In humans, second-degree atrioventricular block, or 2AVB, can cause serious concern – the atrioventricular node, a cluster of cells in your heart that helps set the rhythm of beats, does not count time properly and delays electrical signals moving from the upper to the lower chambers of the heart.
Certain kinds of 2AVB can be fatal if left untreated and often necessitate a pacemaker.
Except, of course, if you are an elite athlete. Then this glitch in the heart’s electrical wiring is downright common and most likely no cause for alarm. Scientists are not certain why athletes are more predisposed to atrioventricular node problems or why they seem to take them in stride.
To help answer this question, researchers at the University of Copenhagen looked to natural athletes – horses. “They are by nature athletic animals in contrast to most animals,” says Rikke Buhl, Professor of Veterinary Sciences at the University of Copenhagen.
The results of their study, published in April in the Journal of Veterinary Internal Medicine, help to establish a baseline for the heart health of racehorses and lay the foundation for greater understanding of 2AVB in humans.
Putting a Standardbred trotter through its paces
To learn how exercise affects the equine heart, researchers recruited two groups of horses – Standardbred trotter racehorses that had undergone rigorous training and participated in the racing circuit during the 3 years preceding the study and a control group of untrained horses, which had not been actively trained for at least a year and had not competed in the past 4 years.
Populating the control group was a real challenge, says Sarah Dalgas Nissen, a PhD student at the University of Copenhagen and lead author of the study. “This is a professionel industry, and the horses are not relevant for the sport unless they are intended for racing,” Nissen explains.
Although Thoroughbred horses take the spotlight in the Kentucky Derby in the United States and prominent races in the United Kingdom like the Epsom Derby, Standardbred racehorses are favoured in parts of Europe such as France and Scandinavia. The Thoroughbred horses is designed for explosive speed, but the Standardbred is “more what you’d call a stayer,” Rikke Buhl says, primed for racing longer distances at a trot or pace. They are also more mellow than Thoroughbreds, which can be famously hot-headed.
That gentler temperament was an asset to Sarah Dalgas Nissen and the master students charged with measuring the resting heart rate and arrhythmia burden in the more than 250 horses included in the study. The researchers used electrodes attached to the horses’ chests to monitor the electrical activity of their hearts for 2 hours in the afternoon or evening, when the horses were at rest in their stables.
To get an even closer read on the activity of the atrioventricular node, the researchers inserted a tiny catheter into one of the large veins of the neck in 13 horses. Special pads placed on the horses’ flanks created a magnetic field the scientists could use to steer the catheter and a small electrical sensor into the heart itself. Any movement of the catheter creates a minute distortion in the magnetic fields, which Sarah Dalgas Nissen’s instruments were able to interpret into three-dimensional location data.
“That is to visualise where you are, to guide you to move around,” Sarah Dalgas Nissen says. “You create contact fields when you move your catheter around and you touch the surface, and this magnetic field will know the distance and everything and then it can sort of recreate the map on the screen.”
More common than they imagined
The results of the electrocardiograms and measurements from inside the heart revealed that 2AVB was much more common than the researchers had anticipated, with stark differences between trained and untrained horses.
Trained horses were more than twice as likely to experience 2AVB within the 2 hours they were monitored at rest – but even among untrained horses, the incidence rate was about 36%. When the observation time was extended to 24 hours, 86% of untrained horses and 90% of the trained horses had at least one 2AVB.
“You can have a lot of horses who do not have this arrhythmia at all, and then you can have horses that exhibit it all the time,” Sarah Dalgas Nissen says. “We have not identified what the difference is and why some of the horses seem to need it.”
Nissen explains that scientists are not certain whether 2AVB is a feature or a bug for horses — whether it is an electrical change to the heart that somehow improves performance in a way we cannot yet detect or whether it is a glitch horses have adapted to overcome.
“Now that so many horses actually have it, there must be something – an advantage of some kind,” Sarah Dalgas Nissen hypothesises.
Further study will hopefully elucidate whether 2AVB plays a role in sudden unexplained horse deaths. “Sometimes you have fatal incidents on the racetrack,” Sarah Dalgas Nissen explains. “Right now, we do not really know that much about what is going on and how training affects the heart in horses.”
Slow and steady
The study also confirmed something horse trainers have long suspected. Like humans, horses that train vigorously have lower resting heart rates than horses of the same sex and age that are not as active. The gains were modest, however – trained horses only had two fewer beats per minute than untrained horses. But since the average resting heart rate for horses is already so low, about 35 beats per minute, that still accounts for a 7% decrease.
Rikke Buhl says that the difference between a human athlete and someone with a desk job is much more dramatic than the difference between an untrained horse and a trained horse, so whether these findings translate to humans remains to be seen.
“Even if a horse has never been at a racetrack or trained, it is still out in the field every day voluntarily exercising,” Rikke Buhl says. “This is not like an office person who is really sedentary and just walks from the house to the car to the office and back again. They naturally train, they gallop.”
The study’s findings are “very basic measurements, but if we do not know what is going on and what we can expect from regular training, we will not be able to determine whether something is changing too much or if you have a horse that is at higher risk,” Sarah Dalgas Nissen says, adding that baseline measurements are also vital for comparative studies with humans. Sarah Dalgas Nissen and Rikke Buhl say the results of this study could one day improve heart health screenings for athletes, both equine and human.
