Source: Dixit S, Pletcher MJ, Vittinghoff E, et al. Secondhand smoke and atrial fibrillation: Data from the Health eHeart Study [published online ahead of print August 25, 2015]. HeartRhythm. doi:10.1016/j.hrthm.2015.08.004. See AAP Grand Rounds commentary by Dr. Richard Wardrop III (subscription required).
PICO Question: Are adults with atrial fibrillation more likely to have been exposed to secondhand smoke during gestational development and childhood than adults without atrial fibrillation.
Question type: Harm/Causation
Study design: Cross-sectional survey
This is an intriguing study of almost 5000 adults who completed an online health survey. The investigators found that in utero exposure to parental smoking or residing with a smoker during childhood were associated with a mild increase in risk for atrial fibrillation in adulthood. Other studies have suggested explanations (i.e. biologic plausibility) for an association of second-hand smoke (SHS) with AF, such as induction of inflammation, interference with atrial structure remodeling, or effects on the autonomic nervous system. I was fascinated that such exposure might extend so far back in life, for a disease that develops much later in life.
Whenever I see a survey study, my antennae always perk up looking for sources of bias, which are inevitable when we ask our subjects to self-report any behavior, much less behaviors that occurred decades ago and even prior to their own birth! I was pleased to see that the authors provided online access to the survey tool itself. I had trouble right from the start: the first question was "When your mother was pregnant with you, did either of your parents smoke?" The answer choices were Mother, Father, Both, Neither, or Don't Know. I don't know if my mother smoked during her pregnancy with me; I don't think she was ever a heavy smoker, but I do recall seeing her with an occasional cigarette when I was young. I know that my father was a heavy smoker up until about age 50, so I certainly was exposed to SHS as a child. I guess I would have checked the Father box, but I can see some in my same situation might choose Don't Know. (Note, so far I haven't developed AF.)
The authors discussed 6 primary study limitations: 1) the cross-sectional study design (making a case for causality more difficult); 2) potential confounders not identified, particularly obesity in this case; 3) the nature of self-reporting, as above; 4) this was a yes/no exposure history, with no attempt to quantify amount of SHS exposure (which would have been incredibly difficult); 5) recall bias (maybe I'd prefer not to report my parents' smoking, at some subconscious level); and 6) lack of racial and socioeconomic diversity in their sample (making generalizations to other populations tenuous). I think they did a good job discussing how these limitations might have changed (or not changed) their findings.
They left out another major limitation of their study, however. Because they used an internet-based survey, and they required subjects to have a working email address, they excluded individuals with less electronic literacy. Since AF is a disease in later years of life, and use of digital tools is somewhat less common in older populations, this might also induce bias into the data collection.
Still, the report has interesting findings, and certainly we should encourage our smoking parents and family members to give up smoking, for their own health as well as their children's.