By combining data from 3 prospective cohort studies, we found that Caucasian race was associated with greater atrial fibrillation prevalence and that African American race was associated with substantially lower atrial fibrillation prevalence. These differences were not materially changed after adjustment for potential confounders. To our knowledge, this is the first study to use data derived from standardized protocols and electrocardiographically proven atrial fibrillation to focus specifically on the question of race and atrial fibrillation. By comparing echocardiographic characteristics in the Heart and Soul subgroup without atrial arrhythmias, we found that left atrial diameter was significantly larger in Caucasians than in African Americans.
The initial suggestion that atrial fibrillation might be less common in African Americans arose from studies of patients with stroke.16,17
Go et al2
then demonstrated through an analysis of medical records and International Classification of Diseases, Ninth Revision, codes that African Americans enrolled in the Kaiser Permanente system were less likely to have atrial fibrillation. Other studies have been consistent with this,18,19
but no previous study has data derived from predefined and uniform study protocols to confirm these findings. Given that race may influence the way an individual undergoing clinical care is treated,4–7
the data collected in a clinical setting may bias findings connecting diagnoses and race. Within each of the studies included in our analysis, all participants had the same evaluation regardless of race, with data collected in a uniform fashion according to predetermined protocols.
To investigate possible mechanisms by which African Americans might be protected from atrial fibrillation, we performed an analysis of echocardiograms in subjects enrolled in the Heart and Soul Study. Because atrial arrhythmias can cause atrial remodeling, we excluded participants with known atrial fibrillation from this analysis. Of note, no African Americans in the Heart and Soul Study had atrial fibrillation. The ventricular septum was thicker in African Americans, a finding that, if anything, would be expected to be associated with a higher risk of atrial fibrillation: A thicker ventricular septum can represent cardiac remodeling due to hypertension (and therefore might represent occult or more severe forms of hypertension) or a higher left ventricular end-diastolic pressure, both factors that would generally increase one’s predisposition to atrial fibrillation. Despite no differences in atrial volumes or function, Caucasians had a larger left atrial diameter, even after adjusting for potential confounders. Left atrial diameter is a well-established risk factor for atrial fibrillation,20–23
but the larger diameter in Caucasians compared with African Americans is surprising given that atrial volumes, also known to predict atrial fibrillation, 24
were not significantly different. Of note, previous studies have shown that left atrial diameter predicts atrial fibrillation independent of volume,24
and a recent study in Japanese subjects suggests that left atrial diameter might be more important than left atrial volume in predicting recurrence of paroxysmal atrial fibrillation.25
The association between Caucasians and larger left atrial diameter also was maintained after adjusting for left atrial volume. The only previous study that compared echocardiographic atrial characteristics between Caucasians and African Americans used M-mode only,26
but also found a 1.9 mm greater left atrial diameter among Caucasians after adjusting for confounders. Their statistically significant finding was similar to our finding of a 2-mm difference. It is therefore possible that African Americans have less atrial fibrillation because of a smaller left atrial diameter, potentially due to genes that govern left atrial shape or the shape of anterior or posterior structures (eg, the aorta, chest wall, or spine) that may restrict anterior–posterior atrial expansion.
It also is possible that our finding related to left atrial volume reflects a type I error (a false-positive association). In fact, whereas the difference in atrial fibrillation prevalence is substantial (Caucasians having an ~4-fold greater odds of atrial fibrillation after multivariable adjustment), the difference in left atrial diameter was small. However, in the Framingham study, the difference in mean left atrial diameter between those with and without incident atrial fibrillation was 2.6 mm, suggesting that small changes in left atrial diameter may be clinically relevant.22
The remarkable difference in atrial fibrillation prevalence is especially interesting given that African Americans had more hypertension, a greater septal thickness, and no other differences in ventricular or atrial measurements. The largely negative echocardiographic findings might point to differences in tissue composition rather than chamber size or shape as an explanation for racial differences in atrial fibrillation.