A heart arrhythmia, or irregular heartbeat, is a performance-limiting condition commonly seen in athletic horses. While some arrhythmias are benign, others may indicate cardiac disease or another underlying health condition.

The horse is a supreme athlete with a powerful heart that can beat up to 250 times per minute during strenuous exercise. Performance horses need to pump large volumes of blood throughout the body and maintain an elevated heart rate for a extended periods during competitions and training.

With regular conditioning, the cardiovascular system adapts to support improved athletic ability. [1][2] Intense training regimes produce structural changes in the heart, including increased cardiac mass. However, repeated exertion can also increase the susceptibility of horses to developing arrhythmias.

Horses affected by an irregular heartbeat may experience reduced stamina, increased fatigue, and an increased likelihood of developing severe heart conditions or experiencing sudden cardiac events.

Arrhythmias in performance horses can also be triggered by electrolyte imbalances, dehydration, agitation, or can occur secondary to another infection or medical condition. Even though many arrhythmias are harmless, they should always be investigated to rule out heart disease.

Arrhythmias in Performance Horses

Cardiac arrhythmia is a common condition, with 25% of horses having arrhythmias that can be detected during veterinary examination. [3] While some arrhythmias limit performance, they often go undetected and unnoticed. [4]

Many benign arrhythmias appear spontaneously without cause and do not impact the horse’s quality of life. These do not require treatment.

Other arrhythmias develop secondary to an underlying condition, such as: [3]

  • Heart disease
  • Congenital defects
  • Metabolic disorders (EMS, PPID, etc.)
  • Systemic inflammation
  • Fever
  • Anemia
  • Toxicosis or medication use

Clinical Signs

Presenting signs for arrhythmias in horses are highly variable between individuals.

Most arrhythmias do not produce clinical signs and remain undetected throughout the horse’s life. Horses with asymptomatic arrhythmias can have successful performance careers without being affected by their condition.

Horses that are severely affected are likely to display signs associated with abnormal cardiovascular function, including: [5]

  • Shortness of breath
  • Anxiety
  • Exhaustion
  • Poor performance
  • Loss of consciousness & collapse
  • Death

If your horse is exhibiting poor performance and other signs of arrhythmia, contact your veterinarian for testing and diagnosis.

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Types of Cardiac Arrhythmias

Various types of cardiac arrhythmias can affect horses, each with distinct characteristics and implications for the equine cardiovascular system.

Some cardiac arrhythmias, such as atrial fibrillation (AF), have been thoroughly studied in performance horses and their influence on athletic ability is well understood. However, less is known about the effects of uncommon arrhythmias on horses. [6]

Sinus Tachycardia

Tachycardia in horses describes a fast heart rate of over 50 beats per minute (bpm) at rest. This occurs when the sinus node (the heart’s pacemaker) generates electrical impulses more frequently than normal.

Sinus tachycardia rarely requires treatment and typically poses no issues. It may be caused by management factors, such as strenuous exercise or drug administration.

Horses that are ill with a fever, anxious, or overly excited may exhibit an elevated heart rate for a brief or prolonged duration. A horse’s heart rate should gradually return to normal within 10 – 15 minutes after exercise. [7]

Atrial Fibrillation

Atrial fibrillation (A-Fib or AF), is the most common type of arrhythmia in performance horses. It can significantly diminish the horse’s athletic capacity and post-exercise recovery time, although it may not be noticeable at rest or during moderate exercise.

Atrial fibrillation is characterized by the irregular beating of the atria (upper chambers) of the heart, leading to incomplete filling. This disrupts the blood flow to the ventricles (lower chambers) and affects circulation throughout the body. [8]

This irregular and rapid heartbeat is caused by an uncoordinated electric current, which stimulates the atrioventricular (AV) node at unpredictable intervals. In some cases, horses may experience atrial fibrillation (AF) without any underlying heart disease, which is known as lone AF. [3]

This condition often goes unnoticed in many horses as it may not induce any noticeable symptoms. It is more prevalent in larger breeds and can be a result of underlying heart disease or electrolyte imbalance. Horses with atrial fibrillation (AF) who do display symptoms may experience the following:

  • Poor performance
  • Exhaustion
  • Shortness of breath & coughing
  • Nasal discharge

