A study published last month in the Journal of the American Heart Association electronic cigarette use concluded that vaping is a cause of heart attacks, raising the risk of having a heart attack about two-fold. One of the co-authors publicized the study on his blog under the title "More evidence that e-cigs cause heart attacks."
This was a cross-sectional study using data from the baseline survey of the Population Assessment of Tobacco and Health (PATH) Study collected in 2013-2014. The investigators examined the relationship between respondents reporting that they ever had a heart attack and that they currently vaped. They found that people who vape were more likely to report ever having had a heart attack in the past.
The problem with drawing a causal conclusion from this cross-sectional study is that there is no way to know which came first: the vaping or the heart attack. As I have previously argued, it is entirely possible that most of the vapers who reported having had a heart attack were people who experienced a heart attack and then began vaping in an effort to stay off of cigarettes. If this is the case, then it is not the vaping that is causing the heart attacks; instead, it is really the heart attacks "causing" the vaping. In epidemiology, we call this "reverse causation," and it is a common limitation of cross-sectional studies, especially when they do not ask about the time course of exposures and outcomes.
Based on what I saw as a likely possibility of reverse causation and on the inability of the study to determine whether the vaping actually preceded the heart attacks, I argued that the investigators went too far in their conclusion and that, in fact, the study does not provide evidence that vaping causes heart attacks.
The Rest of the Story
I was wrong. I have to admit it ...
... I was wrong.
Specifically, I had argued that "there is no way to know which came first: the vaping or the heart attack."
However, Dr. Brad Rodu -- a professor at the University of Louisville -- noticed that in fact, we do know which came first.
Because the PATH survey actually asked respondents not just whether they had ever experienced a heart attack, but when they had the heart attack.
And similarly, the survey asked respondents not just whether they vaped, but when they started vaping.
So Dr. Rodu was able to analyze the same PATH data that the study authors used. Lo and behold, he found that the majority of the 38 vapers who reported having ever had a heart attack experienced that heart attack before starting to vape. In fact, he found that on average, "the heart attacks preceded first e-cigarette use by almost a decade."
There are two major problems here.
The first is that the unsupported, and now debunked, conclusions of the study have influenced many policy makers in their decision to ban the sale of e-cigarettes (while leaving real cigarettes on the shelves, which do cause heart attacks). For example, before voting to ban the sale of e-cigarettes in San Francisco, city council members were told that e-cigarettes are associated with heart attacks.
The second problem is the question of why the study investigators failed to look at whether the vaping preceded the heart attacks, even though they had the information in the survey to make that determination. It would have been easy for them to determine that the majority of the vapers who reported having had a heart attack actually experienced the heart attack before they even started vaping.
The rest of the story is that this new information calls into question not only the conclusions of the research, but also the objectivity of the researchers.
With local, state, and federal policy makers and regulators all considering legislation to regulate vaping, it would be damaging for their decisions to be informed by the now-debunked conclusion that vaping causes heart attacks. The problem is that this conclusion has already been spread widely through the media, and it is very difficult to retract information once it is already out there.