A look at the remarkable equine heart and what can go wrong.
With the high rate of heart disease in humans, most of us are aware of our potential for cardiac troubles. But when it comes to our horses, we tend to be less informed.
Horses don’t have human-style “heart attacks.” Actually, the equine heart is a robust organ that’s not overly prone to problems. But when something does go wrong it tends to be front-page news, as in the 2011 death of 2008 Olympic individual gold-medal-winning Canadian jumper, Hickstead, from an aortic rupture.
In this article, two veterinarians well-versed in cardiac events describe the equine heart and what can go wrong.
The Extraordinarily Variable, Surprisingly Normal Equine Heart
You’ve probably been taught how to monitor your own heart rate, at rest and during exercise: You count the beats over six seconds and multiply by 10 to calculate beats per minute (BPM). Generally speaking, a lower BPM indicates greater cardiac fitness.
Well, that neat formula pretty much goes out the window with horses. For starters, the equine heart has a huge range of “normal.”
“The horse’s heart is extremely well-adapted to its role as an athlete,” says Celia M. Marr, BVMS, MVM, PhD, DEIM, Dipl. ECEIM, MRCVS, European and RCVS specialist in equine internal medicine at Rossdales Equine Hospital and Diagnostic Centre, in Newmarket, England. “At rest, the horse’s heart rate is often as low as 30 (BPM), whereas when it is galloping this can increase to 230. This massive range allows the horse to pump a phenomenal amount of blood—at peak exercise, about 250 liters per minute.”
The heart rate alone is not the only variable. Even the heartbeat’s rhythm can—and, in fact, should—vary substantially in a normal horse, says Peter W. Physick-Sheard, BVSc, Dipl. VetSurg, MSc, FRCVS, equine cardiology researcher and associate professor at the University of Guelph’s Ontario Veterinary College, in Canada.
The heart rate is controlled by the parasympathetic nervous system, which responds to the minutest changes. “For example, if a horse sighs, it will change his heart rate,” he says.
“Variations in heart rate can occur not just minute by minute but also beat by beat,” Physick-Sheard continues. “You might interpret that variation as irregular rhythm, but it rarely is a significant rhythm irregularity.”
What’s more, “That variation from beat to beat is an index of health,” he says. “If it wasn’t varying from beat to beat, that would be a sign of sickness. (The variation) shows you that the heart is responding, as it should, to variations in the animal’s physiologic status. The difference in the horse is that because its resting heart rate is so low, those variations tend to be much more obvious than in a smaller animal with a higher heart rate. It is also obvious because the horse is often quite fit, which can make the heart rate even lower.”
Out of Normal Range: Atrial Fibrillation
With such a wide-ranging definition of normal, how is a veterinarian to know when an arrhythmia is a problem? Physick-Sheard starts by benchmarking the individual horse against known factors that contribute to rhythm variations.
“Is the variation of the type you’d expect in the type of horse you’re examining? Long pauses (between beats) are not unusual in old horses and in very fit horses with a long history of aerobic training,” he says. “In my experience, in the vast majority of cases most variations in rhythm are completely normal. I need to see an extreme degree of variation in an animal that should not have that before I get excited.”
One exception, he says, is the condition known as atrial fibrillation, which causes extremely wide variation in cardiac rhythm.
“Most horses, even if they have a very low heart rate, you can typically tap your foot and expect when the next beat will come,” he says. “Not precisely, but generally there will be a basic resting rhythm. In atrial fibrillation (which veterinarians diagnose using an electrocardiogram [ECG]) there is no rhythm at all.”
“Atrial fibrillation is the most common rhythm irregularity that causes poor performance or exercise intolerance,” says Marr, who is also editor-in-chief of Equine Veterinary Journal. The condition renders the horse’s body incapable of controlling its cardiac output (the amount of blood the heart pumps in a minute), meaning that an affected horse tires easily during intense athletic efforts such as racing or jumping a cross-country course. An estimated 2-3% of horses have atrial fibrillation, and Physick-Sheard says it’s a recurring problem in some animals.
In fact, he says, your horse might have atrial fibrillation, and if all you do is light work you might never know it.
“Horses don’t drop dead from atrial fibrillation, and if you don’t work your horse hard, you may never know he has it unless you listen to the heart,” Physick-Sheard says.
Atrial fibrillation is “one of the most annoying problems in older people,” says Physick-Sheard, and neither the veterinary nor the medical community knows exactly what causes it. “There are genetic factors, tissue-damage factors, heart-rate control factors; even the size of the heart or a high vagal tone (degree of vagus nerve [the longest cranial nerve] activity, which controls various organs in the chest and abdominal cavities, including the heart) predisposes a horse to atrial fibrillation,” he says.
Some horses self-correct the erratic cardiac rhythm, says Marr. Others require intervention, as a human would. Physick-Sheard says 70-75% of affected horses respond to a drug called quinidine sulfate, but its use has drawbacks: “A percentage do not respond or get ill from the treatment, and some have to be treated so vigorously that they become seriously ill from the blood levels of the medication. So it’s not a particularly nice or safe drug.”
Physick-Sheard and colleague Kim McGurrin, DVM, DVSc, Dipl. ACVIM, developed an alternative technique for treating atrial fibrillation that is now a standard procedure at many equine clinics: electrical cardioversion, or shocking the heart back into rhythm. As Marr says, “Electrodes are placed into the heart by threading them down the veins with ultrasound guidance, and then the horse is anesthetized before shocks are given. We use human equipment, but instead of giving the shocks straight on the chest (as doctors do), the horse’s increased size means we have to get the electrodes right into the heart to be effective.”
