Treating Irregular Heartbeats

Chances are you know someone with a heart murmur--you might even have one yourself. Usually it's discovered at birth, and most often it's nothing to worry about. The heart is fine; it just makes a sound associated with blood flow. The same can be said for horses.

A heart murmur is, essentially, the sound the blood flow makes, and flow murmurs or ejection murmurs are common in horses. "Large volumes of blood moving very quickly, particularly with exercise and excitement, is normal," says O. Lynne Nelson, DVM, associate professor of cardiology at the Veterinary Teaching Hospital at Washington State University.

Heart rhythms--the actual heartbeat rate and regularity--deliver the electricity required for the heart to pump properly. Determining if an irregular heart rhythm stems from a heart muscle problem or primary electrical problem can be difficult, and veterinarians will order an electrocardiogram (ECG) to evaluate the heart rhythm and an echocardiogram to look for evidence of underlying structural heart disease.

Nelson notes that some rhythm curiosities viewed as abnormal in other species are considered normal in horses. "Things like heart blocks, where actual beats are skipped, can be very normal in the horse," she says. "There are occasional premature beats arising from the ventricles or the atrium, which can also be normal."

Michelle Barton, DVM, PhD, Dipl. ACVIM, professor of large animal internal medicine at the University of Georgia's College of Veterinary Medicine, estimates that 30% of horses have nonpathologic arrhythmias, such as atrioventricular (AV) block. "Second degree AV block could literally be the result of the nervous system telling the heart, 'you're standing around at rest, so I'm going to slow your heart rate down even more, and I'm actually going to drop a few beats every so often,' " she describes. When the horse exercises, or if he is frightened, the nervous system commands the heart to increase its rate, and the irregularity disappears.

Atrial Fibrillation

There are, however, more serious cases, one of the most common being atrial fibrillation. It's not clear what causes this phenomenon; sometimes it's the result of a larger problem, other times it develops on its own. In a healthy heart the sinus pacemaker controls all of the cells, instructing them to contract at the same time from the top to the bottom of the heart. With atrial fibrillation, Barton explains, "rebel" groups of cells attempt to act as the pacemaker. "Instead of having the atrium--the top of the heart--squeeze all at the same time, it fibrillates, or shakes. You get all of these little islands of atrial heart cells trying to do their own thing. You don't get a nice, controlled squeezing of the atria."

This results in the pumping of a decreased blood supply to the ventricles, which then distribute blood to the rest of the body. While horses with atrial fibrillation can withstand light exercise, optimal performance is restricted.

"A horse with atrial fibrillation is going to have an irregular heartbeat at rest and during exercise," explains Kim McGurrin, DVM, DVSc, of the University of Guelph's Veterinary Teaching Hospital. "What happens with that horse is that he is not able to respond to exercise normally. He doesn't have normal cardiac output; he doesn't kick out as much blood as a perfectly normal horse would. That horse is going to have a minimum 20% restriction on his cardiac performance."

It's not uncommon for atrial fibrillation to go undetected, but then show up during physical examinations such as prepurchase exams. Owners of pleasure horses, which aren't subjected to rigorous conditioning, often remain unaware of this condition until that exam, when a vet picks up on something unusual and orders an ECG. At the performance level, however, a definite intolerance to exercise is demonstrated long before the vet arrives: the horse just doesn't possess as much get-up-and-go as it should. Racehorses that are running along the winning stretch suddenly pull up; jumpers don't have the necessary spring. In more severe instances the horse could collapse.

In some cases the irregular heartbeat can come and go. "We might do something like a Holter monitor (a portable ECG device), where we put a 24-hour ECG, or an ECG on a horse while we are running it on a treadmill, to determine if this rhythm is coming and going and causing the performance issue," Nelson explains. "Sometimes, after a bit of time, the rhythm will be persistent, and we will see it at rest."

Traditionally veterinarians have treated this condition with the antiarrhythmic drug quinidine. The treatment is quick, usually requiring one to three days--just long enough to ensure that all of the atrial cells are working together. The list of side effects, however, is long: colic, diarrhea, laminitis, and congested nasal passageways that provoke breathing difficulties are all possible under the administration of this medication. "The thing that we worry about the most is that when you are using a drug to change the rhythm of the heartbeat ("converting" it), you can sometimes induce more serious, life- threatening heartbeat rhythms," Barton says. "Unexpected death, although rare, can happen with quinidine."

Determining whether a horse is a good candidate for quinidine treatment, therefore, is extremely important. If the heart is already malfunctioning--for example, a bad valve is causing blood to flow back into the atria--the treatment is less likely to work because of the underlying heart disease. "The best candidate is that young horse that has had a sudden onset, and there is nothing else wrong with his heart," Barton says. "With the middle-aged to older horse that has a murmur and a backflow of blood, and that has other signs of heart disease, the atrial fibrillation is another sign that there is something else wrong with the heart, and usually those are unsafe or less safe to try to convert."

The chance for recurrence is also present, especially in cases where the horse wasn't a prime treatment candidate to begin with. "The ones that are harder to convert to begin with, if you do get them successfully converted, are the ones that are more likely to go back into atrial fibrillation," Barton notes.

