by Larry Creswell, M.D.

September has already been a newsy month for triathletes. We’ve had a couple September weekends with big races here in the United States and abroad. What caught my eye, though, was that triathletes were also featured in the medical news from this past week. An interesting study about the structural changes in the heart that accompany triathlon training got some play in the popular press. Several friends here mentioned the study and asked about my take, so I thought I’d share the findings from this study and use the opportunity to talk about the heart’s adaptation, in general, to endurance training.

A New Study
Historically, triathletes have not often been the subject of studies in medical and physiology circles. That’s changing with the increasing popularity of triathlon around the world. This past week, a group of radiologists at the University of Erlangen in Germany reported on a study of 26 elite male triathletes who each had more than six years of high-level training before the study. Each of these athletes underwent cardiac magnetic imaging (MRI) to make measurements of the sizes of each of the heart’s chambers as well as measurements of the thickness of the heart muscle itself. The triathletes were compared to a relatively sedentary group of 27 control subjects (who were “recreationally active three or fewer hours weekly for at least five years”).

The full report will be published in the October issue of the medical journal, Radiology, so I haven’t yet read through the study in its entirety. I do know that the authors wanted to study triathletes in particular because, in their view, triathlon training incorporated both typical endurance training as well as resistance training -- approaches that can produce different adaptations in the heart. I don’t know the authors personally, but I wonder if one or more of them might be triathletes. I’ll have to find out.

The investigators made several observations about the structure of the heart. They found that the triathletes had enlargement of the left atrium (62% larger than the controls), left ventricle (27% larger) and right ventricle (28% larger). In addition, the thickness of the left ventricular wall was also greater in the triathletes (8.2-11.6 mm versus 7.5-10.5 mm). Moreover, the total mass of the heart muscle in the left ventricle and right ventricle were also greater than the controls (by 31% and 30%, respectively). Lastly, the investigators found that the ejection fraction (the fraction of blood ejected from the ventricle each time it fills) was significantly greater in the triathletes.

The other issue that the authors addressed was the issue of atrial arrhythmias -- an abnormal heart rhythm that originates in the upper chamber(s) of the heart. We’ll talk about this topic in more detail in an upcoming article, but it has long been thought that endurance athletes are much more likely than non-athletes to have atrial arrhythmias because of enlargement of the left atrium that occurs as an adaptation to long-term endurance training. Surprisingly, in this study the authors found that the triathletes were no more likely than the non-athletes to have atrial arrhythmias. This is a provocative finding (and, perhaps, good news for us triathletes) that will need to be studied further.

Historical Perspective
The investigators were radiologists who were primarily interested in demonstrating the utility of MRI to make useful measurements of the heart anatomy in an applied setting. For us, the study is interesting because it involved triathletes in particular. But the study might just as well have involved any type of endurance athlete such as runners or cyclists. It turns out that the results are not really surprising. We’ve known for more than 100 years that the heart adapts to endurance training. That observation was first made in 1899 by investigators at the University of Uppsala Sweden and published in a report entitled, “A Study in Sports Medicine.” This was the first reported recognition that athletes (in this case, skiers) developed an enlarged heart in response to exercise. Of course, this issue has been studied extensively since then, in athletes from a variety of sports. Undoubtedly, just because of the sheer numbers of runners, most of these studies have been conducted in runners.

Heart Anatomy and Physiology 101
Here's a quick lesson in heart anatomy and physiology to help you understand the importance of the heart’s adaptation to exercise. The human heart is actually two side-by-side pumps, formed of muscle that contracts with each heartbeat. On the left side, there is a left atrium (a blood reservoir) and a left ventricle (pumping chamber). The left ventricle pumps blood to the body. On the right side, there is a right atrium and a right ventricle. The right ventricle pumps blood to the lungs.

Let’s focus on the left side of the heart, but keep in mind that both sides (pumps) of the heart are working at the same time. With each heartbeat, the left ventricle fills with blood that has collected in the left atrium. The heart contracts and a volume of blood (the stroke volume, or SV) is ejected from the left ventricle into the aorta, where it is carried to the rest of the body.

The cardiac output (CO) is the amount of blood that is pumped by the heart, and is usually expressed in units of liters per minute. The cardiac output is an important concept for athletes because the CO increases dramatically with exercise (in order to supply the increased metabolic demands of the body) and most of the heart’s long-term structural adaptations to exercise are “designed” over time to facilitate raising the CO during periods of exercise. The CO will be equal to the heart rate (HR) times the SV. The increased thickness of athletes’ hearts allows for more vigorous contraction, increasing the SV, and thereby increasing the CO. An increased CO, even at the same HR or SV is possible in athletes because of the increased size of the ventricles.

“Athlete’s Heart”
In the medical community, we use the term “athlete’s heart” to apply to the entire collection of changes -- both structural and electrical -- that occur in the heart because of exercise. I'll share some more about “athlete’s heart” in greater detail in an upcoming article.

Earlier, I covered the structural changes that occur with adaptation to exercise. It’s important to remember that these changes may occur to a greater or lesser degree among various athletes, depending on the type of training they do.

There are also electrical changes that accompany exercise. I'll get into this issue in detail in an upcoming article, but for now, understand that the most common electrical change is a slower resting heart rate.

I suspect that a good many of you have “athlete’s heart.” And that’s a good thing.