Is it possible to do too much cardiovascular exercise?
For those of you who don't know, or may be new to my blog, I had a cardiac ablation done earlier this year (January 2014) to correct a heart arrhythmia that has been present for the past few years. This arrhythmia started to show itself very infrequently at the start, and then began to increase in frequency, symptoms-wise, over a few years.
Considering the symptoms began to arise around the same time I was becoming more and more of a fan of endurance events, I couldn't help by wonder if it was something I have done to cause this. To give you a brief history of events that I have trained and competed in over the past couple of years, along with the presence and increase in frequency of symptoms of my "atrial flutter." Now although I exercise regularly and enjoy physical fitness, I am only including endurance events that I have completed that have lasted longer than 60 minutes in duration:
- Toronto Waterfront Half Marathon (September 2010)
- Toronto Waterfront Half Marathon (October 2011)
- Tough Mudder (August 2012)
- World's Toughest Mudder (November 2012)
- Tough Mudder (May 2013)
I then came across an review article over at NCBI that raises some interesting questions:
A routine of regular exercise is highly effective for prevention and treatment of many common chronic diseases and improves cardiovascular (CV) health and longevity. However, long-term excessive endurance exercise may induce pathologic structural remodeling of the heart and large arteries. Emerging data suggest that chronic training for and competing in extreme endurance events such as marathons, ultramarathons, ironman distance triathlons, and very long distance bicycle races, can cause transient acute volume overload of the atria and right ventricle, with transient reductions in right ventricular ejection fraction and elevations of cardiac biomarkers, all of which return to normal within 1 week. Over months to years of repetitive injury, this process, in some individuals, may lead to patchy myocardial fibrosis, particularly in the atria, interventricular septum, and right ventricle, creating a substrate for atrial and ventricular arrhythmias. Additionally, long-term excessive sustained exercise may be associated with coronary artery calcification, diastolic dysfunction, and large-artery wall stiffening. However, this concept is still hypothetical and there is some inconsistency in the reported findings. Furthermore, lifelong vigorous exercisers generally have low mortality rates and excellent functional capacity. Notwithstanding, the hypothesis that long-term excessive endurance exercise may induce adverse CV remodeling warrants further investigation to identify at-risk individuals and formulate physical fitness regimens for conferring optimal CV health and longevity.
First and foremost, I would like to state that I completely understand that correlation does not imply causality. I have absolutely no concrete evidence proving that strenuous bouts of long-term endurance exercise caused me to have arrhythmia symptoms to the point of needing a cardiac ablation to correct the problem. However, my symptoms first began to appear in the fall of 2010 and slowly but steadily increased in frequency over a couple years following that.
Over the course of 3 years, I spoke with not only my family doctor, but a several other cardiac specialists, technicians, health care workers, etc. and none of them questioned my lifestyle or choice in fitness activities. This is certainly not the point of this blog - to point fingers at those who didn't warn me about this sooner. Instead, the point of this blog is to discuss this topic as openly as possible. Nobody I spoke with told me that I shouldn't exercise, in fact my heart and overall health was heralded on a number of occasions. (One thing that I was told I should probably stay away from is voluntary electric shocks, however.)
I have been an advocate for physical fitness for a long time, and will continue to run this website and blog as long as I can to help promote healthy lifestyles. However, the discussion about what is too much should be taken into account.
Running distances of about 1 to 20 miles per week, speeds of 6 to 7 miles per hour, and frequencies of 2 to 5 days per week were associated with lower all-cause mortality, whereas higher mileage, faster paces, and more frequent runs were not associated with better survival.7 A randomized crossover trial assigned 60 male patients with CHD to ET sessions of either 30 or 60 minutes. The 30-minute exercise sessions produced less oxidant stress and improved arterial elasticity, whereas 60-minute sessions worsened oxidant stress and increased vascular stiffness as measured by pulse wave velocity, mainly in older patients.8
The review discusses the fact that the per-participant risk of sudden cardiac death has remained relatively unchanged over the past 35 years (approximately 1 in 100,000). What has changed, however, is the incidence rate of SCD due to the rising popularity in events such as marathons, triathlons, Ironmans, etc. It should be noted that rates of death due to SCD seem to be mainly caused by previously un-diagnosed heart conditions.
The causes of SCD during or after extreme exertion in individuals younger than 30 years most commonly include genetic causes such as hypertrophic cardiomyopathy, anomalous coronary arteries, dilated cardiomyopathy, and congenital long QT syndrome. In athletes older than 30 years, CHD and acute myocardial infarction16 and ischemia are the predominant causes of exercise-related SCD.17-23
One could argue that there is no point worrying about dropping dead whilst doing an event such as a marathon or triathlon. I completely agree. The incidence rates of such a thing are quite low. It's like the common adage that "you could be hit by a bus and die tomorrow" kind of thing. Sure, you could be afraid of training for and competing in an event like a marathon, but what good does living in fear do?
Again, I would like to stress the fact that correlation does not imply causality, and there is no way to prove that my atrial fibrillation was caused by extensive aerobic endurance training, but there is a pretty strong correlation with me personally, when I first started to partake in long endurance training sessions and when my symptoms first started appearing. Not only that, but as I continued to train for other events, my symptoms began increasing in frequency.
In some individuals, long-term excessive endurance ET may cause adverse structural and electrical cardiac remodeling, including fibrosis and stiffening of the atria, RV, and large arteries. This theoretically might provide a substrate for atrial and ventricular arrhythmias and increase CV risk. Further investigation is warranted to identify the exercise threshold for potential toxicity, screening for at-risk individuals, and ideal ET regimens for optimizing CV health. For now, on the basis of animal and human data, CV benefits of vigorous aerobic ET appear to accrue in a dose-dependent fashion up to about 1 hour daily, beyond which further exertion produces diminishing returns and may even cause adverse CV effects in some individuals.
That's just it, "in some individuals." What happens in one individual may not necessarily ever happen to someone else. We know that some people have genetic variants that pre-dispose them to sudden cardiac death, and this is only exacerbated by doing an event like a marathon. Some individuals seem to accrue biological changes to their hearts based on too much, long-term excessive cardiac exercise. Will any of that happen to you? Who knows. The point of this blog was not to scare you or make you think marathons are bad.
The point of this blog was to share my story and what I believe may have happened to me. I enjoy physical activity. I love being active, resistance training, going for walks, playing with my kids, running, and generally just feeling great from using my body. I am more aware of the fact that long-term aerobic exercise is probably not for me anymore. I still go for runs, but nothing more than about 30 minutes at a time.
Completing an endurance event is exhilarating. I get the fact that it is a life goal for a lot of people. Many have "run a marathon" on their "bucket list." I think that is an amazing goal and accomplishment to strive for. However, just be aware of any potential risks you may be putting your body through, and by all means, talk to you doctor about any weird or unusual changes or feelings you may witness. After all, its always better to err on the side of caution.