Atrial Fibrillation

What is happening in the heart during atrial fibrillation?
In atrial fibrillation, the cardiac impulse becomes confused at the level of the SA node. For reasons that are not entirely clear - again, most likely because of the very large size of the horse's heart, and because of high input from the vagal nerves - 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. The result? The atria never relax or contract properly, and the heart does not get the chance to pump as efficiently as it should.

What causes atrial fibrillation?

• In the majority of horses, there is no discernable underlying cause for the atrial fibrillation. Once again, the horse's large heart and high input from the vagal nerves is to blame - we theorize that these inputs cause the signal from the SA node to become disorganized.
• Some horses do have heart murmurs and underlying valvular disease that precipitate the atrial fibrillation.
• Other factors that may contribute to atrial fibrillation include
  • Viral disease
  • Low potassium levels in the blood - this can occur secondary to using Lasix (a drug used in racehorses that are 'bleeders').
  • Anemia
  • Colic

What effect does atrial fibrillation have on the horse's performance?
Horses have an enormous cardiac reserve - meaning that their hearts are capable of delivering far more blood than they need for mild to moderate work. What does this mean for the horse? It means that horses usually don't show signs of exercise intolerance due to atrial fibrillation until they start to exercise at high levels - for instance, racing, the three-day eventing, or polo.

How do we diagnose atrial fibrillation?
The astute veterinarian can usually be suspicious of atrial fibrillation just on auscultation. The astute veterinarian will notice that the rhythm (unlike, for instance, 2șAV Block) 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. Remember that the 'p' wave reflects electrical activity in the atrium due to the SA node - 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.

My horse doesn't race or play polo - he's just a pleasure horse. Does his atrial fibrillation need to be treated?
In most instances, the answer is 'yes'. Most horses do not have any underlying cardiac pathology that causes a-fib. However, longstanding a-fib can lead to cardiac disease.

Sometimes, the answer is 'no'. The treatment for a-fib (see below) 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.

How do we treat atrial fibrillation?
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 very toxic effects on horses, such as colic, founder, collapse, swelling around the throat, and abnormal heart rhythms (other than a-fib). For these reasons, we advise that horses only be treated at facilities where they can be monitored continually, preferably with a 24-hour ECG monitor.

What is the prognosis for recovery from atrial fibrillation?
The prognosis is excellent (meaning that there is a 95% chance of recovery) if:

• 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% chance of recovery, but a 60% chance that the atrial fibrillation will return) if:

• 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.