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Cardiac Arrhythmias in the Horse

By: Dr. Melissa Mazan

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Many horses, especially fit, athletic horses, have a normal arrhythmia called second degree atrioventricular block. This is characterized by a regularly irregular rhythm. That is, even though there is a break in the normal rhythm, it is very predictable. If you listen, you will hear 'lub dub, lub dub, pause, lub dub'. If you were to tap your foot to the rhythm, you would find that the pause is exactly the length of a normal beat, and the next beat would come in exactly on time.

If you were to look at an ECG of a horse with second degree AV Block, you would see normal 'pqrst' complexes until the time when a pause occurred. At that time, all you would see is a 'p' wave. Then, the normal 'pqrst' would return.

What is happening is that the normal innervation to the SA node, which comes from the vagus nerve, has such a strong input in the horse, that it suppresses some of the impulses that are trying to reach the AV node. So, we see a 'p' wave, but no 'qrst'. Because there is no real abnormality in the heart, the next impulses go through just fine.

In the normal horse, a little bit of excitement or exercise should make the second degree AV block disappear. The exercise or excitement makes the horse's adrenaline levels increase and the vagal influence go down – thus the arrhythmia goes away. This is a benign arrhythmia. An ECG is not usually recommended in these cases.

Occasionally, a horse will have second degree atrioventricular block that doesn't disappear with exercise. This usually indicates that the arrhythmia in this case is not benign, and an ECG and perhaps an echocardiogram is warranted.

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