Atrial fibrillation is most common in draft horse breeds, as well as warmbloods and other large breeds.
The procedure, called electrical cardioversion, delivers an electrical shock internally to a horse's heart to convert the abnormal heart rhythm back to normal.
Dr Debra Sellon, a WSU professor and board-certified veterinarian in equine internal medicine, says the technique is used for horses with a condition called atrial fibrillation, one of the most common abnormal heart rhythm problems in horses. It is especially common in large breed horses like Thoroughbreds, warmblood horses, and draft breeds.
"It is a situation in which the top two chambers of the horse's heart, the atria, have an abnormal rhythm, but the bottom chambers, the ventricles, are still working like normal," she said. "Our goal is to get the top two chambers to synchronize with the others. To perform an electrical cardioversion, a regular defibrillator and specially-made heart catheters are used. The catheters are placed via the jugular vein. One catheter extends into the right atrium and another passes through the right atrium into the large arteries that exit into the lungs.
At the beginning of the procedure, the catheters are placed while the horse is standing, and then it is anesthetized.
"We take chest x-rays to make sure the catheters are placed correctly," Dr Sellon said. "When everything looks good, we connect the catheter to the defibrillator equipment and deliver a fairly low-energy shock. If that doesn't work, we gradually increase the energy the defibrillator delivers until the heart rhythm converts to normal or we reach a maximum acceptable level. If the conversion attempts fail, we can wait a week and try again."
In all, the treatment generally takes one to two hours. The horse is usually under anesthesia for less than an hour of that time.
The WSU equine internal medicine and cardiology team learned to perform electrical cardioversions from Dr Kim McGurrin of the University of Guelph's Ontario Veterinary College in September. Dr McGurrin developed the procedure at that university, and has successfully treated scores of horses herself. She has traveled to numerous universities and private veterinary practices to train veterinary specialists to perform the procedure.
"Dr McGurrin has not experienced any severe complications in the numerous horses she has treated, and all but two horses had their heart converted back to a normal rhythm," Dr Sellon said. "Sometimes a horse's heart won't convert on the first try, but usually does on the second."
The procedure is a welcome alternative to traditional medical therapy with quinidine salts. For some horses, the medication had severe undesirable side effects and it did not always work. Previous to electrical cardioversions, it was the only treatment option available for horses with atrial fibrillation.
Most horses that undergo an electrical cardioversion retain their normal heart rhythm after the procedure.
"About 15 to 20 percent of horses will revert to an abnormal heart rhythm, but that relapse rate is also the same for horses treated with quinidine," Dr Sellon said.
An additional benefit of electrical cardioversions is that horses recover faster and can return to their normal training schedule faster than those that were treated with quinidine.
Signs of atrial fibrillation in athletic horses emerge quickly after onset. These horses will not be able to run as fast or compete to their full potential. Because of these problems, most horses with atrial fibrillation are examined soon after onset of the problem and a diagnosis is made.
Non-athletic horses may go longer before an owner notices a problem, and often a veterinarian discovers the abnormal rhythm in a routine checkup.
Horses referred to WSU for electrical cardioversion treatment generally stay in the veterinary teaching hospital two days before the procedure. During that time, the horse's heart rate and rhythm is monitored and attending veterinarians obtain blood work and perform a physical and complete cardiac evaluation. After the procedure, the horse remains in the hospital for two days.
Courtesy Washington State University College of Veterinary Medicine