If atrial fibrillation occurs during exercise, the horse may abruptly slow down or come to a halt. They may vocalize and exhibit signs of distress. In certain cases, the horse may experience nosebleeds (epistaxis) or cough up blood. [8]

Sinus Bradycardia

Sinus bradycardia is a rare arrhythmia in horses, mediated by the vagus nerve. It typically lacks symptoms and goes unnoticed, but results in a slow heart rate (below 24 bpm). Bradycardia can arise as a secondary effect of other conditions, such as sinoatrial block (SAB), sinus arrest, or advanced sinus arrhythmia. [9]

Another potential cause of sinus bradycardia is an overactive vagus nerve, responsible for regulating the horse’s involuntary processes, including heart rate.

When vagal tone (activity of the vagal nerve) increases, it triggers the activation of the parasympathetic nervous system, reducing the horse’s blood pressure and heart rate. [9] This mechanism enables the horse’s body to rapidly relax following a period of stress.

However, not all instances of a slow heart rate indicate bradycardia. Horses in excellent physical condition may naturally exhibit a consistently slow heart rate.

Causes of Arrhythmias in Sport Horses

When performance horse undergoes strenuous training over an extended period, the cardiovascular system adapts to sustain the demands of high activity levels.

During this adaptation process, the heart undergoes structural changes characterized by an increase in chamber size and thickening of the heart walls. This phenomenon, known as cardiac hypertrophy or “athlete’s heart syndrome” (AHS), refers to the growth of cardiac mass due to heavy training volume. [1]

Horses that engage in repetitive and strenuous exercise essentially subject their hearts to micro-damage and subsequent repair. This repetitive stress may render them more susceptible to developing arrhythmias, murmurs, and even sudden cardiac death.

How the Equine Heart Works

The heart is a muscular organ with hollow chambers responsible for pumping blood throughout the horse’s body. It consists of four chambers: the left and right atria located in the upper part of the heart, and the left and right ventricles situated in the lower part. [2]

The right side of the heart receives blood that is low in oxygen and nutrients and pumps it to the lungs. In the lungs, waste products such as carbon dioxide (CO2) are expelled, and the blood is replenished with oxygen before the blood is brought back to the left side of the heart.

The left side of the heart pumps oxygenated, nutrient-rich blood from the lungs out to the body’s tissues. While circulating, the blood brings oxygen to tissues and accumulates waste materials to continue this perpetual cycle.

The Heartbeat

A healthy horse’s heart beats at a rate of approximately 28 – 44 times per minute while at rest. Healthy foals have a higher heart rate, ranging from 60 – 110 bpm. This varies depending on the horse’s size and level of fitness. [10]

The heartbeat consists of two phases. During the diastole phase, the heart muscles relax, and the chambers fill with blood. During the systole phase, the muscles contract, pushing blood out of the heart and into circulation. [2]

The sinoatrial (SA) node, often referred to as the heart’s pacemaker, is located in the right atrium. It is responsible for generating the electrical signals that initiate the heart’s contractions. These occur rhythmically, at a rate of approximately 30 signals per minute, leading to the contraction of the atria.

Subsequently, the electrical signal passes through the atrioventricular (AV) node, which triggers the ventricles to contract. Any deviation from this normal heart rhythm can be considered an arrhythmia.

Second-Degree Atrioventricular Block

Second-Degree Atrioventricular Block (2AVB) is extremely common and poses little concern for horses. As many as 40% of horses may exhibit 2AVB, with the condition being particularly prevalent in trained sport horses. [11]

2AVB occurs when there is a delay in the transmission of electrical signals through the atrioventricular (AV) node, leading to a consistently slower heart rate. It is thought that this arrhythmia could be an adaptation, rather than a disadvantage for sport horses.

Ventricular Tachycardia

Premature ventricular complexes (PVCs) are abnormal heartbeats that originate from the ventricles, which are the lower chambers of the heart. These beats occur earlier than expected in the regular cardiac rhythm, disrupting the normal sequence of electrical signals. [3]

During a PVC, an electrical impulse is generated in the ventricles before the next expected regular heartbeat. This causes a premature contraction of the ventricles, leading to a momentary pause in the heart’s regular rhythm. Following the PVC, the heart resumes its normal rhythm.

