Page 726 - The Veterinary Care of the Horse

or AV node

• damage to the muscle of the heart wall leading to abnormal impulse conduction and
VetBooks.ir • muscle contraction

electrolyte imbalances e.g. in exhausted endurance horses.

There are a number of recognized arrhythmias in the horse and they are not all of clinical

significance. Indeed, some are considered to be ‘normal’ in the resting horse. ECG traces are
essential for the accurate identification of arrhythmias.

ATRIAL FIBRILLATION

This is a relatively common arrhythmia in the horse. There is no regularity of rhythm at all
and the heartbeat is often described as ‘irregularly irregular’. Instead of contracting in a

synchronized fashion, the cardiac muscle cells of the atria contract in a random and
inefficient manner. Consequently, the amount of blood entering the ventricles and pushed out
by the heart is significantly reduced.

Causes

When a wave of muscle contraction passes through the atria of a normal heart, the muscle

cells do not respond to nerve impulses for a short period afterwards. This is known as the
refractory period. In horses such as Thoroughbreds that have large hearts, some of the cells
may have a shorter refractory period than the others and be ready to contract again before the

previous wave of contraction has died out. If there is a disturbance to the normal conduction
of nerve impulses, the muscle fibres can contract individually, rather than in a co-ordinated

fashion. Any event or underlying heart disease that interferes with the normal electrical
activity of the atria can predispose to atrial fibrillation. It occurs in horses which have
enlarged atria as a result of valvular disease, especially mitral regurgitation. It may be

triggered by extreme physical exertion. The condition is much more common in large horses;
it is not seen in ponies.

Clinical signs

These include the following.

• In a horse that is used for light exercise there may be no signs at all.

• Loss of stamina and performance in horses that work at speed, e.g. racehorses, point-to-
pointers, hunters and endurance horses. When maximally exerted, the horse may
suddenly pull up with breathing difficulties and a staggering gait.