The Equine Heart

and reduce cardiac output. These are by slowing down the primary pacemaker the sinus node the atrioventricular AV node which lies between the atria and ventricles in the middle of the hearts conduction system or by using both. The result is a wide variation in instantaneous heart rate and rhythm.
What IS Abnormal?
Theres a small group of rhythm disturbances that come from the atria and are clearly a problem. These are thirddegree heart block atrial fibrillation atrial fluttertachycardia and frequent atrial ectopic beats arising from multiple foci with random timing. None of these disturbances will disappear with stimulation and all have a performance impact however its unlikely they would lead to death and with the exception of atrial fibrillation they are not common. In fact some of these may not be noticed unless the horse is examined by a vet or is asked for a maximal performance. Atrial fibrillation is quite common and can be treated in most cases. Third degree heart block is thankfully very uncommon and usually untreatable. Atrial flutter is also very uncommon as are complex atrial ectopic beats but in horses they can often be managed.
Ventricular Rhythm Disturbances
Rhythm disturbances involving the ventricles or lower part of the heart in contrast to those in the atria or upper part always generate anxiety. Because some can indeed be very serious and all are often associated with systemic disease there is cause for concern. It might help if we try to clarify the role and significance of these disturbances by adopting some loose categories. These are 
Benign variations on normal
Problems associated with systemic disease and heart muscle disease and
Issues associated with intense exercise.
Benign Variations
Benign variations come mostly in one shape occasional single ventricular ectopic beats. These are benign especially if they are always the same shape and especially if they are consistently linked to the preceding normal beat. They are probably nothing to worry about especially if they only appear in very limited heart rate ranges. You need an ECG to confirm all of this but with light exercise these extra beats usually disappear or only appear within fixed ranges. Research shows these same single extra beats can occur even at maximum heart rates without having any apparent impact on performance. The issue here is usually abnormal impulse generation.
Rhythms in Systemic and Myocardial Disease
Ventricular rhythm abnormalities that are not so benign consist of problems with impulse generation and impulse conduction
in various combinations and are quite common in severe systemic disturbances. These dont necessarily indicate primary problems with the heart but rather problems with homeostasis the normal balance of body fluids. Severe systemic disturbances do leave the heart very vulnerable however because it is simultaneously working harder when it is itself compromised.