When three or more repetitive PVCs occur, it leads to an abnormal rhythm known as ventricular tachycardia (VT). [3] VT impairs the efficient pumping of oxygenated blood to the body’s tissues, depriving them of vital nutrients. [12]

While rare compared to atrial fibrillation (AF), VT can be a serious or life-threatening condition. It is often associated with factors such as drug administration or an underlying condition such as cardiac disease, colic, infectious disease, or electrolyte abnormalities (e.g., hypokalemia). [13][14]

Horses with complex ventricular arrhythmias and PVCs should not be ridden or driven, as they are at risk of sudden cardiac death (SCD) and can potentially collapse. This poses a significant danger to the rider and other horses in the vicinity. Types of complex ventricular arrhythmias include VT, ventricular fibrillation (VF), and idioventricular rhythms (IVR). [3][15]

Ventricular Fibrillation

In extremely rare cases, a dangerous arrhythmia known as ventricular fibrillation (VF) can occur. VF is often preceded by VT and is typically a life-threatening condition. It is associated with heart irritability, severe cardiac disease, severe electrolyte imbalance, hypoxia, or drug toxicosis. [16]

VF can be readily diagnosed with an electrocardiogram (ECG). However, once VF develops and cardiac collapse ensues, death is usually imminent. Because VF is very rare, there is limited research on its effects in equids. Further research is needed to better understand this condition. [16]

Diagnosing Cardiac Arrhythmias

The diagnosis of cardiac arrhythmias typically involves a combination of methods, including differential diagnoses, auscultation, electrocardiography (ECG), and exercise testing.

If you suspect your horse has a cardiac arrhythmia, ask your veterinarian for an evaluation to accurately diagnose and assess the condition. [3]

Differential Diagnosis

Symptoms of cardiac arrhythmia can resemble those of other conditions, making diagnosis challenging. Vague symptoms such as poor performance and exhaustion can stem from various causes, including respiratory and metabolic diseases, as well as orthopedic pain. [8]

Thorough testing is crucial to identify the underlying cause of poor performance. In addition to cardiac arrhythmia, other conditions that can contribute to exercise intolerance include:

Auscultation

When blood flows rapidly in and out of the heart, it creates vibrations that can be heard through a stethoscope. These audible sounds, detected during auscultation, are commonly known as the heartbeat.

During auscultation, a veterinarian listens to these sounds with a stethoscope to identify any deviations or abnormalities. This enables them to diagnose cardiac arrhythmias, murmurs, or other related conditions. [17]

The veterinarian’s ability to determine the location and characteristics of the abnormality is crucial for diagnosing cardiac arrhythmias. Any instances of fast, slow, or irregular heart rates may indicate the presence of a secondary or lone arrhythmia.

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Electrocardiogram (ECG)

An electrocardiogram (ECG or EKG) is a diagnostic test that records the electrical signals produced by the heart to identify any abnormalities. While ECG testing is commonly used in the equine industry, it often does not provide a definitive prognosis and has certain limitations. [4]

Due to the horse’s large heart and significant cardiac reserve, abnormalities in the heart’s electrical activity may only become apparent during periods of intense exercise. Many ECGs are typically conducted at rest, which may not accurately capture arrhythmias that manifest exclusively during strenuous activity.

If a horse is suspected of having an underlying heart defect or disease, an ECG is recommended as part of the diagnostic process. It can provide valuable information to aid in the identification and evaluation of cardiac conditions. [3]

Exercise Testing

Since many cardiac abnormalities are only apparent during strenuous exercise, a treadmill test may be conducted. This test is easy to standardize and can offer very accurate readings. However, it can take a horse several sessions to become habituated to running on a treadmill. [18]

Horses are fitted with safety equipment prior to running on a treadmill, which can be increased to various different speeds and inclines to increase intensity.

To record heart activity, the electrodes from the ECG are usually attached on the left side of the thorax, under the saddle and under the girth. Heart rate data is commonly transmitted to a watch attached to the horse while the horse is running. [1]

A field exercise test may be suitable for a horse that cannot travel to a facility for a treadmill test. This test could prove more accurate, as it is conducted to a competition or racing setting where the horse is likely to display arrhythmia symptoms.

