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The equine heart has four chambers: two atria and two ventricles. At rest an adult horse’s heart rate is typically 28 to 44 beats per minute. During maximal exercise heart rate can reach 220 to 240 beats per minute and cardiac output can rise to several hundred liters per minute. The combination of a relatively large heart, a large blood volume, and the ability to mobilize red cells from the spleen makes the horse an exceptional athlete. Heart size varies with body size and breed. Thoroughbreds tend to have proportionally larger hearts than many other breeds. The widely repeated claim that Secretariat’s heart weighed 10 kg is an estimate made by the pathologist at necropsy; the heart was never actually weighed. Larger hearts can support higher cardiac output, but many other factors (conformation, training, genetics, and luck) also determine performance. Auscultation is performed on both sides of the chest. Soft (grade 1–2) flow murmurs are common in fit horses and are often physiological. Exercise electrocardiography can reveal arrhythmias that are not apparent at rest.
Physiological flow murmurs are frequent in athletic horses. Pathologic murmurs or arrhythmias (atrial fibrillation is the most common significant arrhythmia) can affect performance and require further investigation. Exercise testing or continuous ECG may be needed to detect intermittent problems.
## 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, elim
## Overview A cardiac murmur is an abnormal heart sound produced by turbulent blood flow. Mitral and aortic regurgitation are the most common significant murmurs in older horses (mitral is often equa
## Overview Aortic rupture is a spontaneous tear in the wall of the aorta, the main artery leaving the heart. In most breeds the tear occurs at the aortic root and is rapidly fatal due to cardiac tam
## Overview Myocarditis is inflammation of the heart muscle, often secondary to viral or bacterial infection. It can cause arrhythmias, exercise intolerance, fever, and elevated cardiac troponin I. C
## Overview A ventricular septal defect is a congenital hole between the left and right ventricles and the most common congenital heart defect in horses. A loud holosystolic murmur is typical. Small
Know your horse’s normal resting heart rate. A persistently elevated resting rate, irregular rhythm, exercise intolerance, or a sudden drop in performance warrants veterinary examination. Do not rely on heart size alone as a predictor of ability.
## 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. Signs include: exercise intolerance; poor performance; irregular heart rhythm on auscultation; elevated heart rate during work; occasional epistaxis or sudden deceleration
## Overview A cardiac murmur is an abnormal heart sound produced by turbulent blood flow. Mitral and aortic regurgitation are the most common significant murmurs in older horses (mitral is often equal or more frequent than aortic in studied populations). Many murmurs are incidental and do not affect performance; others indicate clinically important valvular disease that can limit athletic use or, rarely, lead to heart failure. ## Causes Degenerative valve disease (mitral and aortic valves are both commonly affected), congenital defects such as ventricular septal defect or patent ductus arteriosus, and endocarditis are the main causes. Physiologic (innocent) murmurs also occur, particularly in fit young horses. ## Diagnosis Auscultation detects the murmur and characterizes its timing, location, and intensity. Echocardiography with Doppler is required to determine the valve involved, the severity of regurgitation, and the effect on chamber size and function. ## Treatment Most murmurs require only monitoring. Horses with significant regurgitation may need exercise restriction and periodic re-evaluation. Specific medical or surgical therapy is rarely indicated except for selected congenital lesions or endocarditis. ## Recovery and Prognosis Prognosis depends on the lesion. Mild valvular regurgitation (whether mitral or aortic) is often well tolerated for years. Severe regurgitation or myocardial dysfunction carries a guarded prognosis for continued athletic work and, in advanced cases, for life. ## Prevention No specific prevention exists for degenerative valve disease. Regular auscultation allows early detection. Signs include: abnormal heart sound on auscultation; exercise intolerance in significant cases; elevated resting heart rate; jugular pulsation or edema in advanced heart failure
## Overview Aortic rupture is a spontaneous tear in the wall of the aorta, the main artery leaving the heart. In most breeds the tear occurs at the aortic root and is rapidly fatal due to cardiac tamponade or massive hemorrhage. In Friesian horses a breed-specific form occurs in the aortic arch near the ligamentum arteriosum and frequently creates an aortopulmonary fistula; some of these horses survive for months to years with progressive signs. No effective treatment exists once rupture has occurred. ## Causes The cause is unknown in most cases. In Friesians, disorganization of collagen fibers in the aortic wall has been documented and is believed to predispose to rupture at the arch. In other breeds, rupture is often associated with older age and strenuous exercise, although many horses show no premonitory signs. ## Diagnosis Most horses are found dead or die within minutes of onset. Survivors (primarily Friesians with aortopulmonary fistulation) may show tachycardia, bounding pulses, jugular pulsation, peripheral edema, coughing, or recurrent colic-like episodes. Echocardiography can identify the fistula or pericardial effusion in living horses. Definitive diagnosis is usually made at necropsy. ## Treatment No treatment reverses the rupture. Supportive care is unrewarding. Humane euthanasia is recommended once the diagnosis is confirmed in a living horse. ## Recovery and Prognosis Prognosis is grave. Acute root ruptures are almost uniformly fatal within minutes. Friesian horses with chronic aortopulmonary fistulas may survive for a variable period but remain at high risk of sudden death. ## Prevention No preventive measures are known. Breeding decisions in Friesians may eventually be informed by ongoing genetic research. Signs include: sudden death; collapse; tachycardia; pale mucous membranes; bounding arterial pulses; jugular pulsation; peripheral edema (chronic cases)
## Overview Myocarditis is inflammation of the heart muscle, often secondary to viral or bacterial infection. It can cause arrhythmias, exercise intolerance, fever, and elevated cardiac troponin I. Continued exercise during active myocarditis risks fatal arrhythmias. Rest is the cornerstone of treatment. ## Causes Viral infections (including equine herpesvirus), bacterial disease, or other systemic illness. ## Diagnosis Auscultation of arrhythmias, electrocardiography, elevated cardiac troponin, and echocardiography. Viral testing may identify a cause. ## Treatment Complete rest, anti-inflammatories, and treatment of any underlying infection. No specific antiviral is routinely effective. Return to work is gradual and only after cardiac markers and rhythm normalize. ## Recovery and Prognosis Variable. Mild cases often recover fully. Severe myocarditis can be fatal or end an athletic career. ## Prevention Vaccination and biosecurity for respiratory viruses where applicable. Signs include: arrhythmias; exercise intolerance; fever; elevated cardiac troponin I; weakness
## Overview A ventricular septal defect is a congenital hole between the left and right ventricles and the most common congenital heart defect in horses. A loud holosystolic murmur is typical. Small defects are often well tolerated and allow moderate work. Large defects cause exercise intolerance and can lead to heart failure. ## Causes Congenital. ## Diagnosis Auscultation and echocardiography. ## Treatment None for small, uncomplicated defects. Management of heart failure if it develops. Affected horses should not be bred. ## Recovery and Prognosis Good for small defects. Poor for large ones causing failure. ## Prevention Do not breed affected horses. Signs include: loud holosystolic murmur; exercise intolerance if large; may be well tolerated if small
Physiological flow murmurs are frequent in athletic horses. Pathologic murmurs or arrhythmias (atrial fibrillation is the most common significant arrhythmia) can affect performance and require further investigation. Exercise testing or continuous ECG may be needed to detect intermittent problems.
Know your horse’s normal resting heart rate. A persistently elevated resting rate, irregular rhythm, exercise intolerance, or a sudden drop in performance warrants veterinary examination. Do not rely on heart size alone as a predictor of ability.
See the Heart on our interactive 3D horse model. Open 3D Model