New procedure restores racing performance by shocking horse's heart back into normal rhythm

A THOROUGHBRED is a racing machine, and like a precision Formula One race car, a racehorse sometimes can get out of tune.

The horse's heart--an organ that functions like a car's fuel-injection system, which squirts fuel into the throttle for that high-performance hum and split-second, powerful acceleration--depends on the timing of regular electrical impulses to keep it running smoothly. If those electrical impulses get out of sync, the heart begins to beat irregularly and, as a result, the body is deprived of a constant, smooth flow of blood. In essence, the horse's body develops a "miss" and performance suffers. That miss is called an arrhythmia, or an irregular heartbeat.

Heart muscle is unique in that special conducting pathways direct electrical current through the heart that stimulates a wavelike contraction of muscles that pumps blood through the chambers. Electrical impulses must fire in precise order for the heart to function properly. Any disruption in that circuitry could lead to chaos when the equine heart races up to 240 beats per minute during peak competition.

A racehorse is particularly susceptible to developing arrhythmia because of the size of its heart and how fast it beats during peak performance. Even a minor irregularity in the way the electrical signals conduct through the heart may throw the horse into atrial fibrillation, a type of arrhythmia that disturbs the even flow of blood through the heart but is not life-threatening.

Easy diagnosis

The first sign of atrial arrhythmia is a marked decrease in performance. Typically, a trainer will check the horse for obvious problems that might throw the horse off his game: fever, wind problems, foot ailments, subtle lameness, or heat and swelling in a joint.

Next, the trainer usually will ask a veterinarian to perform a complete physical on the horse. During the routine physical examination and using just a stethoscope, the veterinarian should be able to detect the irregular heartbeat. A closer listen to the sounds made by the heart will enable the veterinarian to identify where the arrhythmia originates in the heart. Absence of the sound associated with the contraction of the atria is another clue. A noninvasive, ultrasound device called an electrocardiograph that renders a tracing of the horse's heart function is used to confirm the diagnosis.

Early intervention is best because the longer the atrial arrhythmia exists, the more difficult it is to reestablish normal heart rhythm.

Just hit reset

Until recently, the only way to restore proper heart rhythm was through the administration of the drug quinidine. Given via nasogastric tube because of its high toxicity, quinidine can produce serious side effects. Its administration is sometimes fatal even in healthy horses, yet some horses might require as many as six doses given at two-hour intervals to achieve the desired effect.

A horse undergoing quinidine treatment must be monitored constantly via electrocardiograph, and special attention is paid to notice any signs of toxicity, such as colic or diarrhea. Moreover, quinidine treatment is unsuccessful in 20% of cases.

Earlier this year, the University of Florida became the first veterinary hospital in the United States to perform an electroshock procedure to reestablish a horse's natural heart rhythm. The novel procedure was developed at the University of Guelph in Ontario by Kim McGurrin, D.V.M., who traveled to Gainesville, Florida, to train the veterinarians there in the proper technique. At Guelph, McGurrin has used electroshock to successfully treat more than 50 horses with atrial fibrillation, 44 of which came to Ontario from the U.S. to undergo the procedure.

Two board-certified veterinary cardiologists who are assistant professors at the University of Florida, Amara Estrada, D.V.M., and Darcy Adin, D.V.M., perform the procedure, called intracardiac electrical conversion. They are assisted by Associate Professor of Equine Medicine Steeve Giguere, D.V.M., Ph.D., and other large-animal internal medicine specialists on staff.

The technique entails threading two catheters specially made for equine use through the horse's jugular vein and into the atrium, the upper right chamber of the heart, and into the pulmonary artery on the left side of the heart. A short electrical shock is delivered between the two poles formed by the electrodes at the ends of the catheters, which stops the heart action for an instant and causes all the cells to begin to work again in synchronicity. Adin said a good analogy is pressing the reset button on an electronic device.

Surgeons use an echocardiograph to guide the proper placement of the catheters in the heart.

