Cardiac Arrhythmias in Horses

Detailed Information on Cardiac Arrhythmias in Horses for NAVLE Preparation

General Overview

• Cardiac Arrhythmias: Deviations from the normal heart rhythm that can impact performance and potentially pose risks to both horses and riders. Arrhythmias are classified based on their origin (atrial vs. ventricular) and the type of rhythm disturbance (bradyarrhythmia, tachyarrhythmia, premature depolarizations, fibrillation).

Etiology

• Physiologic Arrhythmias: Typically related to high vagal tone, such as sinus arrhythmia and second-degree atrioventricular (AV) block. These usually disappear with exercise or stress and are not clinically significant.

• Pathologic Arrhythmias: May result from underlying cardiac diseases like valvular disease, congenital defects, myocardial damage, or non-cardiac conditions such as electrolyte imbalances, hypoxemia, or endotoxemia.

Pathophysiology

• Abnormal Impulse Formation: Enhanced automaticity or triggered activity leads to premature depolarizations or tachyarrhythmias.

• Abnormal Impulse Conduction: Includes blocks (e.g., AV block) and reentry circuits, leading to sustained arrhythmias such as atrial fibrillation (AF).

Clinical Signs

• Asymptomatic: Many horses with physiologic arrhythmias show no clinical signs.

• Poor Performance: Pathologic arrhythmias, especially AF and ventricular arrhythmias, can lead to exercise intolerance, reduced performance, and fatigue.

• Syncope or Collapse: Severe arrhythmias, particularly ventricular tachycardia (VT) or third-degree AV block, can cause episodes of fainting or collapse.

Diagnostics

1. Electrocardiography (ECG):

   • Resting ECG: Identifies arrhythmias like AF, premature depolarizations, and AV block.

   • Exercise ECG: Useful for detecting exercise-induced arrhythmias that may not be present at rest.

   • 24-hour Holter Monitoring: Provides continuous ECG recording to capture intermittent arrhythmias.

2. Echocardiography: Assesses underlying structural heart disease, chamber enlargement, or myocardial lesions.

3. Blood Tests: Evaluate electrolyte levels, cardiac biomarkers (e.g., troponins), and acid-base status.

Specific Arrhythmias

1. Atrial Fibrillation (AF)

   • Definition: The most common clinically significant arrhythmia in horses, characterized by chaotic atrial electrical activity and loss of effective atrial contraction.

   • Causes: May occur in horses with underlying heart disease (e.g., mitral valve regurgitation) or in otherwise healthy horses due to atrial dilation or favorable electrophysiologic conditions.

   • Clinical Signs: Irregular heart rhythm, poor performance, and exercise intolerance. Some horses may have paroxysmal AF that resolves spontaneously, while others develop persistent AF.

   • Diagnosis: ECG showing absence of P waves, presence of fibrillation waves (f-waves), and irregular RR intervals.

   • Treatment:

     • Medical Treatment: Quinidine sulfate (22 mg/kg via nasogastric tube every 2 hours) for conversion to sinus rhythm, but with significant risks of toxicity (e.g., hypotension, tachycardia, QT prolongation).

     • Transvenous Electrical Cardioversion (TVEC): High success rate (>95%) for converting chronic AF, requiring general anesthesia and specialized equipment.

     • Post-treatment Monitoring: Echocardiography and 24-hour ECG monitoring are recommended to assess for recurrence and atrial function recovery.

   • Prognosis: Varies depending on the presence of underlying heart disease. Recurrence of AF is common, especially in horses with structural heart disease.

2. Atrial Premature Depolarizations (APD)

   • Definition: Premature atrial contractions that can predispose horses to AF.

   • Clinical Impact: Often asymptomatic, but may lead to atrial tachycardia (AT) or AF in some cases.

   • Management: Antiarrhythmic drugs like sotalol (2-3 mg/kg PO twice daily) may reduce the risk of recurrence in horses with a history of AF.

3. Ventricular Arrhythmias

   • Ventricular Premature Depolarizations (VPD): Isolated early beats originating from the ventricles, which may be benign or indicate underlying myocardial disease.

   • Ventricular Tachycardia (VT): Three or more consecutive VPDs, associated with a high risk of sudden death.

   • Management: Lidocaine (0.25-0.5 mg/kg IV bolus, followed by a continuous infusion) is the first-line treatment for VT. Rest and corticosteroids may be used for less severe cases.

4. Atrioventricular Block (AV Block)

   • First- and Second-Degree AV Block: Often physiologic in horses due to high vagal tone and resolve with exercise.

   • Third-Degree AV Block: Complete dissociation between atrial and ventricular activity, leading to severe bradycardia and collapse. Pacemaker implantation is the definitive treatment.

Medications

• Quinidine Sulfate: Class IA antiarrhythmic for AF conversion, with significant risks of toxicity.

• Sotalol: Class III antiarrhythmic for reducing recurrence of AF.

• Lidocaine: Class IB antiarrhythmic for treating VT, with careful monitoring for adverse effects.

• Amiodarone: Used for refractory arrhythmias, with both intravenous and oral formulations available, though it requires careful monitoring due to potential side effects.

Prognosis

• AF: Horses with AF and no underlying heart disease may return to athletic function after successful cardioversion, though recurrence rates are high. Horses with structural heart disease or persistent AF have a guarded prognosis.

• Ventricular Arrhythmias: Prognosis depends on the severity of the arrhythmia and the presence of underlying disease. Horses with structural heart disease and VT have an increased risk of sudden death.

Reference:

• Durando, M. M., Bonagura, J. D., & Reef, V. B. (2019). Arrhythmias: Diagnosis and treatment. Veterinary Clinics of North America: Equine Practice, 35(1), 1–25.