help correct the hyperkalemia, especially preoperatively. If insulin is used, blood glucose concentrations should be monitored closely, especially in young ruminants, since they are at greatest risk of becoming hypoglycemic. Potassium plasma concentra­tions should also be monitored closely following the combina­tion of insulin and diuresis, since many ruminants will become hypokalemic postoperatively, especially if they remain anorexic for some time.

The fluid rate should be tailored to the patient's hydration needs, level of azotemia, and urine production. Although overhydration is rarely a concern in adult cattle with normal kidney function, small ruminants with acute renal failure should be monitored closely to avoid pulmonary edema. An initial rate of administration of 1.5 to 2 times maintenance is usually used until the azotemia is resolved.