Fluid Therapy for Competitive Endurance Horses

C. Langdon Fielding

Approximately 4% to 9% of endurance horses are eliminated from competition for metabolic reasons that include colic, rhabdomyolysis, synchronous diaphragmatic flutter, and exhaus- tion.^1,2 Veterinarians are responsible for the emergency treat­ment of horses with metabolic derangements, and fluid therapy is an essential component of the therapeutic management of critically ill endurance horses.

GENERAL APPROACH TO FLUID MANAGEMENT OF ENDUR­ANCE HORSES. Endurance horses experience significant fluid losses during competition; even horses that successfully complete rides lose approximately 5% of body weight in water.^3,4 Horses presented for treatment of metabolic problems often exhibit clinical signs of hypoperfusion. In evaluating sick endurance horses, the following seven clinical perfusion parameters should be evaluated:

1. Mentation

2. Heart rate

3. Pulse quality

4. Mucous membrane color

5. Capillary refill time

6. Extremity temperature

7. Jugular fill

Urine production, when apparent, can be used as an additional perfusion parameter. In the absence of polyuric renal failure, urine output signifies renal perfusion, which is one means of assessing organ perfusion. Horses with altered menta­tion, persistent tachycardia (>60 bpm), poor peripheral pulse quality, pale mucous membranes, capillary refill time greater than 2 seconds, cool extremities, or lack of urine production should be considered candidates for fluid therapy. Laboratory parameters consistent with dehydration, such as increased PCV, increased total protein, and increased serum creatinine, have been identified in endurance horses at risk for metabolic failure.⁵

The rapid administration of large volumes of crystalloids is the basis of treatment at many ride events. Volume loading increases preload, thereby enhancing stroke volume, cardiac output, and subsequent oxygen delivery.

Several types of intravenous crystalloids are available for administration to endurance horses. LRS and Normosol-R are balanced polyionic crystalloids commonly administered to treat hypovolemia in these horses. Specific electrolyte abnormalities may dictate the use of isotonic saline (when hypochloremic alkalosis is present) or fluid additives (potassium chloride or calcium gluconate when hypokalemia or hypocalcemia is identi­fied, respectively). However, in one study, 0.9% sodium chloride resulted in hyperchloremia when administered to eliminated endurance horses.⁶ The electrolyte and serum biochemistry profiles of endurance horses that fail to finish a race are not necessarily markedly abnormal and allow the use of commercial fluids such as LRS and Normosol R.⁴ A range of electrolyte abnormalities has been identified in competing endurance horses and includes hypokalemia and an increased strong ion difference (SID).^8,9 Calcium supplementation is common during fluid therapy of eliminated endurance horses. However, results of studies examining the concentration of total and ionized serum calcium in horses during endurance rides have been equivocal as to calcium status.^3,4,10 If calcium is used, it can be added to fluids through calcium gluconate (1 mL/kg of 23% calcium gluconate at a rate of ≤50 mL/L of crystalloid).

Endurance horses with prolonged ileus or anorexia may benefit from dextrose supplementation of fluids. A dose of 0.5 to 1 mg/kg/min of dextrose is well tolerated by most adult horses and is equivalent to 1.5% to 3% dextrose in fluids administered at a rate of 1 L/h for a 500-kg horse. Ideally, blood glucose concentrations should be monitored in horses administered dextrose to avoid hyperglycemia.

Medications commonly used in the treatment of endurance horses with metabolic derangements may affect fluid balance and should be considered in horses receiving fluid therapy. Seda­tives and tranquilizers such as α₂-adrenergic agonists (xylazine, detomidine, romifidine) and phenothiazines (acepromazine) have deleterious effects on cardiac output and/or blood pressure.^13,14 Drugs such as dimethyl sulfoxide (DMSO) and α₂-adrenergic agonist sedatives also affect urine output and can therefore alter fluid balance.^15,16

The following metabolic conditions are common metabolic disorders of endurance horses.

EXERTIONAL MYOPATHY. Intravenous fluid therapy is the most important feature of treatment for rhabdomyolysis. See the Fluid Therapy for Horses with Rhabdomyolysis section earlier in this chapter. Fluids should be administered in 10-L (20 mL/kg) boluses until urine output is achieved and the urine is grossly clear. Low doses of flunixin meglumine (0.5 mg/ kg IV) are indicated for inflammation once hypovolemia is resolved and diuresis (i.e., urination) is achieved. Adequate fluid resuscitation is indicated before release of affected horses, because exertional rhabdomyolysis has been associated with severe renal failure in both humans and horses.^17,18 Horses with myopathies should be monitored closely and reevaluated within 48 hours for azotemia and progress of increased muscle enzymes.

FAILURE TO RECOVERY. Horses that are eliminated from ride events because of persistent tachycardia warrant close monitoring. Such horses often require treatment at a later time. The seven clinical perfusion parameters should be evalu­ated thoroughly, and any animal with equivocal circulatory or hydration status should be reevaluated frequently to make sure that it is improving through voluntary eating and drinking, rather than deteriorating further. Those with clear hypovolemia or failure to improve with rest should be administered intra­venous fluids as described earlier.

SYNCHRONOUS DIAPHRAGMATIC FLUTTER (THUMPS). Syn­chronous diaphragmatic flutter is typically associated with hypocalcemia, and concurrent metabolic alkalosis, hypochlo­remia, and hypokalemia are often also present and have been implicated in its development.¹⁹ Many affected horses are otherwise stable hemodynamically and meet all the criteria for adequacy of perfusion. Despite these findings, horses with thumps often require treatment with intravenous fluids supple­mented with calcium for resolution of hypocalcemia and clinical signs. Calcium gluconate can be added to 5-L bags of crystalloid (0.5 mL/kg of 23% calcium gluconate per 5-L bag for a 500-kg horse), and fluids with dilute calcium can be administered as a bolus. Adverse effects of rapid calcium administration include bradycardia, and therefore calcium warrants slow administration. Many mildly affected horses would likely resolve synchronous diaphragmatic flutter with oral electrolytes and/or the consump­tion of feed (particularly alfalfa).

COLIC. Colic is a common problem identified in eliminated endurance horses and can result in significant mortality.²⁰ All horses exhibiting signs of colic should have a nasogastric tube passed, as nasogastric reflux (likely resulting from ileus) is a common finding in these horses.²⁰ Intravenous fluids should be administered as outlined previously until the horse is no longer producing gastric reflux and it is passing manure regu­larly. Flunixin meglumine (0.5 to 1 mg/kg) can be administered intravenously once hydration is restored. Calcium gluconate can be added to 5-L bags of crystalloid (0.5 mL/kg of 23% calcium gluconate per 5-L bag for a 500-kg horse), and fluids with dilute calcium can be administered as a bolus. Managing endurance horses with colic can be challenging because many show significant levels of pain. It is important to remember that endurance horses with colic rarely have a surgical lesion and typically respond to aggressive medical therapy.^9,20