"We have one person doing the ultrasound [echocardiograph], and another person passes the catheters," Adin explained. "On the ultrasound, we can see the one catheter go into the pulmonary artery and the other go into the right atrium. Once we're pretty happy with the positioning by ultrasound, the horse will undergo anesthesia at that point and then have chest X-rays taken to confirm placement."

The shock is delivered using a biphasic defibrillator similar to the type used to treat arrhythmias in humans. The procedure takes about two hours to complete. "We start with a very low shock," said Adin, "then we go up incrementally because it's difficult to predict how much a particular horse is going to need."

Causes of arrhythmias

In explaining what causes arrhythmias, one theory holds that depletion of electrolytes, particularly potassium, through dehydration from sweating or excessive use of the diuretic furosemide (Salix) could be a triggering factor in arrhythmias. Potassium plays an important role in the conduction of current through the heart. Although not much is known about the particular action of potassium in the equine heart, in other species the movement of potassium along specific channels from the inside of a heart cell to the outside of that cell determines the duration of the heart muscle contraction.

Imbalances in calcium, magnesium, chloride, and sodium also have been theorized to predispose the horse to arrhythmias. But neither Adin nor Estrada believes this to be true.

"The only thing furosemide would do to the heart rate, if too high a dose were given, is it could dehydrate the animal and cause the heart rate to go up, but it shouldn't be a predisposing factor for atrial fibrillation," Adin said. "We don't know what the underlying cause is," said Estrada. "We see a lot of racehorses that go into atrial fibrillation. It is just a function of them having very large hearts and very elevated vagal tone [a high resting heart rate] and high sympathetic tone [large atrial muscles] that sets the stage for fibrillation to start."

"Horses are really large animals that have large atria even in the absence of cardiac disease," said Adin. "Then they also have high resting vagal tone because of the nature of the species and also because of their athleticism. So because of those two factors, horses can spontaneously go into atrial fibrillation without necessarily having heart disease. They certainly can go into atrial fibrillation if they have heart disease, but most of them don't."

Adin said atrial arrhythmias are more prevalent in racehorses because the atria play such an important role in cardiac output.

"The atrial contraction contributes about 20% to 25% to the amount the heart puts out," Adin said. "That's not important for a horse that is a pasture pet, but a Thoroughbred or Standardbred that is racing needs every little bit of cardiac output they can get. So once they develop atrial fibrillation, it would be fine if they didn't have an athletic demand on them, but as soon as they start racing with that arrhythmia, it reduces their performance dramatically."

Safety of the procedure

"There are certainly dangers in putting catheters into the heart and delivering a shock," said Estrada. "Dr. McGurrin, who developed the technique, has done it in many horses, and I don't think she's ever had a horse have an adverse reaction."

"Anytime we have catheters in the heart, there is a potential for arrhythmias just by the presence of catheters tickling the heart," Adin added. "That doesn't seem to be a major problem. But there also is the potential to cause ventricular fibrillation [which can be fatal], but, fortunately, we would be right in there to defibrillate the heart."

Adin said the use of ultrasound to guide the defibrillation to the proper site enables the cardiologist to avoid areas that may be dangerous to shock. "The procedure is very quick," said Adin, "and it certainly has fewer side effects than quinidine."

There are inherent dangers anytime a horse is placed under general anesthesia. The most common complication is fracture during recovery.

Horses fresh from training and on high levels of nutrition are at greater risk for developing complications when undergoing anesthesia than unconditioned animals, according to a study by R. E. Clutton, B.V.Sc., head of veterinary anesthesia at the University of Edinburgh, Scotland.

Barring complications, horses emerge well from intracardiac electrical conversion.

"Theoretically, a horse should be able to go back into training right away," Estrada said. "It takes only a couple of days for the anesthesia drug to get out of their system."

According to Adin, the cost of intracardiac electrical conversion is comparable to that of quinidine treatment, and it promises to be more effective. Horses whose atrial arrhythmias are not solved by repeated treatments of quinidine have had their normal heart rhythm restored by the new technique.