During a total of 480,509person-years of follow up (median 14.4 years, IQR 13.7, 14.8), we observed 968 incident cases of AF, for an overall incidence rate of 2.01 cases per 1,000 person-years. Of the 968 cases, 743 (76.8%) were confirmed by electrocardiogram and 225 (23.2%) were confirmed by a physician’s report in the medical record. Forty three cases were characterized as lone AF.
Baseline characteristics stratified by baseline physical activity level are displayed in . As anticipated, women who reported higher levels of physical activity tended to have lower body weight and body mass index than those with lower levels of physical activity. The proportion of women with masters or doctorate degrees increased as physical activity level increased, while the proportion with some college education declined. Women with high levels of physical activity were more likely to drink at least one alcoholic drink per week. Many risk factors for coronary heart disease, including hypertension, diabetes, hypercholesterolemia, and current smoking, were concentrated in women with the lowest levels of physical activity. Unadjusted AF incidence rates were highest amongst those with the lowest physical activity levels (2.43 cases per 1,000 person-years) and ranged from 1.81 to 1.87 cases per 1,000 person-years among women in the top three quintiles (≥5.9 MET-hrs/wk).
Baseline characteristics of the study population
After adjustment for age, cholesterol, current and past smoking, alcohol use, diabetes, race, and randomized treatment, increasing levels of physical activity were associated with a statistically significant reduction in the rates of AF (P-trend = 0.007; ). Specifically, relative to women in the lowest category of physical activity, the rates of AF were 18% lower among women in the highest category of physical activity (hazard ratio (HR) 0.82, 95% CI 0.66–1.01), a finding of borderline statistical significance (P = 0.06). After adjustment for hypertension, the effect of physical activity on the rates of AF was somewhat attenuated, but the trend remained statistically significant (P-trend = 0.03; HR for extreme quintiles 0.87, 95% CI 0.70–1.07, P = 0.18). By contrast, after adjustment for body mass index, no statistically significant relationship between physical activity levels and incident atrial fibrillation was observed (P-trend = 0.22; HR for extreme quintiles 0.99, 95% CI 0.80–1.23, P = 0.91). The addition of hypertension to a model including BMI did not substantially alter those point estimates (). Tests for deviation from linearity and an age-activity level interaction were not statistically significant. After adjusting for all covariates except hypertension and BMI and censoring follow-up at the time women developed CVD, estimates of the HR for each of the physical activity quintiles were similar to those calculated without censoring for incident CVD (Supplementary Table 1
Risk of incident atrial fibrillation according to level of physical activity.
The risks of AF for women whose physical activity level met or exceeded the United States government’s recently published guidelines for physical activity are displayed in . In updating models adjusting for all covariates except hypertension and BMI, the rate of AF was 14% lower in women with at least 7.5 MET-hrs/wk of physical activity (HR 0.86, 95% CI 0.75–0.98, P=0.03). While this risk estimate did not change substantially after adjusting for hypertension (HR 0.89, 95% CI 0.78–1.02, P=0.09), the relationship was no longer statistically significant. By contrast, adjusting for BMI markedly attenuated the reduction in AF risk observed with physical activity (HR 0.96, 95% CI 0.84–1.10, P=0.57). Results from analyses that censored women at the time they developed CVD were not substantially different (Supplementary Table 2
Risk of incident atrial fibrillation according to the United States government’s recommended level of weekly physical activity.
reports the adjusted risk of incident atrial fibrillation for women who reported engaging in vigorous activity. As shown, we did not observe an increased risk of atrial fibrillation for any of the levels of vigorous activity as compared with no vigorous activity. For example, when compared to women who did not engage in any vigorous physical activity, those reporting the highest levels of vigorous activity (tertile 3) were at modestly lower, statistically non-significant risk of AF (Model 1 HR 0.90, 95% CI 0.75–1.08, P=0.26). That relationship was attenuated somewhat by the addition of either hypertension (HR 0.93, 0.81–1.17, P=0.44) or BMI (HR 0.98, 95% CI 0.81–1.18, P=0.76) to the model. Time spent jogging or running was not associated with AF risk. We did not observe an association between lone atrial fibrillation and any threshold of vigorous activity, although the total number of lone AF cases was small (N=43), so our power to detect such an association is low.
Table 4 Risk of atrial fibrillation according to three categories of weekly vigorous activity. Vigorous activity is defined as those activities that require at least 6 metabolic equivalent task units of energy expenditure per hour, such as running, jogging, swimming, (more ...)
Increasing frequency of strenuous activity at baseline was associated with a statistically significant reduction in the risk of AF in models adjusted for age, cholesterol, current and past smoking, alcohol use, diabetes, race, and randomized treatment(). Women who engaged in strenuous physical activity 1–3 times/week appeared to be at the lowest risk (HR 0.78, 95% CI 0.67–0.91, P=0.002). This relationship was largely unchanged after adjusting for hypertension (HR 0.80, 95% CI 0.69–0.94, P=0.006), and was attenuated but remained statistically significant after adjustment for BMI (HR 0.85, 95% CI 0.73–1.00, P=0.04). In models adjusted for both hypertension and BMI, women engaging in strenuous activity 1–3 times/week remained at statistically lower risk of AF (HR 0.85, 95% CI 0.73–1.00, P=0.05). Tests for a U-shaped trend across all categories of aerobic exercise frequency in each model were not significant.
Risk of incident atrial fibrillation according to baseline frequency of strenuous physical activity, such as swimming, aerobics, cycling, and running.