Nov 24 2011

Snow Shoveling and Heart Attacks

People notice patterns. Our brains evolved to be really good at pattern recognition. This strength can become a weakness, however, when we notice patterns that are not there, that are illusions or statistical flukes with no basis in reality. The answer, however, is not to ignore apparent patterns we think we see, but rather to treat them properly – as hypotheses rather than conclusions.

That simple rule can wipe away a lot of cognitive mischief. Anecdotes, testimonies, and our everyday experience can be valuable, as long as we treat that information as a way of generating hypothesis that may or may not be true. We then need to confirm the hypotheses with some sort of systematic and controlled way of looking at data (you might call that science).

For example, even as a medical student in my earliest days in a clinical setting, I noticed that many people who come into the ER or got admitted to the hospital with a heart attack were engaging is some strenuous activity at the time. Further, this activity was not something they do on a regular basis, but rather something they do only occasionally. In the summer this activity was often gardening or yard work. In the winter, this activity was often shoveling snow. I soon found out that this observation is common, in fact it is a pattern that is taken for granted  – yard work and snow shoveling provoke heart attacks.

The association between snow shoveling and heart attacks has become a bit of common medical wisdom, quickly learned by anyone in the profession. But then again, so is the observation that emergency rooms are busier with more bizarre cases during a full moon. Both beliefs are based on anecdotal observation. The former observation is highly plausible and fits well with out models of cardiac disease. The latter is highly implausible without any clear mechanism. But that’s not enough – such plausibility arguments are useful for setting the threshold for how much evidence would be convincing, but we still need to objectively look at some evidence to be confident in our conclusions that one or the other pattern is real.

There have been over 100 studies looking at the alleged lunar effect, trying to correlate lunar cycles with a number of phenomena, like crime, births, and ER visits. These studies have failed to find any correlation. At this point we can comfortably conclude that the lunar effect is an illusion of confirmation bias – one of those fake patterns humans are good at detecting, but without any basis in external reality.

What about the alleged correlation between snow shoveling and heart attacks? Dr. Adrian Baranchuk noticed that this correlation has never been documented in the literature. It is another one of those things that everyone knows but is not based upon any solid evidence. So he and his colleagues did a basic study looking at 500 consecutive patients with an acute coronary syndrome (ACS). They found:

Our study population included 500 patients with ACS, mean age of 65.7 ± 13.4 years (range 31-94) and 66.7% of the events occurred in males. A total of 35 (7%) events were documented to have occurred following snow-shoveling. Between patients with snow-shoveling-related and non-related events there were no significant differences in the prevalence of diabetes, hypertension, hypercholesterolemia or sleep apnea. Logistic regression did not show any significant group differences in age and known coronary artery disease; however, those suffering a snow-shoveling-related event were 3.6 times more likely to have a family history of premature cardiovascular disease (p = 0.001) and were 4.8 times more likely to be male (p = 0.01).

That 7% of patients coming in with ACS had their symptoms while shoveling snow is probably meaningful. It’s a pretty strong correlation which supports the hypothesis that snow shoveling can trigger a heart attack. It’s not definitive, however, because there is no comparison group. For example, it would have been interested to query 500 patients who came into the ER with non cardiac symptoms and find out what percentage of them were shoveling snow. If there’s snow on the ground a lot of people might be shoveling snow.

That a causal connection exists between snow shoveling and ACS is supported by the fact that those having a snow-shoveling related event were more likely to have a family history of premature cardiovascular disease. It was also associated with being male, but it seems likely this is simply due to the fact that men are more likely to be shoveling snow than women.

What this single study tells us is that the anecdotal observation that snow shoveling is associated with heart attacks probably has some basis in reality. But further study is needed to confirm the association and to see what it really means – how much risk does snow shoveling really present, and in which populations? For now, however, it seems prudent that if you are an older male with a family history of premature heart disease, you may want to hire someone to shovel your snow for you, whether or not there’s a full moon.



Happy Thanksgiving to all my American readers!

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