Sudden loss of performance, reduced energy levels and/or a lack of activity can be attributed to many possible causes. Culprits can include a virus, change of routine, ageing, etc. But when faced with these symptoms, how many owners would suspect complications arising from cardiac arrhythmia? The term literally means an irregular heartbeat, and one common cause of this condition is atrial fibrillation. A famous equine sufferer is Parcival, the Dutch dressage superstar who was diagnosed with the condition before the Olympic Games last year.
The horse’s heart
To understand how AF can impact a horse’s performance, it’s important to understand how the horse’s heart works.
The heart is essentially a pump which is divided into two sides which work in parallel. Each side has two chambers; the ventricle, which is the pumping mechanism, and the atrium, which collects blood. The left side of the heart supplies oxygenated blood to the muscles and vital organs while the right side sends blood returning from the body to the lungs where it can be replenished with oxygen and carbon dioxide is disposed of.
The heart’s rate and rhythm are controlled by electrical pulses generated by special cells high in the wall of the right atrium. The pulses travel through the atrial walls; pause in the heart’s dividing wall for half a second before reaching the ventricles. Once in the ventricles, the pulse spreads rapidly causing an instantaneous contraction of the muscular ventricle walls. This action forces blood from the heart into the aorta and pulmonary arteries.
Atrial fibrillation (AF) disrupts the electrical activity of the atria, which throws the coordinated pumping mechanism completely out of sync. The ventricles are then bombarded with electrical gibberish resulting in a random, chaotic rhythm.
The result of all this is a dramatic reduction in the power output from the heart. Many horses continue to function quite adequately under work such that the condition goes unnoticed by their riders. Horses which are in hard work, however, will struggle; affected racehorses fade dramatically towards the end of races for example.
What causes AF?
Heart disease or abnormalities can predispose a horse to suffering AF and very big horses seem to be more prone to developing the condition because of the size of their hearts. Just a single extra beat at the wrong moment could cause a short circuit which could easily lead to AF.
Another possible cause is the loss of potassium in the horse’s body caused by excessive sweating during hot and humid weather. A major leak in one of the heart’s valves could also lead to AF. In this case there would be no treatment as heart failure would almost inevitably result from such a serious condition.
Treatment and prognosis
Sometimes AF simply resolves itself. In cases where the rhythm does not return to normal, drug therapy using quinidine is the preferred option. The drug is administered by stomach tube and works by suppressing the electrical activity in the heart which allows the normal rhythm to return. There may be side effects including colic, depression and diarrhea.
The other alternative, which is not used commonly as only a few centres in Europe have the necessary equipment needed to carry it out, is electrical cardioversion where an electric shock is passed across the heart to restore rhythm. Unlike in humans, it is not possible to carry out this procedure externally. The horse’s heart is obscured by other organs deep within the thorax meaning that the only option is to carry out the procedure internally while the horse is under a general anesthetic.
Special catheters are passed through the jugular vein and thence to the heart where an electric shock is then used to return the heart rhythm to normal. The procedure lasts for around 40 minutes and is 95% successful.
Provided a regular rhythm is restored and there is no underlying heart defect, the horse can return to its previous job. The condition does sometimes recur, often years later, but the majority of horses suffer it once and never again. In some cases it may be recommended that the horse is returned to work in a less demanding role.
Disclaimer: Of Horse! and sponsors do not endorse nor validate the accuracy of a blog post. Each article is the opinion of the blogger.