Arrythmia’s
The heart rate of a horse is usually 25-42 beats per minute and is determined by the electrical conduction running through the heart to provide controlled, rhythmic contraction. The electrical impulse starts at the sinoatrial node in the muscle wall of the right atrium and passes across the atria. The atrio-ventricular (AV) node is then activated. From here the impulses pass through a collection of fibres known as the Bundle of His and then spreads through the ventricles causing contraction. This conduction of electrical impulses is seen using an electrocardiogram (ECG). An arrhythmia is simply an irregular beat, some of these are of little significance where as others can cause a reduction of the cardiac output and hence signs of heart disease.
AV Node Blocks
1st Degree Av Block
This is slowed conduction through the AV node and simply causes a reduced heart rate and is of no clinical significance.
2nd Degree Av Block
This occurs when there is intermittently no conduction through the AV node causing no contraction of the ventricles. As with 1st degree AV block a reduced heart rate is heard and is of no clinical significance.
3rd Degree Av Block
This arrhythmia is caused by no conduction through the AV node whatsoever causing the atria and ventricles to contract completely out of sequence. This arrhythmia is serious and requires prompt treatment which is usually removing the causing factor. The causes are kidney failure, halothane (drug used for general anaesthesia) and inflammation or fibrosis of the node.
Atrial Fibrillation
Atrial fibrillation can either be an incidental finding with no signs of heart disease or can be alongside poor performance and cardiac disease. The arrhythmia is characterized by an irregularly irregular beat, i.e. appears completely random with a series of fast beats followed by no beats at all. The atria contract partially and rapidly and the ventricles contract occasionally and irregularly.
Diagnosis
Although listening to the heart sounds with a stethoscope can be enough to make a diagnosis electrocardiography (ECG) is required to make a definitive diagnosis by identifying the electrical disturbances.
Treatment
Quinidine sulphate can be given orally to convert the arrhythmia to a normal heart beat if the horse is showing poor performance with minimal cardiac disease. If, however, there is more severe cardiac disease this is not appropriate.
If there are no other signs then no treatment is required.