Fatigue as a result of prolonged exercise, such as endurance racing and eventing competitions, depends on the duration and intensity of the activity, liver glycogen stores, ambient temperature, and mechanisms of central fatigue. Excessive losses of fluids and electrolytes through sweat can also contribute to fatigue, through metabolic imbalances and alterations in systemic circulatory patterns. The diagnosis of exhausted horse syndrome is based on clinical signs of dehydration, characteristic changes in serum electrolytes, myopathies, and changes in mentation. Treatment is centered on external cooling, restoration of the circulating fluid volume, and supportive care.
Prolonged exercise mainly depends on aerobic metabolism, unlike high-intensity exercise, which relies more on anaerobic sources of energy. Fatigue during prolonged exercise has been associated with hypoglycemia and with depletion of glycogen stores in muscle and liver. Intramuscular glycogen provides 50% of the energy during the first 30 minutes of submaximal exercise but drops to < 20% after 1 hour. Blood glucose makes a smaller contribution, providing only 10% of total energy used. Although circulating fatty acids may provide an energy source during prolonged exercise, fatigue will set in before these fat stores are completely exhausted.
In prolonged exercise, the heat generated during aerobic ATP resynthesis imposes a high thermoregulatory demand on the animal. Only 20%–25% of the total energy produced by the muscles is converted to mechanical energy, leaving 75%–80% of that energy that must be removed as heat.
Physiological responses to heat production include sweating and panting to remove excess heat from the body. Complications from these compensatory mechanisms include dehydration, acid-base and electrolyte disturbances (which are implicated as causes of fatigue), exhaustion, and even death (which can occur after prolonged exercise).
Exhausted Horse Syndrome
Exhausted horse syndrome, known in endurance racing as the “metabolic,” is a condition related to severe muscular fatigue, coupled with massive fluid and electrolyte losses through sweating. The lack of adequate tissue perfusion leads to hypoxia, acid-base derangements, and clinical signs of multiorgan failure. Blood work can identify the level of dehydration and electrolyte changes, which can be affected by both the strong ion difference and lactic acid production. Treatment is focused on addressing the fluid and electrolyte imbalances with fluid administration and cooling treatments. Additional treatments to specifically address myopathies, cardiac arrhythmias, coagulopathies, cerebral edema, and renal failure may be required.
Horses occasionally develop severe clinical signs of fatigue at endurance events, despite current preventive practices which include evaluation of recovery at rest stops. Horses that compete in sports such as 3-day eventing, endurance rides, or combined driving are also at risk of developing life-threatening exhaustion.
Etiology
Exhausted horse syndrome occurs when the physiological costs of sustained submaximal exercise exceed the horse's ability to maintain and recover its fluid and electrolyte balance. During prolonged exercise in hot conditions, horses may lose body fluids at a rate of 10–15 L/hour through sweat. Urgent treatment of fluid and electrolyte deficits and hyperthermia (rectal temperatures > 40.5°C [104.9°F]) may be required. Exhausted horses may lose up to 10% of their body weight in water, with some having body fluid deficits of up to 40 L, depending on their size.
Water lost as sweat is mainly lost from the extracellular fluid and circulating plasma. Decreased blood volume decreases perfusion to vital organs and hampers thermoregulation. In severe cases, cardiovascular compromise may result in multiorgan failure, including damage to the kidneys, GI mucosa, and lamina of the hoof.
Pathophysiology
Unlike human sweat, equine sweat is hypertonic; as such, sweating in horses is associated with more dramatic loss of electrolytes than in other species. Electrolyte deficiencies of sodium, potassium, and chloride, as well as calcium and magnesium, can occur. Alterations in muscle electrolytes can contribute directly to clinical signs of fatigue. The most common acid-base alteration resulting from these electrolyte imbalances is metabolic alkalosis (see Metabolic Alkalosis video).
During endurance events, aerobic energy metabolism produces minimal amounts of lactic acid; therefore, alkalosis due to alterations in strong ions (hypochloremia and hypokalemia) predominates. Depletion of magnesium and calcium may contribute further to neuromuscular dysfunction, causing ileus, cardiac arrhythmias, and synchronous diaphragmatic flutter.
