How often does your heart skip a beat? The answer may explain why fewer blacks have AF than whites
Recent studies have determined that blacks have a lower risk of atrial fibrillation (AF) compared with whites despite having a higher prevalence of risk factors such as hypertension and heart failure. A new study in HeartRhythm found evidence that blacks experience statistically significantly fewer premature atrial contractions (PACs), but with similar frequencies of premature ventricular contractions (PVCs). Both are common causes of early, skipped, or irregular heartbeats. This difference in atrial ectopy by race may help explain the substantial differences in AF rates between blacks and whites.
"We all have premature or irregular beats that arise from our heart, and research from our group and others has demonstrated that early beats arising from the top chamber of the heart (PACs) are important predictors of atrial fibrillation," explained Gregory M. Marcus, MD, MAS, of the University of California, San Francisco, who led the study. "However, we really know nothing about why some people have more of these beats than others. We and others have previously shown that blacks have a substantially lower risk of AF compared with whites, but we did not know why. By demonstrating clear differences between two races, these data are the first to suggest that the frequency of these early beats may be heritable."
In the current study, investigators analyzed PAC burden using 24-hour ambulatory ECG monitoring (Holter monitoring) of nearly 1,000 participants randomly selected from the Cardiovascular Health Study. To assess whether PACs specifically were important, they also analyzed PVC frequencies for comparison.
The Cardiovascular Health Study Cohort is a prospective community-based cohort of older adults: From 1989 to 1990, an initial group of 5,201 adults (94.7% white, 4.7% black, 0.6% other race) aged 65 years or older were recruited from a random sample of Medicare beneficiaries at four U.S. sites (Forsyth County, NC; Sacramento County, CA; Washington County, MD; and Pittsburgh, PA). An additional 678 blacks (and nine of other races) were recruited from 1992 to 1993. A major advantage of this study is that it was community-based and included participants from four different communities across the U.S. Because these participants were not patients complaining of a particular ailment, this study is one of the few that has captured Holter data from the general population. One criticism of previous studies is that perhaps varying levels of healthcare access among participants of different races might have resulted in undiagnosed cases of AF. However, all individuals in the current study received the same continuous monitoring, essentially eliminating differential access to care as an explanation of the results.
Twenty-two percent of participants developed AF over a median follow-up of 11 years. After adjustment for potential confounders, whites exhibited twice as many PACs as blacks, and black race was associated with a 42% lower risk of AF. No detectable difference in PVC frequencies were observed between the two groups. The investigators conclude that the differential PAC burden, without differences in PVCs, suggests that identifiable common exposures or genetic influences might explain the differences in PACs.
Co-first authors Matthew A. Christensen, MD, and Kaylin T. Nguyen, MD, of the University of California, San Francisco, observed that, "It is interesting that there was a significant difference in PACs between races, but no observable differences in PVCs. This shows that these racial differences appear to be more pronounced in the atria than the ventricles."
By demonstrating that the frequency of early beats varies across different races, these data suggest that the characteristics of these common extra heart beats likely have a heritable component. These results open the door to considering genes that might be pertinent to both PACs and PVCs, including genes related to their frequency, location, and relation to subsequent heart disease. "Identifying those genes could then provide useful data related to the prediction of various forms of heart disease and uncover novel mechanisms relevant to treatment and even prevention of heart disease," Dr. Marcus noted.
In an accompanying editorial, Dan Sorajja, MD, and Win-Kuang Shen, MD, of the Mayo Clinic in Arizona, commend the investigators for revealing this relationship. They also point out that a causal relationship still needs to be explained due to the many risk factors, genetic variants, and substrate contributors to AF. "With this novel finding, the disparity of AF incidence between whites and blacks receives a plausible explanation. However, the association of PACs and race-based differences in AF may just be an epiphenomenon. AF is a multifactorial disease with many roads to get there. What is the significance of more or less PACs in different races? Are PACs one of these roads leading to AF or just a sign along these roads? For now, these PACs appear to be a sign along the road to AF, and not a road itself," they conclude.