We may feel our pulse start to quicken as we watch the field close in on the homestretch in a race, but that’s nothing compared to the fury of pounding going on inside those horses’ chests as they make their way towards the finish. It’s no surprise that racing places heavy demands on a horse’s heart, yet until now we have known very little about what actually goes on with this most essential muscle during the extreme conditions of live racing. A recent University of Guelph study conducted at Mohawk Racetrack provides an inside look – and with some surprising results.
By Lindsay Day, REMT
It’s a rare phenomenon that has mysteriously punctuated the history of many organized sports. A young, fit athlete collapses during (or just following) physical exertion from sudden cardiac death – an abrupt and fatal loss of heart function that occurs without warning. While heart problems are less common in horses than people, equine athletes too can fall victim to this silent killer, whose underlying causes often remain as puzzling to experts as its incidence.
“The sudden death of a racehorse is a shocking and tragic event,” says Peter Physick-Sheard, an equine cardiologist in the Department of Population Medicine at the University of Guelph. “It doesn’t happen very often, but it has a big impact when it does – especially when it happens on the track.”
Based on current estimates, sudden exercise-related deaths are thought to account for roughly 12 to 14 per cent of fatalities among actively competing horses – and that excludes those that are attributed to fractures, accidents or some other injury. Though small in number, one of the principal concerns regarding such “non-traumatic” deaths is a lack of understanding of the precipitating factors at play – they happen to otherwise healthy horses that typically show no prior signs of illness or disease. While severe heart rhythm disturbances are speculated to be the underlying cause in many of these deaths, scientists have been at a loss to confirm this or to explain just how and why they occur.
In a recent landmark study conducted at Mohawk Racetrack, Physick-Sheard and his colleague Kim McGurrin, together with their research team, recorded the heart rate and rhythm of nearly 300 standardbred horses before, during and after racing at scheduled meets. “The hope is that as we learn more about what’s going on during racing we might gain some insight into the contributing factors that lie behind these sudden deaths,” says Physick-Sheard.
The study, which was published in The Journal of Veterinary Internal Medicine in 2010, represents the first systematic analysis of this type to be done during live competition in any species, and as Physick-Sheard and McGurrin state in their report, “[the] findings were unexpected.”
All systems go
Peak heart rates during racing
Covering a mile on the trot or pace in under two minutes, with sulky and driver in tow, is not an insignificant challenge. Fueling this extreme effort, a horse’s heart reaches a rate of 230-270 beats per minute, which represents about a seven-fold increase over resting values. That translates to roughly four beats per second, with each contraction capable of pumping out a litre of oxygen-rich blood to supply the muscles and vital organs.
“It was pretty spectacular to see just how hard some of these horses were working,” says Physick-Sheard. “While we saw some horses tick on right through the entire race at 230 [beats per minute], that tended to be in races with older horses, and we know from research that mature horses don’t tend to have the same peak heart rate as very young horses.”
On the other end of the spectrum, sustained heart rates as high as 260-270 beats per minute were recorded.
“We even saw the odd horse transiently reach 320 [beats per minute], which is unheard of,” he says. “One thing that was very obvious is that some of these horses were working exceptionally hard. We certainly saw this in the case of some individual horses, but sometimes it was across the entire field. I mean these horses were really hustling – they were really maxing out.”
The bumpy descent
Heart rhythm disturbances during recovery
While heart rate tracks the number of times the heart beats per minute, heart rhythm refers to the regularity of that beat. A heart rhythm disturbance, called an arrhythmia, means there is an irregular heartbeat, and it’s generally not considered a good thing.
While no rhythm disturbances were detected during racing, the study did reveal that they were occurring in a number of horses post race.
“About 18 per cent of the horses in the study showed severe heart rhythm disturbances between the time they finished the race to the time their harness came off,” reports Physick-Sheard. “This was a big surprise for us – we had no idea these sorts of arrhythmias were so common. And when I say these were serious rhythm disturbances, I mean that if you were to see them in a resting horse, or a horse that’s sick, you would be really concerned – you wouldn’t want to stand too close.”
Physick-Sheard and his team were initially concerned enough to alert the trainers of these horses to what they were seeing. But as the arrhythmias produced no observable ill effects and the horses went on to race fine in subsequent weeks, they realized it was unnecessary.
