English
Atrial Fibrillation in horses is a cardiac rhythm disorder, characterized by rapid and irregular beating of the atria, the upper chambers of the heart.
This condition can affect horses of any breed, age, or discipline, but it is more commonly seen in performance horses and breeds including Standardbreds, draft horses, and Warmbloods.
Signs of atrial fibrillation in athletic horses include poor performance, exercise intolerance and difficulty breathing.
Treatment involves medication to re-establishing a normal cardiac rhythm in affected horses. Horses with atrial fibrillation generally have an excellent prognosis and can usually return to work within one week of treatment.
Atrial Fibrillation in Horses
Atrial fibrillation (AF) refers to excessive contraction of the atria, the components of the horse’s heart that collect blood from the lungs and body for further distribution.
AF is one of the most important heart arrhythmias (irregular heartbeat) causing poor performance in athletic horses. [1]
Normal Cardiac Function
The atria are large chambers within the heart that collect blood returning from the lungs or body. These structures contain an electrical node that produces an electric current.
The atrial node is under the control of the autonomic nervous system, the component of the nervous system that regulates involuntary tasks such as breathing and digestion.
Under normal conditions, the autonomic nervous system stimulates the atrial node to produce electrical currents at a regular interval. The electrical current from the atrial node spreads through the atrial heart muscle and reaches another electrical node (atrioventricular node) at the base of the ventricles, the heart chambers that pump blood.
This electrical signal triggers the ventricles to contract, producing a complete heartbeat. Atrial fibrillation develops when there is a disruption of the atrial nodes, causing them to produce more electrical currents than normal.
Causes
Atrial fibrillation is broadly characterized by an irregular heart rhythm. Causes of this condition can include: [2]
- Electrolyte imbalances
- Structural changes to the heart
- Certain medications
- Imbalance of the autonomic nervous system in response to exercise or anesthesia
When the abnormal electrical current reaches the atrioventricular node, it often cannot respond appropriately as it is still recovering from the previous heartbeat. [2] This results in reflection of the electrical current back through the atria towards the atrial node, stimulating production of another irregular electrical current. [2]
Ultimately, this means that one abnormal current from the atrial node can result in production of more irregular currents, and the development of AF.
Risk Factors
There are several risk factors for developing AF in horses: [1], [3]
- High performance sports such as racing, polo, or eventing
- Breeds such as Standardbreds, draft horses, and Warmbloods
- The presence of heart murmurs, particularly mitral valve regurgitation
There is some evidence to suggest that AF may be hereditary in Standardbreds. [4] Similar studies in Thoroughbreds show low heritability in this breed. [5]
Medications such as furosemide (Lasix), supplements containing sodium bicarbonate, and thyroid hormones are also known risk factors for AF. [6]
Types of Atrial Fibrillation
Atrial fibrillation in horses can be classified into different types based on the duration and persistence of the condition.
Paroxysmal Atrial Fibrillation
Paroxysmal refers to a sudden bout of a disease, that usually resolves quickly. In horses, paroxysmal AF usually causes a single episode of poor performance. [3]
Horses may exhibit symptoms such as: [7]
- Sudden deceleration during a race or performance activity
- Vocalization
- Nosebleeds or coughing up blood
- Difficulty breathing
- Weakness
Symptoms usually resolve quickly after the horse stops exercising, and the arrhythmia usually disappears by 24 to 48 hours after the event. [3]
The cause of paroxysmal AF is usually unknown but may be related to potassium depletion in horses treated with diuretics prior to a race or performance event. [3] Usually there are no structural abnormalities in the heart associated with paroxysmal AF. [3]
Persistent and Permanent Atrial Fibrillation
Both persistent and permanent AF last longer than 24 to 48 hours after the first arrhythmia event. [3]
- Persistent AF is treatable and resolves once the horse receives appropriate treatment. [3] Horses may have multiple episodes of persistent atrial fibrillation, each requiring treatment. [3]
- Permanent AF continues even with treatment, leading to a chronic arrhythmia that may last the horse’s remaining lifetime. [3]
These types of AF are often collectively referred to as sustained AF. [3] Sustained AF usually indicates an underlying functional or structural change in the heart, preventing a return to a normal cardiac rhythm. [3]
Sustained AF is also self-perpetuating, as the arrhythmia itself can cause electrical, structural, and functional changes in the heart that further contribute to the arrhythmia. [3]
Horses with sustained AF are usually unable to exert themselves fully, leading to symptoms such as: [8]
- Poor performance
- Exercise intolerance
- Rapid breathing or difficulty breathing
Some horses performing at low levels may show no symptoms, as they are not required to perform high-intensity exercise. [7]
Diagnosis
An initial diagnosis of AF usually occurs during physical examination of the horse. The veterinarian will hear an irregular heartbeat when listening to the heart with a stethoscope, suggesting the presence of AF. [3]
Further diagnostic work-up involves placing an electrocardiogram machine on the horse to measure the heart’s electrical activity. Based on this test, the veterinarian can confirm the presence of AF. [3]
Additional diagnostics aim to identify possible causes or the severity of heart damage. These diagnostics can include: [3][6]
- Bloodwork to measure electrolytes
- Ultrasound of the heart to identify structural changes
- Exercising electrocardiogram
Treatment
After identifying AF, treatment can begin. Treatment involves cardioversion to re-establish a normal cardiac rhythm.
The primary strategies include drug cardioversion and electrocardioversion.
Drug Cardioversion
The main medication for drug cardioversion is quinidine sulfate. Other drugs such as flecainide or amiodarone may also be part of a treatment protocol. [3][9]
The veterinarian administers quinidine through a nasogastric tube or intravenous injection. [3] There are several dosing rates available for quinidine, however most veterinarians administer the drug every 2 hours for 2-4 doses, followed by every 6 hours until the horse’s arrhythmia resolves.