Unlike endurance riding, anaerobic metabolism predominates during events such as 3-day eventing and combined driving, resulting in metabolic acidosis (see Metabolic Acidosis video). After recovery, which can range from 30 minutes to 2 hours after the event, lactate is oxidized and the acidosis resolves. Metabolic alkalosis will then predominate. It is important to recognize that horses in these events, with multiple intermittent periods of maximal intensity exertion, may also show clinical signs of both exertional heat illness and exhausted horse syndrome.
Clinical Signs
Horses affected by exhausted horse syndrome demonstrate a range of clinical signs, including the following:
perfusion abnormalities and clinical signs of dehydration
sweating, which may be inappropriate or absent
mentation changes, listlessness
alterations in gait, ataxia
recumbency
abdominal pain
laminitis
clinical signs of myopathy (hard muscle bellies, pain on palpation)
persistent tachycardia and tachypnea, despite adequate rest
elevated rectal temperature (≥ 42°C [107.6° F])
Treatment
External and internal cooling treatments
Restoration of the circulating fluid volume
Rapid Cooling
Cooling should be initiated by moving hyperthermic horses to shade and treating with cool- or cold-water sponge baths, cold hosing, or misting fans. Water should be removed from the hair coat with a sweat scraper and the process repeated to prevent an insulating layer of warm water from forming on the skin. Application of ice water should be avoided, as should alcohol baths or direct ice application. Cool-water enemas, or peritoneal and gastric lavage, may help to decrease body temperature in severe cases.
Fluid Therapy
Isotonic balanced electrolyte solutions may be provided for dehydration by nasogastric intubation if the horse has normal borborygmi. Horses may receive up to 8 L initially, with subsequent administration of 4–8 L every 1–2 hours, as needed. Commercial electrolyte mixtures for horses are suitable; however, hypertonic, hypotonic, and alkaline solutions should not be used.
In severe cases, IV fluid therapy is preferred. A shock dose of a balanced electrolyte solution should be provided initially (20–40 mL/kg bolus) with addition of 100 mL 23% calcium gluconate per 5 L and 5% dextrose (ie, 500 ml of 50% dextrose per 5 L of fluids). Additional fluids and additives should be based on reevaluation of serum chemistries and hydration status. Diuresis may be needed in cases with renal insufficiency or concurrent myoglobinuria.
Additional treatments may include NSAIDs for muscle pain and colic, administered simultaneously with fluid therapy to prevent renal injury, and phenothiazines for anxiety resulting from any myopathies. Anticonvulsant medications may be required, and dexamethasone may help to decrease cerebral edema. Dimethyl sulfoxide administered IV may be useful to further decrease inflammation, and low-molecular-weight heparin can be administered SC to treat coagulopathies.
Prevention
Environmental temperature and humidity have a major impact on the severity of disturbance of the horse's fluid balance during prolonged exercise. It is important to ensure adequate hydration before an event, especially after long trailer rides to the competition, and to provide access to fluids during and after exercise to decrease the likelihood of dehydration. Administration of supplementary fluids, electrolytes, and glucose before and during competition, when allowed by doping regulations, may decrease the incidence of exhausted horse syndrome.
Key Points
Prolonged exercise relies on aerobic metabolism, which is inherently inefficient, producing large amounts of heat that must be removed by the body.
Exhausted horse syndrome is characterized by dehydration, myopathies, abdominal pain, and changes in mentation. It has been observed in horses competing in endurance sports or 3-day events and is a greater risk when ambient temperatures are high.
Treatment of exhausted horse syndrome should focus on rehydration strategies and rapid external cooling with cool hosing and sweat scraping.
Additional treatments may include sedation, NSAIDs, and glucocorticoids.
For More Information
Foreman JH. The exhausted horse syndrome. Vet Clin North Am Equine Pract. 1998;14(1):205-219. doi:10.1016/s0749-0739(17)30220-1
Geor RJ, McCutcheon LJ. Thermoregulatory adaptations associated with training and heat acclimation. Vet Clin North Am Equine Pract. 1998;14(1):97-120. doi:10.1016/s0749-0739(17)30214-6
Flaminio MJBF, Rush BR. Fluid and electrolyte balance in endurance horses. Vet Clin North Am Equine Pract. 1998;14(1):147-158. doi:10.1016/s0749-0739(17)30217-1
White SL. Fluid, electrolyte, and acid-base balances in three-day, combined-training horses. Vet Clin North Am Equine Pract. 1998;14(1):137-145. doi:10.1016/s0749-0739(17)30216-x
