In this analysis of the ARIC cohort, we show that incidence of AF is higher in whites than in African-Americans, despite risk factors for AF being more prevalent in the latter group. Incidence rates of AF among whites in the ARIC study were comparable to those obtained in other population-based studies conducted in the US (4
). Consistent with previous publications, AF rates increased exponentially with age and were higher in men than women. Finally, we estimated the cumulative risk of AF at age 80 to be twice as high in white men as in African-Americans, with white women having an intermediate risk. No temporal trends were evident in the incidence of AF.
Previous reports consistently suggest that African-Americans have lower prevalence and incidence of AF compared to whites. In the population aged 50 or older enrolled in Kaiser Permanente of Northern California, AF prevalence in African-Americans was 32% lower than in whites (3
). Similarly, elderly African-Americans in the Cardiovascular Health Study had a 53% lower risk of incident AF compared to whites (7
). Other data sources provide parallel results. In the US National Hospital Ambulatory Medical Care Survey, rates of visits to an emergency department with AF as the primary diagnosis were higher in whites than in African-Americans (9 vs. 5 per 10,000 person-year) (9
). The National Hospital Discharge Survey, which compiles data on discharges from nonfederal hospitals in the US, also shows a higher incidence of AF hospitalizations in whites than in African-Americans (17
). This racial difference in the incidence of AF has been also evidenced in heart failure patients enrolled in the Epidemiology, Practice, Outcomes, and Costs of Heart Failure (EPOCH) study, where African-Americans had a 50% lower prevalence of AF than whites even after adjustment for known risk factors for AF (18
). Our findings are consistent with previous evidence, providing for the first time age-specific incidence rates of AF in a population-based study of white and African-American middle age adults.
We can only guess at the mechanisms underlying the reported lower burden of AF in African-Americans, particularly considering that prevalence of some risk factors for AF, such as hypertension, is higher among this racial group. Underascertainment of AF in African-Americans because of poorer access to medical care could explain the divergence (19
). However, this would not be operable in the finding of racial differences in the Kaiser Permanente study, where all participants presumably had similar access to healthcare (3
), and could not explain the lower risk of AF in the ARIC African-American sample when including only cases identified through ECGs done at study visits. Differential mortality, also, might account for part of the discrepancy. Total and cardiovascular disease mortality is higher among African-Americans than whites (20
), which in turn could lead to lower AF incidence in the former group if the increased mortality affected disproportionably individuals susceptible to develop AF. In this regard, it has been shown that post-myocardial infarction mortality is higher among African-Americans than whites (35% v. 30% among patients admitted to hospitals with revascularization service) (21
), increasing the proportion of myocardial infarction survivors, and hence patients at higher risk of AF, among whites. Nonetheless, the magnitude of this difference is not enough to explain a 50% lower cumulative risk of AF among African-Americans. Another explanation for the racial differences could be a relative higher incidence of paroxysmal (vs. permanent) AF among African-Americans compared to whites (22
). In the Group Health Cooperative study, a population-based study of newly detected AF in a large health plan, the proportion of whites among sustained AF cases was higher than among transitory/intermittent AF cases, though these differences were small and not significant (23
). Geographic disparities in medical practice across the different ARIC sites could explain differences between races, though the only study site including both racial groups, Forsyth County, had a 78% higher rate of AF in whites than African-Americans, not supporting this hypothesis. Finally, pathophysiological mechanisms could account for the differences. For example, in the Cardiovascular Health Study, white men had on average a larger left atrium than their African-American counterparts (24
), and a larger left atrium was associated with an increased risk of AF in this same population (7
). Also, genetic traits associated with increased risk of AF could be disproportionally prevalent in whites. No published studies, however, have compared genetic risk factors in whites and African-Americans.
Previous reports suggest that the burden of AF in the population might be increasing. In the Framingham Heart Study, age-adjusted prevalence of AF in men aged 65-84 increased from 3.2% in 1968-1970 to 9.1% in 1987-1989, and from 2.8% to 4.7% in women during the same period (25
). Similar results were observed in Olmsted County, Minnesota, where age-adjusted prevalence of AF in men and women rose from 5% and 4%, respectively, in the decade 1960-69 to 12% and 8% in the decade 1980-89 (26
). These observations could result from an increase in AF incidence, from longer survival of patients with AF, or from both. An analysis of the National Discharge Survey, in the United States, found a 2- to 3-fold increase in hospitalization rates for AF from 1985 to 1999 (5
), but this survey did not differentiate between incident and prevalent cases. More interestingly, recent findings from Olmsted County, Minnesota, implied that incidence of AF could be on the rise: In an analysis including 4,618 cases, AF incidence increased 12% between 1980 and 2000 (4
). In the ARIC cohort, we did not find a significant increase in AF incidence. This could be due to the limited number of cases, to differences between the studied populations, such as age and race distribution or prevalence and treatment of AF risk factors, or to the shorter period of time included in the ARIC follow-up.
A major limitation of the present analysis is the method of AF ascertainment, mostly based on hospital discharge codes. This could lead, on one hand, to underascertainment of AF cases not requiring hospitalization. Unfortunately, no published evidence provides information to estimate the proportion of AF cases missed because of lack of outpatient diagnoses. On the other hand, our hospital-based AF ascertainment could lead to false positives. In the Cardiovascular Health Study, 98% of the AF cases identified through hospital discharge codes were confirmed through review of discharge summaries (7
). In the ARIC cohort, however, we were able to confirm only 89% of a sample of 125 discharge summaries. Additionally, we estimated that the sensitivity of the ICD code of AF in the hospital discharge summary of participants with suspected stroke was 84%, slightly higher for whites but not enough to explain the important difference in incidence rates. Still, AF incidence and lifetime risk in the white ARIC participants were extremely similar to those from other populations, such as the Mayo Clinic study in Olmsted County, Minnesota, or the Framingham Heart Study (4
The present analysis has several important strengths. First, its large sample size and extended follow-up allowed the estimation of precise incidence rates, specifically in the African-American population. Second, the ARIC cohort is approximately representative of the underlying populations, providing excellent generalizability of the results. The race-specific incident rates of AF presented here could be used to update projections of AF prevalence in the United States, taking into account the lower burden of AF in African-Americans.
In conclusion, we report incidence rates of AF in a population-based sample of four different US communities including a sizeable number of African-Americans. These results highlight the high impact of AF in the general population, particularly among older individuals, and serve as a call to action for the development of effective preventive strategies. Future studies comparing risk factors for AF in whites and African-Americans are needed to provide insights into the mechanisms explaining the racial discrepancy.