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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • An abnormally high body temperature in which the hypothalamic temperature setpoint is not altered but the heat-dissipating mechanisms are overwhelmed or fail.
  • Different from a fever, in which the body temperature is elevated because of upward resetting of the setpoint with the heat-dissipating mechanisms usually remaining intact.
  • Heat stroke occurs when hyperthermia is combined with CNS dysfunction.
  • Exertional heat illness, or post-race distress syndrome, is a severe hyperthermic state in performance horses that occurs following exercise in adverse environmental conditions and results in CNS and GI system dysfunction

Pathophysiology!!navigator!!

  • The normal physiologic body temperature is set by the hypothalamic thermoregulatory center at 37.5–38.5°C (99.5–101.5°F).
  • With hyperthermia, mechanisms of heat dissipation are overwhelmed or inadequate.
  • Body heat is generated by metabolic processes, working muscles, and absorption of solar radiation.
  • During exercise, massive heat loads are produced by muscles. In the horse, approximately 80% of energy expended is in the form of heat.
  • Heat can be dissipated by conduction, convection, radiation, and evaporation.
  • Horses have a low surface area to body mass ratio and an insulating haircoat. Loss of heat is primarily evaporative through sweating and breathing.
  • Heat can also be dissipated in part by transfer of heat from the body surface through convection and radiation.
  • Heat from the body core is conducted to the cooler surface tissues by circulation of blood.
  • A body temperature >39.5°C (103°F) can be considered hyperthermia.
  • A body temperature >41–42°C (106–108°F) leads to heat denaturation of cellular proteins.
  • The CNS is most sensitive to hyperthermic damage. As the temperature continues to increase and/or persist, other organs and systems also become affected, leading to MODS-L.
  • Decreased perfusion of the GI tract as blood is shunted to muscles and the periphery can lead to translocation of bacteria and endotoxins, resulting in systemic signs of endotoxemia and systemic inflammatory response syndrome.
  • Profuse sweating during hyperthermia results in loss of water and electrolytes, leading to dehydration and metabolic derangements

Systems Affected!!navigator!!

All systems are susceptible to damage by hyperthermia.

Genetics!!navigator!!

Genetic predisposition to hyperthermia and rhabdomyolysis during anesthesia (i.e. MH).

Incidence/Prevalence!!navigator!!

Greater in hot, humid weather.

Geographic Distribution!!navigator!!

Any area, especially those that have hot, humid weather.

Signalment!!navigator!!

  • Breeds with a higher body size to skin ratio have increased heat production and a smaller surface area for heat dissipation.
  • Dark-colored haircoats (increased absorption of radiant heat).
  • Long, thick haircoat (reduces evaporative and conductive heat loss)

Signs!!navigator!!

Historical Findings

  • Excessive sweating or lack of sweating.
  • Prolonged muscular exertion (endurance rides).
  • Maximal exercise in warm/humid environmental conditions.
  • Weakness.
  • Stilted gait.
  • Fatigue.
  • Depression.
  • Impaired performance.
  • Respiratory distress.
  • Seizures.
  • Anesthesia.
  • Transport.
  • Foal on macrolide antibiotic

Physical Examination Findings

  • Elevated temperature associated with dyspnea, tachypnea, tachyarrhythmia, and tachycardia.
  • Parameters remain elevated in spite of exercise cessation.
  • Excessive, patchy, or lack of sweating.
  • Dull mentation.
  • Weakness.
  • Congested mucous membranes.
  • Prolonged capillary refill time.
  • Dehydration.
  • Dilated cutaneous vasculature.
  • Muscle rigidity.
  • Ataxia.
  • SDF.
  • Colic.
  • Ileus.
  • Signs of endotoxemia.
  • Seizures.
  • Collapse

Causes!!navigator!!