Audible Problem: Heart Murmurs
The heart is like an engine, with valves that open and close to regulate blood flow. Because of its complex structure, the cardiac blood flow in a normal equine heart has minor disturbances, similar to the little ripples produced when you pull a boat oar through water, Physick-Sheard explains. The human ear cannot detect these normal disturbances with a stethoscope, he says.
Occasionally, however, the blood flow is sufficiently disturbed that it produces an audible sound, he says. This condition is called valvular regurgitation (heart murmur)—a leaking heart valve.
“Usually the valve stops the reverse blood flow,” he says. “If the valve becomes thickened and doesn’;t seal properly, blood leaks when it’s supposed to be closed.” As a result the heart operates less efficiently.
A murmur, like atrial fibrillation, might go undetected in many horses and never interfere with performance. “Mild forms are not necessarily very significant and, in fact, are quite common in racehorses and other sport horses, having very little effect on the horse,” says Marr. “It becomes more important in older horses, which tend to get age-related scarring in their heart valves, which causes the valves to leak. Fortunately in older horses the condition tends to progress very slowly, and although it may affect the horse’s exercise ability, he can often be used for light riding.”
In the typical racehorse’;s short career, Physick-Sheard says, a murmur probably won’t progress enough to become a hindrance. But, “It might be a factor for a retired racehorse in a second career. It could be a concern for an eventing prospect: At time he hits his career peak, at around age 10 or 11, there’s been more time for it to progress.” The less-efficient heart might prevent that horse from reaching the highest levels of his sport, he says.
“There is no cure or treatment for a murmur,” Physick-Sheard continues. “It’s a bad engine and you can’t replace it.”
“The most important thing with valvular regurgitation is to have an echocardiogram, which allows the vet to look inside the heart and determine the extent of disease, which in turn allows advice on appropriate management,” adds Marr.
Scary but Rare: Sudden Cardiac Death and Other Anomalies
In rare instances horses can succumb to what’s known as sudden cardiac death. “This term implies that there has been a fatal cardiac arrhythmia,” says Marr, who adds that the same thing can happen to humans, especially athletes.
Typically, the fatal arrhythmia (in both humans and horses) occurs following peak athletic effort, as the heart rate is beginning to come down, says Physick-Sheard. Complicating the diagnostic process in equids is the fact that many apparently healthy horses show what he calls “a very bizarre variation in heart rhythm: Instead of the rate slowing down smoothly, it comes down in steps—drop suddenly, rise, drop suddenly. It’s like a staircase with sloping steps. The message is that, during cardiac deceleration, it’s quite normal for the heart rate to be irregular as it slows.”
But, he adds, “In that same period when that normal deceleration is occurring, we know of some additional variations that occur in there. We know these may be of clinical significance in horses that drop dead, but we don’t know whether this is a bizarre part of being a horse or whether it represents serious pathology.”
Fortunately, Physick-Sheard says the probability of sudden cardiac death in show and pleasure animals is extremely low. “They are far more likely to be hit by lightning,” he says. “It appears to be a risk factor in animals used for intense exercise, but even there the probability is less than 1% per thousand.”
We mentioned the jumper Hickstead at the beginning of this article. At age 15, the champion Dutch Warmblood stallion had just completed a grand prix course in Verona, Italy, when he collapsed under rider Eric Lamaze and died while still in the ring. Veterinarians determined the cause of death to be an aortic rupture—what Physick-Sheard calls “a recognized problem in older stallions.”
In a vascular rupture, “a major blood vessel (such as the aorta) bursts, causing severe hemorrhage,” Marr explains. “Horses that die because of vascular rupture generally appear to have died for no reason; however, post-mortem exam will typically identify the problem.”
Physick-Sheard says that although older stallions are somewhat more susceptible, “a ruptured aorta can occur in any horse. It can occur with the rupture of the uterine artery in mares after foaling. The horse has a mild species predisposition to degeneration of walls of arteries that can result in spontaneous rupture.” Fortunately, it shouldn’t concern the average horse owner—most horses are at very low risk.
There are a few other extremely rare, oddball sorts of equine cardiac ailments. For one, “Horses can be born with congenital defects, like human babies,” says Marr. “Fortunately this is not common, but if the defect is severe it can be life-threatening.
“There are rare (cardiac) disorders that are linked to lack of selenium, but that is a muscle disease that will be obvious in very young foals,” Marr adds. “There are also rare cases of horses being poisoned with a chemical called monensin, which is used as an antimicrobial in cattle and poultry with no ill effects in those species; but when horse feed is contaminated, monensin damages the heart muscle.” She stresses that veterinarians typically identify both problems easily because they are so severe.
Physick-Sheard points out one more (also thankfully rare) situation with a potential cardiac secondary effect: “Every time a horse gets sick, one of the organs that has to work harder is the heart. If a horse is severely anemic, the heart is taxed. If the horse has a fever or is toxic, the heart is compromised.”
Be Heart Smart
“The heart is an amazing thing,” says Physick-Sheard. Given its complexity, it is remarkable that most equine tickers just keep on ticking and never cause problems, he says.
But inasmuch as cardiac problems can have performance implications, it’s a good idea to have your veterinarian give the heart a listen as part of a horse’s prepurchase exam or semiannual wellness exam. And the more intense your equine athletic goals, the closer the cardiac scrutiny should be.
To keep your horse’s heart as healthy as possible, strive for cardiac fitness, Physick-Sheard advises. “Train in a sensible way—no sudden changes,” he says. “The less fit the horse is, the more important it is to be consistent. Don’t expect more from the horse than the horse can give you.”