Cardiac disease in the performance horse: Atrial Fibrillation and Equine Valvular Disease
Erin M Beasley DVM, PhD, DACVIM-LAIM
University of Georgia, Large Animal Medicine
Objectives
This article will review common cardiac conditions that affect the performance horse. The impact of each cardiac problem on athletic performance as well as recommendations for treatment, management and prognosis for future athletic performance will be discussed.
Cardiac disease in the performance horse
The prevalence of cardiac conditions in the performance horse appears to vary with breed and discipline. In one study investigating a variety of horse breeds and disciplines in a hospital population the reported prevalence of each of the following conditions was: mitral regurgitation (4.4%), atrial fibrillation (2.3%), aortic regurgitation (2.1%), and tricuspid regurgitation (1.7%). Cardiac murmurs are common in athletic horses and are not uncommonly physiologic, meaning they are related to the response of the heart to training rather than to valve pathology. However, it shouldn’t be assumed that murmurs are related to training and further investigation consisting of a cardiac work up should be pursued in most cases. Horses appear to have a propensity for developing arrhythmias related to the large size of the heart, influence of vagal tone and management practices. In many cases arrhythmias occur in the absence of structural heart disease. Horses with atrial fibrillation and valvular insufficiency can have successful careers even at intense levels of exercise and training. However, an accurate diagnosis, treatment when warranted and follow-up examinations are key components to successful management, safety of the horse and rider, and longevity in the competition world.
Cardiac evaluation
The components of a comprehensive cardiac work-up in the horse consist of the following:
- Physical examination and minimum data base (including cardiac troponin I and urinalysis).
- Echocardiogram with continuous electrocardiogram (ECG).
- Exercising electrocardiogram [indicated in most but not all cases.
- Resting 24-hour electrocardiogram ideally with video recording of horse activity [indicated for horses with suspected dysrhythmia or collapse (syncopal) events.
The echocardiogram should be performed by a board-certified internal medicine clinician, in either the specialty of Large Animal Medicine or Cardiology. The reason that this is so important is that examination requires capturing mechanics and structural changes in the heart that might not always be apparent in the standard set of images that are taught at most continuing education courses. During an echocardiogram, the specialist must use their knowledge and skills to adapt their imaging approach in order to highlight and evaluate the cardiac pathology present (or not present) in each individual case. The problem with the general practitioner acquiring and sending images and video loops for interpretation by a remote specialist is that an echocardiogram is evaluating a dynamic system retrospectively. It is not uncommon that vital information is missing that could impact the prognosis or treatment recommendations for the horse, but more often results in an inability to formulate any recommendations without repeating the exam which leads to owner frustration.
Arrhythmia
The most common physiologic arrhythmia in horses is second-degree atrioventricular (AV) block. The hallmarks of this arrhythmia are a regularly, irregular rhythm that is abolished with an increase in sympathetic tone (e.g. exercise, excitement, fear). This arrhythmia, therefore, always goes away rapidly if the horse is walked, exercised, or even when it becomes nervous as an unfamiliar person approaches. Atrial fibrillation (AF) can be difficult to distinguish from second degree AV block at very low heart rates. The hallmark of atrial fibrillation is an irregularly, irregular heart rhythm. This rhythm does not change with exercise, but can be more difficult to detect at rapid heart rates. Electrocardiography is necessary to establish the diagnosis. There are two ways practitioners can acquire this information. One is with a portable electrocardiographic machine paper printout and the other is with the AliveCor Vet, an adaptation that fits to the iPhone in combination with an application (App) downloaded to the phone that can digitally capture the heart rhythm. This technology has made it easier than ever to capture arrhythmias in horses. The results can be texted or emailed for quick consult.
A diagnosis of atrial fibrillation is made upon examination of an ECG that has absent p-waves, normal QRS complexes and an irregular R to R interval. Fibrillation or “f” waves are often present but can vary in appearance. In AF, the electrical activity of the heart has become erratic such that the atria are fibrillating (can think of it as continual spasms of the atrial muscle) and the only meaningful electrical conduction is initiated at the AV node with the action potential then propagated through the ventricles.
