We often talk about certain horses “having a big heart,” because they give all they can when asked to run or jump. But a horse’s heart is more than a metaphor.

The heart can’t be discussed in isolation; it is just one part of the cardiovascular system, and it’s not the driver – it’s the slave. Ontario Veterinary College professor emeritus Dr. Peter Physick-Sheard says, “The heart doesn’t control the system; instead, it responds to the change in pressure in the animal’s arteries, which in turn is under the control of tiny valves in the smallest of blood vessels.”

Causes of sudden death

When all the valves open wide to allow blood to flow into the muscles, as will happen when the horse is racing, jumping, or otherwise exercising intensely, the heart has to work hard to maintain the correct blood pressure. Horses don’t have “heart attacks” the way humans do. But they can sometimes die suddenly, especially after strenuous activity such as racing or eventing, and while an aortic aneurysm may often prove to be the culprit, the cause isn’t always known. These sudden deaths are thankfully rare, but usually involve some aspect of the heart or cardiovascular system.

Physick-Sheard has been researching questions like this for most of his career, with a focus on disturbances in the heart rhythms (arrhythmias) of racehorses during the immediate recovery times after races. By putting monitors on racehorses before, during, and after they run, he’s been able to detect arrhythmias in about 16 per cent of the horses in the hours immediately following the race. “Despite these arrhythmias, none of the horses we studied died, and they all went back to continue racing,” he said.

His initial studies were done on Standardbreds, but he has now followed up with a larger study involving racing Thoroughbreds. He expects his data to hold up with horses competing in other intense sports such as eventing and show jumping as well, since sudden unexpected deaths have also been seen during or following those events. He is now analyzing the data from this latest study, and doing additional research addressing other related areas – such as the connection between changes in activity in the autonomic nervous system and the arrhythmias he’s observed.

Tell-tale blood markers

A Ph.D student, Tania Rossi, who is working with Physick-Sheard, is studying markers in the blood that can only come from heart muscles that have been damaged. Tests to detect these markers are used in humans to determine if someone has had a heart attack. Rossi is determining which of the available tests work best in horses, and what might be the normal range in a healthy horse after intense exertion. She will also follow up with horses who have arrhythmia after a race for 48 hours to see if there is any relationship between these heart muscle biomarkers and the disturbed heart rhythms.

Arterial calcification

Another OVC professor, Dr. Luis Arroyo, is studying calcifications in the arteries of equine athletes. He examined 101 racehorses, both Thoroughbreds and Standardbreds, who had died from a variety of causes (such as broken legs, internal bleeding, and sudden death). He found calcium deposits in the major arterial walls in 82% of the horses. The average age of the horses with these deposits was 3.84 years for the Thoroughbreds and 5 years for the Standardbreds, but there was no significant difference between males and females.

“This condition is clearly very prevalent in racehorses,” Arroyo added, “and we see it at a young age. It’s much less common in horses that are not worked hard.”

What do these areas of calcification mean to the horse? Arroyo is conducting additional studies to understand the possible consequences. He explained: “The artery wall has three layers, and the thickest one is made up of smooth muscle cells that contract and relax to help adjust blood flow. The artery is supposed to be elastic. Having these lesions present in the arteries makes them stiffer, and they may predispose the horse to failure of the arterial wall.” Most have been seen in the pulmonary arteries, which take blood to the lungs.

To better understand the possible risks of these calcifications, Arroyo is measuring pulse wave velocity – the rate of speed at which a wave of pressure moves along an artery. This test is often used in humans: the wave will move more slowly along the artery if it is healthy and flexible, but moves quickly through an artery that has become stiff. That fast pulse wave velocity in humans is considered a predictor of risk for heart problems and death. He is also planning to investigate if the calcifications are also linked to damage of the smaller blood vessels.

“We don’t know at this point what is causing this calcification, and what the effects are, but we think the elastic fibres are being damaged by the repeated stress from racing,” Arroyo said. While his research is not complete, he feels the presence of these lesions in the arteries of very young horses may be significant. He commented: “We start racing these horses when they are very young – they are babies, really. It is possible that they are basically not mature enough physically to handle this.”

Physick-Sheard points out that horses are much tougher than humans and their cardiovascular systems are designed to handle significant stress. However, it may be that the demands we are placing on them are putting them at risk. “It’s the horse owner’s responsibility to keep a horse at a consistent level of exercise and not increase the demands suddenly. Any increase in intensity should be done very gradually. If the horse is doing intense work [such as upper-level eventing or show jumping] his heart and cardiovascular system should be checked by a vet annually.”