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Basics ⬇

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BASICS

Overview!!navigator!!

EAAs are common in athletic horses, often as the heart rate speeds up or slows down following intense exercise. Arrhythmias during strenuous exercise are of greatest significance.

Signalment!!navigator!!

EAAs are detected most often in performance animals and are occasionally identified in horses engaged in lower level athletic performance.

Signs!!navigator!!

  • EAAs can be associated with signs of poor athletic performance or found incidentally
  • Some individuals with the more severe arrhythmias such as ventricular tachycardia or paradoxical atrial fibrillation present with marked exercise intolerance, distress, or collapse at exercise
  • Cardiac arrhythmias are often assumed to account for sudden death at exercise, but this is difficult to prove

Causes and Risk Factors!!navigator!!

  • Some EAAs, particularly in the pre- and postexercise periods, represent physiologic variants as the heart rate speeds up and slows down
  • Dynamic airway obstruction, arterial hypoxemia, electrolyte disturbances, particularly depletion of total body magnesium, calcium, and potassium, are potential risk factors
  • Some horses with hyperkalemic periodic paralysis develop ventricular arrhythmias at exercise and these horses may also have dynamic airway obstruction
  • Myocardial pathology can lead to arrhythmias at rest and exercise

Diagnosis ⬆ ⬇

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DIAGNOSIS

Differential Diagnosis!!navigator!!

Upper airway disorders, exercise-induced pulmonary hemorrhage, lameness, and myopathy.

CBC/Biochemistry/Urinalysis!!navigator!!

  • Cardiac troponin I is increased only with myocardial necrosis
  • Arterial oxygen measurements, taken during treadmill exercise, may reveal hypoxemia
  • Serum electrolyte concentrations and fractional excretion of electrolytes can identify electrolyte disturbances

Imaging!!navigator!!

  • ECG during exercise is mandatory to identify the specific form of arrhythmia
  • Once EAAs have been identified, 24 h continuous ECG is used to determine whether the arrhythmia is also present at rest
  • Owing to their portability smartphone ECG devices can be useful. Recording after exercise may identify individuals that warrant more extensive ECG studies

Other Diagnostic Procedures!!navigator!!

Dynamic airway endoscopy can identify predisposing respiratory disorders.

Pathologic Findings!!navigator!!

Gross and histologic examination of the heart is often unremarkable but myocardial necrosis, fibrosis, or inflammation may be identified.

Treatment ⬆ ⬇

TREATMENT

  • With arrhythmias confined to the warmup and postexercise periods, no treatment may be necessary
  • In horses that have multiple premature depolarizations during maximal exercise, the possibility of collapse during exercise and consequent risk to a rider must be considered
  • Treatment should be aimed at predisposing diseases
  • In the absence of obvious causes, rest and corticosteroids are recommended with around a 40% chance that the arrhythmia will resolve

Medications ⬆ ⬇

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MEDICATIONS

Drug(s) of Choice!!navigator!!

  • For treatment of myocardial pathology, corticosteroids such as prednisolone (1 mg/kg PO every 48 h) or dexamethasone (0.05–0.1 mg/kg IV or 0.1 mg/kg PO every 24 h for 3–4 days and then continued every 3–4 days in decreasing dosages) are recommended
  • Vitamin C (20 mg/kg PO every 24 h) and vitamin E (10 IU/kg PO every 24 h) may also be beneficial owing to their antioxidant effect

Contraindications/Possible Interactions!!navigator!!

High-dose corticosteroid therapy has been associated with laminitis, particularly in cases in which other laminitis risk factors such as systemic illness and excessive body condition are present.

Follow-up ⬆ ⬇

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FOLLOW-UP

Patient Monitoring!!navigator!!

Exercising ECGs following treatment are used to assess efficacy.

Possible Complications!!navigator!!

  • Ventricular arrhythmias at exercise can lead to collapse or sudden death with consequent risk to a rider
  • Frequent supraventricular arrhythmias may predispose the horse to the development of atrial fibrillation and more severe exercise intolerance

Expected Course and Prognosis!!navigator!!

  • Many apparently healthy horses have EAAs consisting of atrioventricular block, usually immediately after strenuous exercise, sinus arrhythmia in the warmup and postexercise phases, or isolated supraventricular or ventricular premature depolarizations at any stage, providing there are no more than 2 at strenuous exercise
  • Horses with frequent premature depolarizations during strenuous exercise or with paroxysmal atrial fibrillation will show more severe signs. Paroxysmal atrial fibrillation often occurs only once but frequent premature depolarizations tend to persist and will cause ongoing performance problems if they cannot be treated successfully

Miscellaneous ⬆ ⬇

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MISCELLANEOUS

Abbreviations!!navigator!!

EAA = exercise-associated arrhythmia

Suggested Reading ⬆ ⬇

Slack J, Boston RC, Soma LR, Reef VB. Occurrence of cardiac arrhythmias in Standardbred racehorses. Equine Vet J 2015;47:398–404.

Author(s) ⬆

Author: Celia M. Marr

Consulting Editor: Celia M. Marr and Virginia B. Reef