New Treatment Option

One recent alternative to quinidine is direct current conversion--a twist on the ventricular defibrillators that one sees the fictitious doctors using on television. Developed at the University of Guelph's Veterinary Teaching Hospital by Peter Physick-Sheard, BVSc, MSc, FRCVS, McGurrin, and Dan Kenney, VMD, Dipl. ACVIM, this technique uses catheter-mounted intracardiac electrodes that deliver an electric jolt to the heart. The electrodes are fed down through the jugular vein and directly into the heart while the horse is under anesthesia. Traditional use of defibrillators isn't a feasible option because the size of a horse's chest would require too much current, burning his skin and muscles in the process.

McGurrin and her team started treating client-owned horses with this technique in 2002. So far, out of the 103 treated, 102 have responded. There have, however, been recurrences. "It does cure it at that point in time; they return to normal, and they can return to function," she says. "However, 15-20% of horses will have a recurrence. It will happen again, and we can't predict to whom it is going to happen, or when."

With this group of horses, the earliest recurrence happened three hours after treatment, although McGurrin underlines that was a rare case. On average, recurrences take place several months later; one horse that was treated in 2002 experienced a recurrence in 2006.

While McGurrin dissuades the administration of quinidine to horses that have had atrial fibrillation for more than six months because the chance for successful conversion is less, she has witnessed success with this new technique. "We treated a horse that was in atrial fibrillation for 7½ years, and we treated him successfully," she relays. Another, she adds, was in atrial fibrillation for three years and responded favorably. "It seems that they will respond to the new treatment more easily."

Equine cardiologists say converted horses are more likely to develop atrial fibrillation again--the longer the duration of the arrhythmia, the greater the chance for recurrence.

At Washington State veterinarians attempt drug treatment initially, and they move onto direct current conversion if the horse doesn't respond to a combination of quinidine and another common drug, digoxin. Medical professionals are unsure as to how long the patient will maintain a regular heartbeat after direct current conversion. "The question is still quite out there as to whether this is good for the long haul," Nelson says. "Do these horses maintain normal rhythm for the long haul, or do they reconvert themselves back into fibrillation? Many people opt for this choice and find that horses are fine for months to years before they convert back into atrial fibrillation, and we will convert them back again if the owner still wants to use them."

Ectopic Beats

Another relatively common category of irregular heartbeats is ectopic atrial or ventricular beats, which involve another group of cells outside of the pacemaker that is attempting to set the beat. This usually means that there is a larger problem, such as poor electrolyte levels in the blood, infection, inflammation (caused by colic or influenza), or scar tissue in the heart.

How life-threatening it is depends on whether the rhythm is originating from the top (atria) or bottom (ventricle) chamber of the heart, how big of an area (it's affecting) and what it's doing to the heart rate, says Barton. "The faster the heart is beating, the more dangerous it is, because there is less blood coming out of the heart," she explains.

Treatment for this category of conditions depends on the cause: if the electrolyte levels are low, sometimes all that's needed is an increase of sodium, potassium, calcium, or magnesium. "The treatment depends on what you think caused it and its effect on the heart rate," Barton says. "If the heart rate is not that increased, and we are pretty confident that it's just one little area that is irritated, that horse may just require stall rest for 30 to 60 days. Sometimes we may put them on anti- inflammatory drugs, like banamine (flunixin meglumine), and maybe a steroid."

If the heart rate is abnormal and clinical signs of reduced blood flow out of the heart are present, then special antiarrhythmic drugs are often needed.

Horse owners who know their horses well will notice signs that their horse is not as perky as normal, and they can begin their investigation by listening to the horse themselves. "Any horse owner can feel their horse's pulse or use a cheap stethoscope and listen and get familiar with what your horse's normal heart rate and rhythms are," Barton says. "If you have a horse that is showing signs of reduced performance, shortness of breath or rapid breathing, or that they take a really long time to recover after exercise, listening to their heart might tip you off."

"Horse owners can get to know their horse's heart rate and know what they feel like--or, using a stethoscope, sound like--at rest, after light work, and after heavy work," Nelson advises. "They can hear how quickly the heart rate comes down after work. Get a good feel of what's normal for your horse, and you are more likely to pick up abnormalities."

And, if you feel that something's not right, seek medical help. "Anytime a horse has something wrong with its heart, you've got to be cautious," McGurrin instructs. "You've got to be aware that the horse has something wrong with it, and be a little bit more careful."

Take-Home Message

Nelson advises horse owners to ask their veterinarians to listen to their animal's heart during routine visits. "It's a good idea to get routine health checks and to ask the veterinarian to listen to the heart rate and rhythm," she says. "Even if they were listened to last year, things can change in a year." These routine exams help the specialists determine how long the problem has been in existence.

"Sometimes long- standing problems are a lot harder to deal with than if it's a recent occurrence of a rhythm disturbance," she adds.

About the Author

Carolyn Heinze

Carolyn Heinze ( is a freelance writer/editor. She currently works from her pied à terre in Paris, France, where she continually dreams of convincing the French Republican Guard to let her have a go-round on one of its magnificent horses. One can dream, can't they?

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