Thursday, June 22, 2017

Running and Your Heart, Part III: Coronary Artery Disease

Coronary arteries, as seen via cardiac catheterization.
Before we get into it, let's just reiterate that this post (or anything else you read on the blog) is NOT to be construed as medical advice.  This is for informational and general-knowledge purposes only.  Furthermore, while I have a pretty good grasp on this stuff, I'm NOT a cardiologist, and anything you might read here is subject to my own interpretations (or mis-interpretations).  As such, this blog should not be taken as a substitute for medical care by a qualified professional.  I'm happy to provide information and try to answer people's questions.  But I AM NOT YOUR DOCTOR.

That being said...this blog is getting awful science-y.

In my continuing effort to either confuse the shit out of you, or freak you out (no, no, JK), I thought we'd delve a bit more into the relationship between distance running and heart health/disease by focusing on the coronary arteries.  Last time I talked about the normal adaptations the heart makes to endurance exercise ("the athlete's heart") and how these adaptations can be both beneficial and, in some cases, harmful.  In that post we quickly glossed over the coronary arteries, but today we're going to examine that aspect of the cardiovascular system in greater detail, because it's extremely important and because a lot of recent research has examined this relationship closely.

Recall from last time that the arteries carry oxygen-rich blood from the heart to the various muscles, tissues, and organs of the body, supplying them with the oxygen necessary to perform their particular functions.  The coronary arteries run directly over the heart muscle itself, bringing oxygen to the heart tissue and allowing the heart muscle to fulfill its ceaseless task of pumping blood throughout the body.  Given the heart's position of primacy in the body, you can see how the entire system relies to large extent on the uninterrupted flow of blood to its muscle.

As discussed previously, the term "heart disease" can encompass a wide variety of problems with the various physiologic systems at play in the heart: anatomic, structural, electrical, etc.  But most commonly, when someone refers to "heart disease," they mean an abnormality within the coronary arteries that compromises the flow of blood and the delivery of oxygen to the heart muscle.  This can take the form of stenosis, a hardening and narrowing of the artery that can disrupt blood flow.  Such narrowing occurs when various junk, usually cholesterol, builds up within the lumen of the artery.  (Picture a pipe or a hose that gets clogged with dirt and how that affects the flow of water through it.)  Over time, these plaques can harden and calcify, causing the artery to narrow and stiffen.  These stiff, narrow arteries thus lose their ability to dilate (expand) in response to an increased demand for oxygen--for example, during exercise.  So when a heart with narrow, inelastic coronary arteries is placed under the stress of exertion, the arteries cannot expand to meet that increased demand, and the heart muscle suffers from a lack of the necessary oxygen, called ischemia.  (This is the most common reason someone would have chest pain with exertion, also termed angina.)  Sometimes, a piece of these plaques can break off and become dislodged from the inner wall of the artery, travel downstream, and get stuck in a narrower part of the artery, causing a near-complete or complete cessation of blood flow to a particular part of the heart.  If prolonged, this can lead to infarction, or death of this part of the heart muscle: what is commonly known as a "heart attack."

So why do people get coronary artery disease?  Well, part of it is genetic; if your parents or siblings have coronary artery disease, you're more likely to suffer from it as well, and obviously you can't do anything about that.  But there are many modifiable risk factors for coronary stenosis, such as high blood pressure, diabetes, and smoking, that you can do something about.  And running helps with these factors: regular aerobic exercisers have lower rates of high blood pressure and diabetes, and are less likely to smoke.  But here's the kicker: despite the fact that distance running unquestionably reduces your risk factors for coronary disease, it may not actually reduce the chances of developing coronary disease.

CT scan reveals calcification of the coronary arteries.
One of the problems with standard screening tests for coronary artery disease--namely, EKGs and stress tests--is that they are not particularly sensitive in detecting underlying coronary disease among fit individuals.  A routine exercise stress test aims to induce strain on the heart by gradually increasing the heart rate via exertion in a laboratory setting; patients are then assessed for symptoms of heart disease, or changes in blood pressure or heart monitor patterns.  For regular endurance exercisers, the limitations of this test are obvious.  If an athlete is increasing their heart rate via exercise on a daily basis without adverse symptoms, why would any abnormalities appear when she does it on a treadmill, in front of a physician?  However, in the past decade advances in technology have made high-resolution CT scanning widely available for the detection of underlying coronary artery disease.  A CT scan is not without downside--it does involve exposure to ionizing radiation, which is carcinogenic in high doses--but this modality can help identify at-risk individuals who might otherwise be missed by more traditional assessments of cardiovascular health.

Applying this test to an athletic population, researchers have discovered some surprising findings.  Despite having a lower incidence of hypertension, diabetes, and obesity, long-term marathon and ultramarathon runners actually have a higher incidence of coronary artery calcification than non-exercisers in the general population.  (Interestingly, runners who regularly train and compete at shorter distances do not demonstrate this finding.)  This paradoxical relationship has been reported as early as 2008, and has been validated several times since (including by yours truly and colleagues earlier this year).

Why does this happen?  We're not really sure, though several theories have been advanced that might account for this process.  One idea is that repeated high-intensity aerobic efforts subject the coronary vessels to more turbulent blood flow, which over time can lead to chronic inflammation and calcification.  Free radical formation, causing chronic oxidative stress, may also play a role. 

So, does this mean we should all stop running ultras?  Not necessarily.  No one has demonstrated as yet that this increase in coronary calcification leads to an increase in clinical signs of heart disease, or to an increase in mortality (we'll address this further in a subsequent post).  There is some thinking that the calcifications commonly seen in long-term distance runners are firmer and more stable than the softer plaque often seen in the general population, and therefore less likely to break apart and cause the downstream problems I talked about earlier.  Also, research demonstrates that long-term training leads to larger coronary arteries, with more ability to dilate (open up) than those in untrained subjects.  This might serve to counteract the narrowing effect of coronary calcification.  (If your hose is getting clogged, make the hose bigger, and water will be able to flow through more easily.)

OK, this isn't the most reassuring post of all time.  But let's sum up with what we actually know:

  • long-term endurance exercise reduces your risk of high blood pressure, diabetes, high cholesterol, and obesity.  Since heart disease is only one of the issues that can arise in people who suffer from these ailments, this fact alone is probably reason enough to keep training.
  • despite this, people who regularly train for and participate in marathons and ultramarathons appear to have a higher rate of coronary calcification than those who don't.  
  • this higher incidence of calcification may or may not be clinically relevant.  But all things being equal, you'd rather it wasn't there.
  • even runners with higher levels of coronary calcifications may not show signs or symptoms of disease, and standard screening tests may not pick up underlying disease in these people.
Therefore, my take-home point is not that we should all freak out and stop running.  But we should realize that we're not immune to coronary artery disease, even though we are invariably "healthier," on average, than non-runners.  For those of us entering our masters running careers, and who have been at this for several years or more, we should be cognizant of this risk.  Talk to your physician about the pluses and minuses of a CT scan of the coronary arteries, particularly if you have a family history of coronary artery disease in a close relative.  And check back next week when I'll tackle the "running versus mortality" question and try to debunk some of the negative press coverage you may have seen recently.