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Cardiac Arrhythmias in the Horse

By: Dr. Melissa Mazan

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Atrial fibrillation is marked by rapid randomized contractions of the atrial muscles, causing an irregular, often rapid, ventricular rate. The cardiac impulse becomes confused at the level of the SA node. For reasons that are not entirely clear, the signal from the SA node does not travel smoothly through the atria to the AV node and then to the ventricles. Rather, multiple signals come from various areas of the atria that begin to act like many small SA nodes. Only a few of these signals manage to get to the AV node and then to the ventricles. As a result, the atria never relax or contract properly, and the heart does not get the chance to pump as efficiently as it should.


In the majority of horses, there is no discernable underlying cause for the atrial fibrillation, but the horse's large heart and high input from the vagal nerves are factors. Some studies have shown that horses with atrial fibrillation have large atria and stretching injuries (scarring) or inflammation in those atria. These problems likely promote the conduction disturbance of atrial fibrillation, whereby the electrical pathway encircles the atria, rather than exiting the atria as normal, to the ventricles.

Some horses do have heart murmurs and underlying valvular disease that precipitate the atrial fibrillation. Other factors that may contribute to atrial fibrillation include:

  • Low potassium levels in the blood that can occur secondary to using the diuretic Lasix
  • Viral disease
  • Anemia
  • Colic

    Effect on Performance

    Horses have an enormous cardiac reserve. This means that their hearts are capable of delivering far more blood than needed for mild to moderate work. Consequently, horses usually don't show signs of exercise intolerance due to atrial fibrillation until they start to exercise at high levels, as in racing, the three-day eventing or polo.


    The astute veterinarian can usually be suspicious of atrial fibrillation just by listening to the heart (auscultation) and will notice that the rhythm is irregularly irregular. Veterinarians often refer to it as sounding like sneakers in a dryer or bongo drums.

    The definitive diagnosis is made with an electrocardiogram (ECG). Remember that the p wave reflects electrical activity in the atrium due to the SA node, and if the SA node isn't firing properly, then there won't be a recognizable p wave. Instead, there are multiple fibrillation or f waves – perhaps 20 or 30 of them where there ought to be only one 'p' wave.


    Most horses do not have any underlying cardiac pathology that causes atrial fibrillation. However, long-standing atrial fibrillation can lead to cardiac disease.

    The treatment for atrial fibrillation is potentially dangerous. So, if your horse is debilitated or is at high risk for toxicity for other reasons, you may choose not to treat. If your horse doesn't need to do high-intensity work, you may never notice that he has a problem.

    The treatment of choice is a drug called quinidine. Quinidine has to be given with a nasogastric tube every 2 hours throughout the treatment period. This drug can have toxic effects on horses, such as colic, founder, collapse, swelling around the throat, and abnormal heart rhythms, other than atrial fibrillation. For these reasons, horses should only be treated at facilities where they can be monitored continually, preferably with a 24-hour ECG monitor.


    The prognosis is excellent, meaning that there is a 95 percent chance of recovery, if the following conditions are in effect:

  • The resting heart rate is less than 60 beats per minute
  • The atrial fibrillation has been present for less than 4 months
  • There is no associated heart murmur, or the murmur is of low intensity (grade 2 out of a possible 6, or less)

    The prognosis is guarded to good (meaning that there is an 80 percent chance of recovery, but a 60 percent chance that the atrial fibrillation will return) if these conditions are present:

  • The heart rate is greater than 60 beats per minute
  • The atrial fibrillation has been present for more than 4 months
  • There is an associated heart murmur that is grade 3 out of 6 or more

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