  • Excessive muscular activity, prolonged work.
  • Maximal exercise in warm/humid environmental conditions.
  • Generalized seizures.
  • Hypocalcemic tetany associated with transit or lactation.
  • Drugs—macrolides, halothane anesthesia (i.e. MH), phenothiazine tranquilizers, furosemide.
  • Endophyte-infested tall fescue.
  • Anhidrosis.
  • Confinement in closed trailers or buildings during hot weather

Risk Factors!!navigator!!

  • Poor physical fitness, insufficient conditioning, and lack of acclimatization.
  • High ambient temperature, especially >30°C (86°F), and high relative humidity.
  • No air movement and high solar radiation.
  • Dehydration—no access to drinking water during work.
  • Anhidrosis.
  • Foal on macrolide antibiotic, especially in hot ambient temperatures and exposed to direct solar heat.
  • Large body mass relative to body surface area.
  • Long haircoat (i.e. PPID).
  • Obesity.
  • Airway disease (cannot dissipate heat).
  • Possible risk of hyperthermia during anesthesia of horses with HYPP.
  • MH genetic predilection.
  • Halothane anesthesia

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Fever from infectious, inflammatory, and neoplastic diseases (usually does not exceed 41°C or 106°F).
  • Rule out heaves (severe equine asthma), anhidrosis, exertional rhabdomyolysis, and PPID

CBC/Biochemistry/Urinalysis!!navigator!!

  • CBC—stress, hemoconcentration.
  • Biochemistry—elevated creatine kinase and aspartate aminotransferase with exertional rhabdomyolysis.
  • Decreased Ca2+, Mg2+.
  • Sodium can be increased or decreased.
  • Increased or decreased K+.
  • Metabolic alkalosis or acidosis.
  • Respiratory alkalosis with hyperventilation.
  • Azotemia with dehydration, elevated renal and hepatic enzymes with organ damage.
  • Urinalysis (may be oliguric)—concentrated urine; possibly myoglobinuria

Other Laboratory Tests!!navigator!!

  • Intradermal epinephrine/terbutaline test for decreased sweating—anhidrosis.
  • Genetic marker blood test for HYPP or MH.
  • Blood gas disorders—possible alkalosis or acidosis.
  • Clotting profile—development of DIC, liver failure, thrombocytopenia, prolonged clotting time, and elevated fibrinogen degradation products.
  • Halothane—caffeine contracture test to identify individuals with MH

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

N/A

Pathologic Findings!!navigator!!

N/A

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

Immediate emergency management on farm then, depending on severity, may require inpatient medical management.

Nursing Care!!navigator!!

  • Provide shade or remove from direct sunlight.
  • Enhance cooling through fans or misting fans.
  • Repeatedly applying copious cold water can promote rapid cooling. Between applications of water, use sweat scraper to remove warmed water from haircoat.
  • Monitor rectal temperature and continue cooling until <40°C (104°F) for at least 30 min.
  • Correct dehydration by providing drinking water if patient is not critical.
  • IV saline, lactated Ringer's solution to restore blood volume and perfusion.
  • Identify and correct electrolyte and acid–base derangements. Oral and IV sources of K+, Na+, Cl–, Ca2+, Mg2+ as indicated.
  • Clip long haircoat.
  • Identify and treat accordingly other conditions—rhabdomyolysis, renal failure.
  • SDF—administer 50–100 mL/L of 23% calcium borogluconate to isotonic fluids

Activity!!navigator!!

Stop exercising/activity immediately.

Diet!!navigator!!

Withhold feed until hyperthermia and relevant secondary complications (i.e. colic, ileus) are resolved.

Client Education!!navigator!!

  • Ensure horses are acclimatized to high ambient temperature and humidity prior to prolonged or high exertional activity.
  • Ensure horses are hydrated and have ready access to water, especially during prolonged exercise.
  • If exercising in heat and humidity, ensure that cold water stations are readily available for rapid cooling of horses. If possible, avoid the hottest times of the day

Surgical Considerations!!navigator!!