Atrial fibrillation is the most clinically important arrhythmia of horses and directly impacts the ability to perform. The impact on performance is a result of a loss of the atrial contribution to the cardiac output. In AF, about 20% less blood is leaving the heart with each beat. This decreases oxygen delivery to the tissues and at an increasing intensity of exercise becomes performance limiting. It is unusual for horses in atrial fibrillation to have syncopal (collapse) episodes, however, collapse is sometimes reported at the onset of the arrhythmia. When AF is sustained, electrical and structural remodeling changes occur in the myocardium and perpetuate the arrhythmia. Some of these changes are reversible if AF is diagnosed and treated soon after onset. Atrial fibrillation should be considered a “Monday morning” emergency in the performance horse. The reason for this mindset is that the duration the horse is in AF is one of the best predictors of failure to convert to normal sinus rhythm and recurrence of the arrhythmia after successful conversion. Horses that have risk factors including a longer duration of persistent AF and mild to moderate mitral regurgitation have higher reported rates of recurrence (up to 35%). It is important to note that in some horses, AF is paroxysmal (transient). In paroxysmal AF the horse will convert to sinus rhythm on its own typically within 48 hours of detection of the arrhythmia. Therefore, it is recommended to wait at least 48 hours before chemical or electrical rate conversion is performed. Horses with paroxysmal AF are likely to have had metabolic or electrolyte disturbances that triggered the transient arrhythmia.
What are the causes of atrial fibrillation?
Atrial fibrillation is classified as “lone” or related to heart disease. Lone AF is most common in athletic horses, particularly Thoroughbred and Standardbreds, and is related to large heart size, genetic predispositions (Standardbreds) and management practices consisting of bicarbonate “milk shakes,” and potentially administration of supplements that effect thyroid function (iodine containing supplements, Thyro-L). Horses with Lone AF have a good to excellent prognosis for performance with successful cardioversion. Horses with AF that are not converted might be able to continue to perform at a lower intensity of work. Exercising ECG is indicated in all cases of AF to determine if the rate is appropriate for the intensity of work performed and to evaluate for the presence exercise-induced ventricular arrhythmias.
Horses with AF related to heart disease generally have evidence of heart failure most often associated with severe valvular regurgitation and evidence of cardiac remodeling (chamber enlargement). Horses with AF related to underlying cardiac disease are usually not safe to ride and may require rate control to improve quality of life. Rhythm conversion is not indicated in horses with AF due to heart disease.
How do you treat a horse with “lone” atrial fibrillation?
Horses that are candidates for treatment following a cardiac work-up can undergo chemical or electrical cardioversion to reestablish normal sinus rhythm at a referral center. Chemical cardioversion requires intra-gastric administration (usually repeated doses every 2 hours until approach the toxic dose) of quinidine and continuous monitoring of the electrocardiogram to detect life-threatening arrhythmias that can occur as a side effect of quindine. Side effects occur commonly and are variable but some have the potential to be fatal. Reported side effects include: nasal edema, colic, arrhythmia, colitis, laminitis, neurologic signs and sudden death. The primary benefit of quinidine administration over transvenous electrical cardioversion (TVEC) is that general anesthesia is not required. Transvenous electrical cardioversion is well tolerated but requires a brief general anesthestic event. Cardiac catheters are placed in the standing horse and positioning within the left pulmonary artery and right atrium is verified with echocardiography and radiography. The horse is anesthetized and the heart is defibrillated allowing the atrial electrical activity to “reset” and conduct normally through the sinus node. The main side effects of this procedure are related to complications that can occur with indwelling catheters or general anesthetic procedures but are rarely reported. Both procedures have high success rates reported for them 80-95%. However, horses that fail to convert with quinidine are frequently successfully converted with TVEC. Case selection likely plays a role in successful conversion.
Other dysrhythmias
The history or clinical signs of horses with other dysrhythmias can vary widely from poor performance (reluctance, refusal to work or under-performing) to collapse. The application of the Alive Core ECG in these cases can be the difference between having a diagnosis or not because of the transient nature of most arrhythmias. Horses with a history consistent with poor performance, for which other causes have been ruled out, or any horse with collapse or a history of an irregular heart rhythm should undergo a complete cardiac evaluation (see above).
