## Overview Atrial fibrillation is the most common clinically significant cardiac arrhythmia in horses. The atria quiver rapidly and ineffectively instead of contracting in a coordinated manner, eliminating the atrial contribution to ventricular filling (approximately 15 to 20 percent of cardiac output). Many horses have structurally normal hearts (“lone” atrial fibrillation); others have underlying atrial enlargement from valvular disease. Performance horses often show exercise intolerance, while horses in light work may be asymptomatic. ## Causes Large atrial size, high resting vagal tone, and electrolyte disturbances (especially potassium depletion) predispose horses to lone atrial fibrillation. Underlying mitral regurgitation or other cardiac disease can also trigger the arrhythmia. Standardbreds, draft breeds, and Warmbloods appear over-represented. ## Diagnosis Auscultation reveals an irregularly irregular rhythm without fourth heart sounds. Electrocardiography confirms the diagnosis (absence of P waves, irregular R-R intervals, fibrillation waves). Echocardiography is essential to identify structural heart disease before attempting conversion. ## Treatment Conversion to normal sinus rhythm is recommended for performance horses. Options include quinidine sulfate administered by nasogastric tube or transvenous electrical cardioversion under general anesthesia. Horses with significant underlying heart disease are usually not candidates for conversion and are managed supportively or retired. ## Recovery and Prognosis Lone atrial fibrillation of short duration (<4 months) has a high conversion success rate (>90 percent with either method) and good prognosis for return to athletics. Longer duration and concurrent mitral regurgitation increase recurrence risk. Untreated horses can often continue light work safely. ## Prevention No specific prevention exists. Correction of electrolyte imbalances and management of underlying cardiac disease may reduce risk in susceptible horses.
exercise intolerance; poor performance; irregular heart rhythm on auscultation; elevated heart rate during work; occasional epistaxis or sudden deceleration
## Overview Atrial fibrillation is the most common clinically significant cardiac arrhythmia in horses. The atria quiver rapidly and ineffectively instead of contracting in a coordinated manner, eliminating the atrial contribution to ventricular filling (approximately 15 to 20 percent of cardiac output). Many horses have structurally normal hearts (“lone” atrial fibrillation); others have underlying atrial enlargement from valvular disease. Performance horses often show exercise intolerance, while horses in light work may be asymptomatic. ## Causes Large atrial size, high resting vagal tone, and electrolyte disturbances (especially potassium depletion) predispose horses to lone atrial fibrillation. Underlying mitral regurgitation or other cardiac disease can also trigger the arrhythmia. Standardbreds, draft breeds, and Warmbloods appear over-represented. ## Diagnosis Auscultation reveals an irregularly irregular rhythm without fourth heart sounds. Electrocardiography confirms the diagnosis (absence of P waves, irregular R-R intervals, fibrillation waves). Echocardiography is essential to identify structural heart disease before attempting conversion. ## Treatment Conversion to normal sinus rhythm is recommended for performance horses. Options include quinidine sulfate administered by nasogastric tube or transvenous electrical cardioversion under general anesthesia. Horses with significant underlying heart disease are usually not candidates for conversion and are managed supportively or retired. ## Recovery and Prognosis Lone atrial fibrillation of short duration (<4 months) has a high conversion success rate (>90 percent with either method) and good prognosis for return to athletics. Longer duration and concurrent mitral regurgitation increase recurrence risk. Untreated horses can often continue light work safely. ## Prevention No specific prevention exists. Correction of electrolyte imbalances and management of underlying cardiac disease may reduce risk in susceptible horses.
exercise intolerance; poor performance; irregular heart rhythm on auscultation; elevated heart rate during work; occasional epistaxis or sudden deceleration