N/A

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • NSAIDs, such as flunixin meglumine (1.1 mg/kg IV), may be used for their anti-inflammatory, antiendotoxic, and analgesic properties. Antipyretic properties of these drugs are not useful for nonpyrogenic hyperthermia.
  • Glucocorticoids have been used in other species and horses to decrease inflammation associated with heat damage and to aid in cell membrane stabilization; however, supportive research is sparse (dexamethasone 0.05–0.2 mg/kg IV).
  • Sedation may be necessary in many cases of hyperthermia, as horses become anxious, uncooperative, and/or dangerous to work around, thus making cooling efforts impossible.
  • Appropriate adjunctive therapy with rhabdomyolysis, muscle relaxants, anti-inflammatories.
  • MH, dantrolene sodium 10 mg/kg loading dose PO, then 2.5 mg/kg PO every 2 h

Contraindications!!navigator!!

Supplemental K+ in cases of HYPP.

Precautions!!navigator!!

  • Use NSAIDs cautiously in cases of renal compromise or dehydration.
  • Do not administer bicarbonate without knowing blood gas status. Note that these horses are most often alkalotic

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Monitor body temperature frequently.
  • Monitor renal function.
  • Assess hydration, packed cell volume, and total solids.
  • Assess response of electrolyte and blood gas adjustments

Prevention/Avoidance!!navigator!!

  • A period of acclimation (recommend 15 days) to hot and humid conditions will help reduce the risk of heat-related disorders.
  • Condition animals appropriately for the level of work they are expected to do.
  • Avoid riding in hot and humid weather.
  • Identify MH and HYPP individuals prior to anesthesia.
  • Keep foals on macrolide antibiotics in a shaded, cool environment.
  • Clip long haircoats to improve evaporative heat loss

Possible Complications!!navigator!!

  • CNS disorders—seizures, coma, death.
  • MODS-L.
  • Renal and hepatic failure.
  • DIC.
  • Laminitis.
  • Pulmonary edema.
  • May be more prone to subsequent hyperthermia

Expected Course and Prognosis!!navigator!!

Favorable to grave—depending on early detection and reversal of hyperthermia, correction of dehydration and electrolyte derangements, and prevention of organ failure.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

Abortions may occur with prolonged hyperthermia.

Synonyms!!navigator!!

  • Exertional heat illness.
  • Post-race distress syndrome.
  • Exhausted horse syndrome.
  • Heat exhaustion.
  • Heat stress.
  • Heat stroke

Abbreviations!!navigator!!

  • CNS = central nervous system
  • DIC = disseminated intravascular coagulation
  • GI = gastrointestinal
  • HYPP = hyperkalemic periodic paralysis
  • MH = malignant hyperthermia
  • MODS-L = multiorgan dysfunction syndrome—laminitis
  • NSAID = nonsteroidal anti-inflammatory drug
  • PPID = pituitary pars intermediate dysfunction
  • SDF = synchronous diaphragmatic flutter

Suggested Reading ⬆ ⬇

Brownlow MA, Dart AJ, Jeffcott LB. Exertional heat illness: a review of the syndrome affecting racing Thoroughbreds in hot and humid climates. Aust Vet J 2016;94:240–247.

Cohn CW, Hinchcliff KW, McKeever KH. Evaluation of washing with cold water to facilitate heat dissipation in horses exercised in hot, humid conditions. Am J Vet Res 1999;60:299–305.

Hodgson DR, Davis RE, McConaghy FF. Thermoregulation in the horse in response to exercise. Br Vet J 1994;150:219–235.

McCutcheon LJ, Geor RJ. Thermoregulation and exercise-associated heat illnesses. In: Hinchcliff KW, Kaneps AJ, Goer RJ, eds. Equine Sports Medicine and Surgery, 2e. Philadelphia, PA: Saunders, 2013:901–918.

Author(s) ⬆

Author: Ashley Whitehead

Consulting Editors: Michel Levy and Heidi Banse

Acknowledgment: The author and editors acknowledge the prior contribution of Wendy Duckett.