Sunday, March 21, 2010

SCIENCE: Heart function in marathon runners

Published this month in the Journal of Applied Physiology, researchers randomly selected 25 men and women and ran a battery of tests before and after running a marathon. They found that immediately after the race, two markers of heart muscle damage were increased. MRI testing showed that the right side of the heart became enlarged with function that was slightly decreased. Importantly however, the MRI did not show any scar formation with a intravenous dye called gadolinium. Continuous electrocardiogram monitoring did not note any increase in arrhythmias.

So what does this mean for runners?

For starters, we know that the benefits of regular exercise far outstrip any risks associated with running (even including rogue cars and stray dogs). That said, previous studies have noted that there appear to be measurable changes in the heart with marathon running. This study's findings are very similar to those studies in the following ways.

First, increased markers of muscle damage. One of the most sensitive tests of muscle damage doctors use is called the troponin. This is a small molecule that is part of the heart muscle and whenever any heart muscle cells are damaged, a little bit leaks out. Now, when people have heart attacks, troponin goes up, but troponin also can go up if you get in a car accident and hit your chest. The elevation of troponin depends on the context in which it is elevated, and we are not yet sure what it means for martathon runners. The other marker that was elevated, the pro-BNP, is a marker of heart stretching. This is also a very sensitive test and is typically used to diagnose "heart failure". Just like the troponin though, there are many reasons why it would be elevated, and the prognosis depends on the context, which is unclear in marathon runners.

Second, the researchers used electrocardiograms to look for arrhythmias. There are a few good reasons to look for these. Other studies of endurance sports have shown that there may be a small increase in risk of having "sudden cardiac death" from an arrhythmia when participating in endurance sports. These deaths are the type of thing we are trying to avoid by doing pre-sports physicals for children and they are precisely why everyone should see a doctor before starting an endurance training program. Also, people with heart failure and heart attacks (see abnormal markers above) are both at higher risk of arrhythmias. Therefore, if your cardiac markers are up (troponin and pro-BNP), it stands to reason that something might be going on that increases your risk of arrhythmias. Now, I said that your risk is increased, but let's say that the risk for the general public is one in a million, and your risk after running an endurance event is three in a million. While that is technically triple the risk, the absolute increase of risk is trivial. In this case they did not see any arrhythmias, but with only 25 subjects, the likelihood of finding something is very low.

Third, the study found that the right side of the heart becomes acutely (ie: right after the race) enlarged and function decreases slightly. This finding has been described before, but what is really nice about this study is the additional use of gadolinium. This is a drug, given through an IV that is soaked up by heart muscle that is damaged or scarred (ie: after a heart attack). Despite having the abnormal markers I mentioned above, none of the subjects had evidence of damage or scar. This suggests that the markers are elevated through a process that is different from a heart attack.

Again, this is just one small puzzle piece in figuring out what it means to your heart to be a runner and to run marathons. I am intrigued by the study findings, and will still be going out for 4 miles tomorrow morning before work.

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