“We’d basically just found something that nobody had found before because nobody had looked,” says Physick-Sheard. “We can’t really say that the rhythm disturbances are abnormal at this point because that’s one of the things we are trying to do with this study – figure out what ‘normal’ is.”
Heart rhythm disturbance and sudden death
Should we be concerned?
The electrical system of the heart is what allows it to beat in an organized and regular way. Heart rhythm disturbances result from malfunctions of this electrical system and, when sustained, can lead to sudden cardiac arrest. (This is different from a heart attack where artery blockages prevent the heart from receiving the oxygen-rich blood that it requires).
While all the cases of heart rhythm disturbance observed during the study spontaneously resolved without apparent consequence, their timing makes them suspicious.
“One of the things about these rhythm disturbances is that they occur at just the time at which sudden deaths tend to take place – not only in horses but in human athletes as well,” says Physick-Sheard. “It’s a smoking gun.
“When a horse dies suddenly, post mortem examination sometimes reveals hemorrhage inside the chest. In some cases there are definite lesions in the pulmonary artery, so that’s certainly part of the picture. But one of the questions we have is whether the primary problem is a rupture in the vessel or whether something else is predisposing the horse to that outcome.
“One possibility is heart rhythm disturbance. If an arrhythmia happens under the wrong circumstances, then the blood pressure could actually rise quite steeply in the pulmonary artery, which could precipitate a tear in a vessel that might already be weakened.”
In other cases, the post mortems in instances of sudden death don’t reveal anything.
“It’s always presumed that the immediate cause of death in these cases is some severe heart rhythm disturbance – and we know that certainly can cause death – but we never know for sure because no one’s looking at an ECG when they happen,” says Physick-Sheard. “Our findings support this conclusion, but they don’t prove it.”
Are genetics to blame?
A number of sudden cardiac deaths in human athletes have been linked to hereditary or congenital cardiac conditions. Under normal circumstances, such issues may be of little concern, if apparent at all. But when provoked by intense exercise and exertion, they may become a serious risk factor for fatal heart arrhythmias, a consideration that has prompted a number of European countries to introduce pre-participation screening programs for young athletes in organized sports. These types of measures don’t catch everyone, but they are believed to help reduce the risk enough to warrant the effort.
“At this point we don’t know if these sorts of hereditary problems exist in horses,” says Physick-Sheard. “But when we took a closer look at the relationship between the arrhythmias we were seeing and who was getting them, we did find that trotters were markedly predisposed, and that does imply a potential genetic component.”
It’s not just that trotters appear more predisposed to heart arrhythmias either, he adds. “As we go back and look at the Ontario Racing Commission death registry data we are finding that trotters are also over-represented among those horses that die suddenly on the track.”
The stress factor
We know that stress can raise our heart rate and it follows that the same thing happens for our horses too. But is it possible that stress might play a role in the likelihood of a horse experiencing rhythm disturbance during recovery post race?
“One of the reasons Kim McGurrin and I did this study in the first place is that we knew we weren’t getting the full story with treadmill studies,” says Physick-Sheard. “What goes on on the treadmill, or even during training, is nothing like what happens on race day – not only in terms of the intensity of the work, but the psychological component as well.”
Though analysis of the study data is still ongoing, some interesting patterns have emerged so far. “One of the things we found is that horses parked at the half-mile post were three times more likely to have a rhythm disturbance after the race. And that was independent of whether they were trotters or pacers.”
A break in stride at the top of the final stretch was also correlated with an increased likelihood of post race arrhythmia in trotters.
“These findings suggest that in addition to the question of physical effort and genetics, there may also be significant psychological factors at play as well,” says Physick-Sheard.
TOO MUCH FOR THE TICKER?
Physick-Sheard and McGurrin’s study findings suggest that while heart rhythm disturbances are quite common in standardbreds, the vast majority of cases resolve on their own. A number of factors appear to contribute to their incidence, and though they may be associated with sudden death, many questions remain to be answered.
“Each finding is only one part of a very complex puzzle,” concedes Physick-Sheard. “Our ongoing analysis of the data will likely yield further insights, but some questions will have to be answered trackside.”
“Standardbreds are spectacular horses,” he says reflecting on the study findings to date. “They’re tough horses and they work very, very hard.”