Treatment

The treatment of cardiac arrhythmias in horses is determined based on the specific type of arrhythmia, the symptoms experienced by the horse, and any underlying conditions that may contribute to the irregular heartbeat.

Consult with your veterinarian to accurately diagnose the arrhythmia and develop an appropriate treatment plan for your horse. The veterinarian will consider various factors, including the severity of the arrhythmia, the impact on the horse’s overall health and performance, and the horse’s intended work.

Treatment options may include medication to regulate the heart rhythm or interventions such as electrical cardioversion or pacemaker implantation. Regular monitoring and follow-up evaluations are important to assess the response to treatment and make any necessary adjustments.

Medication

Quinidine sulfate is a class IA antiarrhythmic medication. It works by slowing down electrical impulses in the heart, helping to regulate the heartbeat and restore it to a normal sinus rhythm. Quinidine sulfate is commonly used to treat atrial fibrillation (AF) and has an approximate success rate of 88%. [5]

The standard treatment for AF involves administering 10 g of quinidine sulfate through a nasogastric tube every two hours until the horse’s heart rhythm converts back to normal sinus rhythm.

If the horse’s heart rhythm does not convert to normal sinus rhythm with a total daily dose of 40 – 60 g or if the horse exhibits signs of toxicity, the treatment should be discontinued and resumed the following day. [19]

Following quinidine treatment, some horses may experience side effects such as ataxia (loss of coordination), colic, laminitis, or hives. Horse owners must be aware of the risks associated with this medication.

In cases of chronic atrial fibrillation, digoxin may be used as a preventative measure prior to an AF episode. Digoxin has been shown to convert a fast and irregular heartbeat back to normal sinus rhythm. It achieves this by decreasing conduction through the AV node and increasing vagal tone, effectively reducing heart rate.

Further research is necessary to determine whether digoxin can be used to mitigate the negative effects associated with quinidine treatment. It is important to note that quinidine should not be administered to horses with severe heart disease, making digoxin an alternative option in such cases. [20]

Electrocardioversion

Some equine veterinary clinics offer electrocardioversion, also known as transvenous electrical cardioversion (TVEC), as a treatment option for AF in horses that do not respond well to medications. This procedure involves placing the horse under general anesthesia and inserting a catheter into the jugular vein. [3]

Electric leads are then advanced through the catheter and positioned in the atria of the heart. A controlled electrical shock is delivered, aiming to restore the horse’s heart rhythm to normal sinus rhythm. Following the procedure, the horse typically recovers fully and can return to performance sports. [21]

It is important to note that after TVEC treatment, certain medications and supplements should be avoided, including furosemide (Lasix), thyroid medications, and supplements containing sodium bicarbonate. Consultation with a veterinarian is necessary to determine the appropriate post-treatment management for each individual case.

Treatment for Ventricular Tachycardia

Treatment for horses with Ventricular Tachycardia (VT) is warranted if the heart rate exceeds 120 bpm or if sudden loss of effective circulation and an irregular rhythm occur. [12]

The following treatment options are available for VT under the supervision of a veterinarian:

  • Lidocaine: An antiarrhythmic medication and sodium-channel blocker that can help depress ventricular arrhythmias and alleviate symptoms. It is typically administered intravenously (IV) or as a bolus at a rate of 0.5 mg/kg. Lidocaine is often the first-line treatment for horses with VT due to its tolerability and low risk profile.
  • Magnesium sulfate: Acting as an anticonvulsant and antiarrhythmic, magnesium sulfate is another therapeutic option for VT, with minimal negative cardiovascular effects on horses. It is usually given intravenously at a rate of 0.5 – 1 g/hour and can be used in combination with lidocaine.