Murmurs
Murmurs are sounds made by turbulent flow of blood as it is directed through the heart and great vessels (pulmonary artery and aorta). Murmurs should be described by their location, timing, loudness (grade) and sound characteristics. The location should be denoted in terms of left, right, heart base or apex. A scale of 1-6 is used to grade murmurs based on the following qualities: 1. The murmur is quiet, focal and inconsistently heard. 2. The murmur is quiet yet consistently heard. 3. The murmur is the same intensity as the heart sounds and may radiate. 4. The murmur is the same or louder intensity as the heart sounds and radiates. 5. The murmur is loud and there is a palpable thrill. 6. The murmur is loud, with a palpable thrill and can be heard “off” of the chest.
Timing can be challenging to establish, particularly in rapid heart rates. Palpation of an artery (facial, transverse facial or carotid) during auscultation helps determine whether the timing is systolic, diastolic or continuous. The quality of the sound soft, blowing, high-pitched, click, buzz, squeak, musical are all helpful to record in the medical record and can sometimes help the clinician predict the most likely cause.
Systolic murmur:
Left side heart base: Physiologic flow murmur, some cases of mitral regurgitation
Left side apex: Mitral regurgitation
Right side: Tricuspid regurgitation or Ventricular septal defect
Stenosis of the aorta or pulmonic valve will result in a systolic murmur but are exceedingly rare in horses unless associated with severe endocarditis.
Diastolic murmur:
Left side heart base: Aortic regurgitation (rarely pulmonic regurgitation)
Right side: Aortic regurgitation (quieter than on the left)
Continuous:
Left or right: Aortocardiac fistula
The most common murmurs in performance horses are related to valvular disease and regurgitant blood flow. Physiologic flow murmurs occur with exercise and illness (e.g. colic, anemia) and can be loud (3-4)/6! These murmurs are transient which sets them apart from persistent murmurs due to structural heart disease. Horses should be referred for an echocardiogram for systolic murmurs that are not consistent with a physiologic flow murmur, any left sided diastolic murmur, moderate or loud murmurs and murmurs detected during a prepurchase examination.
Valvular disease in the horse is common and is most often due to degenerative change in the valve cusps or leaflets. Minor or major chordae tendinae rupture, endocarditis, and other conditions can cause regurgitation and carry varying prognoses. Echocardiogram is required to make a diagnosis and to estimate the impact on cardiac function of the individual horse. Serial echocardiograms are warranted over the life of the horse. Typically, valvular disease remains static or is slowly progressive over the life of the horse.
However, there are exceptions and in some cases treatment is indicated to slow progression. Angiotensin converting enzyme inhibitors (ACEi) have been evaluated in horses. Benazepril is the most effective ACEi in horses, is affordable in the United States and may slow progression of heart disease related to mitral and aortic regurgitation. Arrhythmogenesis occurs with some valvular disease partially associated with cardiac chamber enlargement which underscores why the exercising ECG is a component of a complete cardiac work-up even when the underlying problem is related to a leaky valve.
The severity of each condition is determined on the basis of the underlying cause, the volume of regurgitation and the structural and functional changes present in the heart that have occurred as a consequence of valvular insufficiency. Horses with mild to moderate disease associated with mitral, aortic or tricuspid regurgitation or a small ventricular septal defect can often perform well at a high intensity of exercise. Horses with echocardiographic evidence of hemodynamic dysfunction, severe cardiac chamber enlargement, exercise-induced arrhythmias and/or other evidence of impending heart failure should not be ridden and may require treatment.
Cardiac emergencies
When to refer on an urgent or semi-urgent basis: Syncope/collapse, atrial fibrillation, aortocardiac fistula (loud murmur, colic and severe tachycardia in Friesian or any breed), tachyarrhythmia, fever with a new cardiac murmur, muffled heart sounds, signs of heart failure (tachycardia, poor peripheral pulse pressure, weakness/lethargy, increased respiratory rate, dependent edema, frothy nasal discharge, distended jugular veins with or without pulsation.
When to refer on a non-urgent basis: New, persistent murmur detected or if there is a change to the quality or character of an existing murmur.
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