Nature of Exercise: Primarily depends on aerobic metabolism, which is inefficient; 75%–80% of energy produced is converted to heat.
Fatigue Drivers:
Substrate Depletion: Hypoglycemia and the depletion of glycogen stores in the muscle and liver.
Thermoregulatory Demand: High ambient temperatures and humidity increase the risk.
Physiological Cost: Occurs when the costs of sustained submaximal exercise exceed the horse's ability to maintain fluid and electrolyte balance.
At-Risk Groups: Horses competing in endurance rides, 3-day eventing, or combined driving.
Pathophysiology (Exhausted Horse Syndrome)
Fluid Loss: Horses can lose 10–15 L/hour of body fluids through sweat, potentially losing up to 10% of their body weight (40 L).
Electrolyte Depletion: Equine sweat is hypertonic, leading to dramatic losses of sodium, potassium, chloride, calcium, and magnesium.
Perfusion & Organ Impact: Decreased blood volume (hypovolemia) leads to hypoxia and potential multiorgan failure, damaging the kidneys, GI mucosa, and hoof lamina.
Acid-Base Derangements:
Endurance (Aerobic): Predominantly metabolic alkalosis due to chloride and potassium loss.
High-Intensity Bursts (Anaerobic): Initial metabolic acidosis from lactate, which often resolves into alkalosis during recovery.
Clinical Findings:
Dehydration & Perfusion: Notable perfusion abnormalities and listlessness.
Neuromuscular: Ataxia, gait alterations, recumbency, and synchronous diaphragmatic flutter.
Myopathy: Hard muscle bellies and pain upon palpation.
Vital Signs: Elevated rectal temperature (≥ 42°C [107.6° F]), and persistent tachycardia/tachypnea despite rest.
Gastrointestinal: Abdominal pain (colic) and ileus.
Treatment Protocols
Rapid External Cooling:
Move to shade; use cool-water sponge baths, hosing, or misting fans.
Crucial Step: Use a sweat scraper to remove water from the coat to prevent it from acting as an insulating layer.
Fluid Therapy:
Oral: Isotonic balanced electrolyte solutions via nasogastric tube (if normal gut sounds are present).
Intravenous (IV): Preferred for severe cases; involves balanced electrolyte shock doses often supplemented with calcium gluconate and dextrose.
Supportive Medications:
NSAIDs: For muscle pain and colic (given with fluids to protect kidneys).
Glucocorticoids: Dexamethasone for cerebral edema.
Others: Phenothiazines for anxiety, anticonvulsants, or heparin for coagulopathies.
Prevention Strategies:
Hydration Management: Ensure adequate hydration before an event, especially after transport.
Access to Resources: Provide continuous access to fluids and electrolytes during and after exercise.
Supplementation: Use of oral electrolytes and glucose before/during competition when permitted by regulations.