The following treatments have been utilized successfully for VT but carry a higher risk of severe adverse reactions in horses. Therefore, they should only be administered in a hospital setting with constant monitoring and access to electrocardiogram:

  • Procainamide: A class IA antiarrhythmic reported to treat VT in horses, but improper administration can lead to toxic effects.
  • Quinidine gluconate: A cost-effective class IA drug effective in treating various types of arrhythmias. However, it can cause several cardiovascular side effects, including blood vessel dilation, and requires close supervision.
  • Amiodarone, Esmolol, Propafenone, and Bretylium Tosylate: These medications have all been successfully administered to treat horses with different forms and severities of VT.
  • Furosemide: Pulmonary edema, or fluid in the lungs, commonly accompanies heart failure. Furosemide, a diuretic, is used to treat this condition by reducing excess fluid in the body.

After successful treatment, the horse should undergo a period of rest for 1 – 2 months. Gradual return to activity and training can be initiated under the supervision of a veterinarian, provided that ECG readings show significant improvement.

Summary

  • A common condition in performance horses, cardiac arrhythmia, or irregular heartbeat, can manifest with no symptoms or cause debilitating effects for horses.
  • Arrhythmias can either be secondary to pre-existing conditions such as heart disease or electrolyte imbalance, or they can occur independently.
  • Diagnosis of cardiac arrhythmias is typically straightforward and involves a combination of methods including differential diagnosis, electrocardiogram (ECG), and exercise testing.
  • While treatments for common arrhythmias in horses are highly successful, there is a notable lack of scientific research and information regarding other types of arrhythmias in equids.

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References

  1. Allen, K. J. et al. Evaluation of heart rate and rhythm during exercise. Equine Vet Educ. 2015.
  2. Cunningham, S. M. Blood Vessel Disorders of Horses. Merck Veterinary Manual. 2019.
  3. Reef, V. B. Recommendations for Management of Equine Athletes with Cardiovascular Abnormalities. J Vet Intern Med. 2014. View Summary
  4. Young, L. E. Recent Advances in Diagnosing Cardiac Abnormalitlies With an EKG During Exercise: A Review. American Association of Equine Practitioners (AAEP). 2007.
  5. McGurrin, M. K. J. The diagnosis and management of atrial fibrillation in the horse. Vet Med (Auckl). 2015.
  6. Alberti, E. et al. Evaluation of Cardiac Arrhythmias before, during, and after Treadmill Exercise Testing in Poorly Performing Standardbred Racehorses. Animals (Basel).2021. View Summary
  7. King, A. Ecker, G. The Horse Health Check: A Systematic Method of Examination. Equine Guelph. Accessed At 2023-06-14.
  8. Ellis, K. L. et al. Poor performance in the horse: Diagnosing the non-orthopaedic causes. Equine Vet Educ. 2022.
  9. Rezakhani, A. et al. Sinus Bradycardia in a Horse. Pakistan Vet J. 2005.
  10. Code of Practice for the Care and Handling of Equines. National Farm Animal Care Council (NFACC). 2013.
  11. Nissen, S. D. Electrocardiographic characteristics of trained and untrained standardbred recehorses. J Vet Intern Med. 2022.View Summary
  12. House, A. M. & Giguère, S. How to Diagnose and Treat Ventricular Tachycardia. American Association of Equine Practitioners (AAEP). 2009.
  13. Hesselkilde, E.Z. et al. Cardiac arrhythmias and electrolyte disturbances in colic horse. Acta Vet Scand. 2014. View Summary
  14. Pratt, S.L. et al. Resolution of sustained ventricular tachycardia in a horse presenting with colic with magnesium sulfate. Equine Vet Educ. 2023.
  15. de Solis, C. N. Ventricular arrhythmias in horses: Diagnosis, prognosis and treatment. The Vet J. 2020. View Summary
  16. Equis – Heart: ventricular fibrillation. Vetlexicon.
  17. Patteson, M. W. & Cripps, P. J. A survey of cardiac auscultatory findings in horses. Equine Vet J. 1993. View Summary
  18. de Mare, L. et al. Standardized exercise tests in horses: current situation and futureperspectives. Vlaams Diergeneeskundig Tijdschrift. 2017.
  19. Reef, V. B. Treatment of atrial fibrillation in horses: new perspectives. J Vet Intern Med. 1995. View Summary
  20. Bentz, G. B. et al. Atrial Fibrillation in Horses: Treatment and Prognosis. Vet Practices Issues. 2002.
  21. Estrada, A. Atrial Fibrillation in Horses. University of Florida Large Animal Hospital.