In humans, dogs, cats and many other smaller mammals, the ECG can tell us not just about the cardiac rhythm, but also about the size of various areas of the heart. The horse's heart, probably because it is so very large, is innervated slightly differently from other animals, receiving the signal through the ventricles all at once, rather than as a gradual spread. The result is that the ECG cannot be used for any measurements other than the cardiac rhythm.
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.
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:
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.