Grueling four-day 'adventure race' becomes lab for testing heart muscle fatigue

Courtesy of Euan Ashley

An entrant pushes on in the “Adrenaline Rush,” a 400-km race that includes kayaking, biking and wilderness hiking.

Cardiologist Euan Ashley wanted to study the hearts of endurance athletes, so he set up a mobile heart lab at the finishing line of the ultra-endurance race "Adrenaline Rush" in the Scottish Highlands and waited for the racers to come in.

At about 2 a.m. on Aug. 1, 2001, the winning team of four athletes collapsed across the finishing line after 90 continuous hours of biking, climbing, swimming, paddling and rope work with virtually no sleep. "We waited in the cold and wet and literally picked them up at the end of the race," said Ashley, who then fed the athletes donuts, tested their hearts and waited for the remaining athletes to come in.

After testing the hearts of about 50 endurance athletes both before and after the 400-km race in Stirling, Scotland, Ashley, MD, PhD, assistant professor of cardiology at the School of Medicine, and his colleagues found that contrary to generally held beliefs, the heart does, in fact, tire with exercise—at least it can under extreme conditions.

"I think it's amazing," said Ashley. "Your heart beats 3 billion times in a lifetime. In the absence of disease, we don't think of an 80- or 90-year-old's heart tiring."

The results of the study, to be published in the Aug.1 issue of the Journal of the American College of Cardiology, not only show that hearts can tire with exercise, but also that one particular gene variant can predict the extent to which the heart will tire.

"It was a bit of a labor of love to find this out," said Ashley, a native of Scotland who recruited scientists from Oxford and Duke, not to mention members of his own family, to help with the study. His wife and his father also waited at the finish line to take blood samples as the racers stumbled in.

"Our place became the hangout," Ashley said. "We had guys sleeping on the floor. We sent out two or three times for more donuts as finishers came in."

Prior to the race, the researchers ran electrocardiograms, conducted ultrasounds and took blood samples of 86 or so race entrants but only about half of the original entrants ever finished the race. Dubbed one of the toughest athletic events in the world, race hazards such as injuries, trench foot and hallucinations due to lack of sleep took their toll.

"A common statistic is one hour of sleep per 24 hours of racing," said Gary Tompsett, an adventure racer from Scotland who helped organize the Adrenaline Rush. "As for the effects on my heart, I would never know. The legs always give up first!"

Still, the scientists determined that the hearts of the athletes who finished the competition pumped 10 percent less blood at the end of the race compared with the amount pumped at the beginning.

Ashley and his colleagues chose to study this group of endurance racers, who hailed from all over the world, because they wanted to test the phenomenon of cardiac fatigue under extreme circumstances. The few earlier studies to look at the phenomenon were conducted on shorter athletic endeavors and found no evidence that the heart muscle actually tires until race lengths stretched to at least 10 hours.

But no researchers had ever tested heart fatigue in a race that stretched over more than four days.

"This is the most extreme event in which cardiac fatigue has been assessed," said Ashley who labeled the race "ultra endurance lunacy in the Highlands of Scotland."

The Adrenaline Rush is an "adventure race," a new style of sport which puts multiple adventure sports—mountain biking, sea kayaking, climbing, hiking and navigation and even horse-riding—back to back into a race format. It's a bit like a four-day, off-road triathlon that uses wilderness as an essential part of its course. Most of the entrants are exceedingly fit with body-fat percentages half that of the normal population. Some are just "a little crazy," Ashley said.

"By the end of the race they are so exhausted that one or two get admitted to hospital," Ashley said. The BBC ran a documentary about this particular event and about Ashley's experiments.

The athletes' hearts that showed signs of cardiac fatigue did return to normal fairly quickly after the race and no permanent damage was done, Ashley said.

Surprisingly, the study also found that the hearts of athletes with a gene that has been dubbed the "fitness gene" actually tired more than those without it, which led to multiple speculations as to why this was so. Scientists found that the hearts of the athletes with the fitness gene pumped 13 percent less blood by the end of the race compared with 8 percent less in those without the gene. "Perhaps the athletes with the fitness gene simply push themselves harder and tire their hearts out more," Ashley said.

In addition to his research and other clinical responsibilities, Ashley runs the new hypertrophic cardiomyopathy clinic at Stanford. Hypertrophic cardiomyopathy is the leading cause of sudden death in athletes.