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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • Hypovolemia and dehydration are by far the most common problems seen and treated in endurance horses
  • More seriously affected horses or those that do not receive appropriate care are at high risk for development of complications
  • A variety of cases require referral to a secondary or tertiary care facility
  • Arrangements for referral should be made before the need arises

Pathophysiology!!navigator!!

  • Total body fluid losses due to profuse sweating for long periods
  • Inadequate replacement fluid intake

Systems Affected!!navigator!!

  • Cardiovascular
  • Musculoskeletal
  • GI
  • Renal/urologic
  • Nervous

Geographic Distribution!!navigator!!

Global

Signalment!!navigator!!

  • Arabian horses and Arab crosses are most commonly used for endurance competitions
  • Most affected horses are 5–18 years old
  • No sex predilection or genetic basis has been noted

Signs!!navigator!!

  • Affected horses appear dull, exhausted, and may be obtundent, stagger, or be reluctant to move. Severely hypovolemic, dehydrated horses often have heart rates ≥70–79 bpm
  • Prolonged CRTs (>3 s), poor mucous membrane color, poor skin turgor
  • Ileus—few or no gut sounds
  • No interest in food or water
  • Hypoglycemic horses may be lethargic and adopt a “sawhorse stance.” Heart rate, CRT, and jugular refill time do not reflect the degree of compromise that one would expect in a hypovolemic horse showing similar clinical signs

Causes!!navigator!!

  • Inadequate conditioning
  • Concurrent lameness
  • Inexperienced, unbalanced riders
  • Inability to pace the horse or recognize fatigue
  • Riders not allowing horses to stop to drink
  • Pushing horses beyond their physiologic limits

Risk Factors!!navigator!!

See Causes.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Fatigue
  • Exertional rhabdomyolysis

CBC/Biochemistry/Urinalysis!!navigator!!

  • Hematocrit values are often 55–69 L/L
  • Elevated total protein levels may reach as high as 14 mg/dL
  • Serum creatinine levels are often markedly elevated
  • Total protein and creatinine measurements provide a more accurate measure of fluid volume deficits than the hematocrit alone, and, along with the physical parameters, provide a helpful guide to the end-point for fluid therapy
  • Hypocalcemia is often present (ionized calcium <1.2 mmol/L)
  • Hypoglycemia may be present, with blood glucose levels of 30–50 g/dL (1.6–2.8 mmol/L)

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

Serial clinical examinations.

Pathologic Findings!!navigator!!

  • Horses that fail to recover may have pathologic findings related to the organ system most affected
  • Cardiomyopathy, renal tubular necrosis, laminar necrosis, cerebral edema, and pulmonary and GI infarcts have all been discovered at necropsy

Treatment ⬆ ⬇

TREATMENT

Aims

  • The primary aim of treatment is to restore fluid deficits and circulating plasma volume to minimize tissue hypoxia, protect organ function, and return homeostasis
  • Most endurance horses with fluid deficits respond well to 10–20 L of IV fluids, administered through a 12 or 14-gauge jugular catheter
  • 0.9% sodium chloride has traditionally been used for fluid replacement in endurance horses, but any isotonic fluid can be used
  • Hypertonic saline should be avoided, except in cases where cerebral edema is suspected
  • Longer catheters (14 cm (5.5 inches)) are preferred to short ones, since they are less likely to be dislodged in case of collapse. Catheters should be sutured in place
  • Replacement fluids via nasogastric intubation are not routinely used because GI stasis is often present, and there is a risk of gastric rupture

Medications ⬆ ⬇

Outline

MEDICATIONS

  • Supplementation of calcium in horses with low ionized calcium (<1.2 mmol/L) or poor auscultable motility hastens recovery of GI motility (23% calcium gluconate, 120–240 mL in 3–5 L 0.9% NaCl IV to effect)
  • Synchronous diaphragmatic flutter can occur without concurrent metabolic compromise, and may not require treatment. If treatment is deemed necessary, 23% calcium gluconate, 100–250 mL in 3–5 L 0.9% NaCl IV usually resolves clinical signs
  • If the heart rate remains elevated (<50 bpm) and ileus is persistent, low-dose flunixin meglumine (0.3–0.6 mg/kg) can be administered, provided that fluid deficits have been replaced, and the horse does not have an elevated serum creatinine level
  • Clinical improvement generally occurs within 30–40 min of flunixin meglumine administration
  • All NSAIDs should be used judiciously and with extreme caution in the endurance horse due to their nephrotoxicity and the propensity for endurance horses to have renal compromise
  • If hypoglycemia is present, 50–100 mL 50% glucose or dextrose diluted in a 3 or 5 L bag of normal saline is given to effect
  • If colic is present and serum creatinine levels are high, butorphanol (0.01–0.02 mg/kg) combined with xylazine (0.3–0.5 mg/kg) or detomidine (0.01 mg/kg) can be used to provide analgesia while fluid therapy is administered

Contraindications!!navigator!!

Serious complications (renal failure) may occur if NSAIDs are used in endurance horses prior to volume expansion.

Possible Interactions!!navigator!!

See Contraindications.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Serial clinical examinations, monitoring heart rate, mucous membrane color, and return of intestinal borborygmi and interest in food and water are important is assessing response to therapy.

Prevention/Avoidance!!navigator!!

Rider education and familiarity with the horse is important in detection of impending metabolic compromise.

Possible Complications!!navigator!!

  • Laminitis
  • Endotoxemia
  • Multiple organ system compromise or failure

Expected Course and Prognosis!!navigator!!

  • The vast majority of endurance horses with hypovolemia and dehydration, when treated promptly and appropriately, resolve without the need for further treatment or follow-up care
  • Those that do not respond to treatment within 4–6 h of appropriate treatment often require referral for a longer period of therapy
  • Horses with concurrent problems also require referral to a secondary or tertiary care facility
  • If small intestinal volvulus is suspected (pain unresponsive to analgesia), referral should be immediate

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

  • Colic—most colic in endurance horses is due to nonstrangulated ileus, but small intestinal volvulus is a recognized postendurance phenomenon
  • Synchronous diaphragmatic flutter
  • Exertional rhabdomyolysis
  • Cardiac arrhythmias
  • Renal failure
  • Heat stroke
  • Enterocolitis
  • Neurologic deficits

Age-Related Factors!!navigator!!

  • N/A

Abbreviations!!navigator!!

  • CRT = capillary refill time
  • GI = gastrointestinal
  • NSAID = nonsteroidal anti-inflammatory drug

Suggested Reading ⬆ ⬇

Cunilleras EJ. Abnormalities of body fluids and electrolytes in athletic horses. In: Hinchcliffe KW, Kaneps AJ, Geor RJ, eds. Equine Sports Medicine and Surgery: Basic and Clinical Sciences of the Equine Athlete, 2e. Edinburgh, UK: Saunders, 2013:881–900.

Fielding CL, Magdesian GK, eds. Equine Fluid Therapy. Ames, IA: Wiley Blackwell, 2015.

Misheff MM. Diagnosis and treatment of metabolic conditions in the endurance horse. In: SIVA Proceedings, XXII International SIVE Congress, Italian Equine Veterinary Society, 2016.

Author(s) ⬆

Author: Martha M. Misheff

Consulting Editor: Jean-Pierre Lavoie