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. 
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.
Atrial fibrillation is broadly characterized by an irregular heart rhythm. Causes of this condition can include: 
- 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.  This results in reflection of the electrical current back through the atria towards the atrial node, stimulating production of another irregular electrical current. 
Ultimately, this means that one abnormal current from the atrial node can result in production of more irregular currents, and the development of AF.
- 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
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. 
Horses may exhibit symptoms such as: 
- Sudden deceleration during a race or performance activity
- Nosebleeds or coughing up blood
- Difficulty breathing
Symptoms usually resolve quickly after the horse stops exercising, and the arrhythmia usually disappears by 24 to 48 hours after the event. 
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.  Usually there are no structural abnormalities in the heart associated with paroxysmal AF. 
Persistent and Permanent Atrial Fibrillation
Both persistent and permanent AF last longer than 24 to 48 hours after the first arrhythmia event. 
- Persistent AF is treatable and resolves once the horse receives appropriate treatment.  Horses may have multiple episodes of persistent atrial fibrillation, each requiring treatment. 
- Permanent AF continues even with treatment, leading to a chronic arrhythmia that may last the horse’s remaining lifetime. 
These types of AF are often collectively referred to as sustained AF.  Sustained AF usually indicates an underlying functional or structural change in the heart, preventing a return to a normal cardiac rhythm. 
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. 
Horses with sustained AF are usually unable to exert themselves fully, leading to symptoms such as: 
- 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. 
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. 
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. 
- Bloodwork to measure electrolytes
- Ultrasound of the heart to identify structural changes
- Exercising electrocardiogram
After identifying AF, treatment can begin. Treatment involves cardioversion to re-establish a normal cardiac rhythm.
The primary strategies include drug cardioversion and electrocardioversion.
The veterinarian administers quinidine through a nasogastric tube or intravenous injection.  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. 
During the treatment period, the veterinarian will continuously monitor the horse’s cardiac rhythm to identify changes indicating either quinidine toxicosis or resolution of AF. 
The veterinarian will usually continue to administer quinidine until the atrial fibrillation resolves or the horse shows signs of quinidine toxicosis requiring treatment to stop. 
Quinidine toxicosis can be quickly treated by administering intravenous sodium bicarbonate, which binds to quinidine and prevents its effects.  Horses given oral quinidine may also benefit from mineral oil or activated charcoal to prevent further absorption of the drug. 
Until the toxicosis resolves, treatment of AF must be stopped. The veterinarian may try drug cardioversion again at a later date or switch to electrocardioversion as the primary treatment. 
Studies show that horses with mild to moderate heart changes have the highest success rate with quinidine treatment alone. The following factors are associated with a greater than 95% likelihood of successful cardioversion: 
- Resting heart rate below 60 bpm prior to treatment
- No heart murmur or only low-grade heart murmurs present
- Atrial fibrillation episodes lasting less than 4 months prior to treatment
Electrocardioversion involves administering electric shocks to the cardiovascular system to reset the electrical error causing the AF.  Electrocardioversion requires specialized equipment, so this treatment option is generally only offered at specialty referral hospitals. 
To administer electrocardioversion, the veterinarian runs long catheters containing electrodes through large vessels until the electrodes are resting within the pulmonary artery and the right atrium.  Placement of the catheters requires a combination of ultrasound and X-rays to ensure correct positioning. 
Due to the intense discomfort caused by the electric shocks, the veterinarian places the horse under general anesthesia for the procedure.  While electrocardioversion itself is considered safe, the risks associated with this treatment are similar to any other general anesthesia procedure. 
Once electrocardioversion is successful, the horse remains under anesthesia for around 10 minutes to ensure that the arrhythmia does not recur. The horse is then recovered from anesthesia similar to other general anesthetic procedures. 
Horses that have structural changes to their heart or prolonged AF episodes have a poorer chance of successful cardioversion, and typically have a higher recurrence rate. 
Emerging evidence from human studies and experimental animal models suggests that long-term use of antiarrhythmic medications may prevent future recurrence of AF. However, these medications have not been thoroughly studied in horses. 
Medications used for this purpose in humans include: 
Further research is required to determine the efficacy of these medications in horses with AF. 
Congestive Heart Failure
For horses that have AF along with signs of congestive heart failure, treating the arrhythmia itself usually has little benefit, as the damage to the horse’s heart is already established. 
Treatment in these cases focuses on slowing the progression of heart failure through medications such as: 
- Diuretics to reduce overall blood volume
- Digoxin to improve the strength of heart contractions
- ACE inhibitors to lower blood pressure
Overall, horses with AF have an excellent prognosis due to the highly successful treatment options available. 
Horses that may have a poorer prognosis include those with: 
- Structural changes in the heart
- Increased heart rate at rest
- Recurrent episodes of AF
- Prolonged episodes of AF
- A history of unsuccessful quinidine treatment
These horses are more likely to develop permanent AF, and typically require changes to their exercise levels if used for performance.
Horses with the poorest prognosis have signs of congestive heart failure, such as: 
- Exercise intolerance
- Rapid heart rate
- Weight loss
- Swelling of the lower abdomen
- Enlarged jugular veins
- Coughing and difficulty breathing
These horses should be immediately retired from performance and managed to reduce the progression of their heart failure.
Although many horses respond to initial cardioversion treatment, there is a risk of developing recurrent AF episodes. One study examining data from multiple referral centers reported a 40% recurrence rate of AF in horses. 
- Previous unsuccessful treatment attempts
- Presence of a heart murmur
- Changes in heart output
- Changes in atrial function detected on ultrasound of the heart
- High atrial fibrillation rate prior to treatment
Return to Exercise
Most horses that respond to treatment can return to performance within 1 week after successful cardioversion.  Before returning to exercise, horses should have a continuous electrocardiogram for 24 hours and an evaluation of atrial function by ultrasound. 
Some horses may require longer periods of rest, up to 4-6 weeks, particularly if they continue to show functional changes in the atria on echocardiogram.  Horses that had prolonged AF prior to treatment are more likely to require a longer rest period before returning to exercise. 
Studies suggest that a previous episode of AF does not impact future performance, with most horses returning to their previous level of performance successfully.  One study in racing Standardbreds showed no significant performance differences before and after cardioversion. 
However, these horses should be closely monitored for any signs of poor performance or exercise intolerance which may indicate a recurrence of AF.  Horses that require furosemide prior to performance should have oral potassium supplemented to prevent an electrolyte imbalance that can trigger recurrence of AF. 
Permanent Atrial Fibrillation
Evaluation of the horse with an exercising ECG is recommended to ensure rider safety at the desired performance level. 
Ongoing use of a heart rate monitor during exercise is recommended for these horses, so that their exercise level can be adjusted to their cardiac capacity and to monitor for any changes.  These horses should also only be used by an informed adult due to the risk of sudden collapse. 
Horses that have an exercising heart rate of over 220 beats per minute should be retired if cardioversion is unsuccessful. 
- Atrial fibrillation in horses is excessive contraction of the atria, resulting in an arrhythmia
- The most common symptom is poor performance or exercise intolerance in athletic horses
- Treatment typically involves medication or electrocardioversion to restore a normal heart rhythm
- Successfully treated horses can return to their previous level of performance within 1 week of cardioversion
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