Published online before print April 13, 2009, doi: 10.1161/CIRCULATIONAHA.108.830042
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(Circulation. 2009;119:2146-2152.)
© 2009 American Heart Association, Inc.
Epidemiology |
Influence of Systolic and Diastolic Blood Pressure on the Risk of Incident Atrial Fibrillation in Women
From the Center for Arrhythmia Prevention (D.C., U.B.T., B.A.K., C.M.A.), Division of Preventive Medicine (D.C., R.J.G., J.E.B., C.M.A.), and Cardiovascular Division (U.B.T., B.A.K., C.M.A.), Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Mass, and Department of Medicine, University Hospital Basel, Basel, Switzerland (D.C.).
Reprint requests to David Conen, Department of Medicine, University Hospital, Petersgraben 4, 4031 Basel, Switzerland. E-mail conend{at}uhbs.ch
Received June 13, 2008; accepted February 20, 2009.
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Abstract |
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Background— The influence of systolic and diastolic blood pressure (BP) on incident atrial fibrillation (AF) is not well studied among initially healthy, middle-aged women.
Methods and Results— A total of 34 221 women participating in the Women’s Health Study were prospectively followed up for incident AF. The risk of AF across categories of systolic and diastolic BP was compared by use of Cox proportional-hazards models. During 12.4 years of follow-up, 644 incident AF events occurred. Using BP measurements at baseline, we discovered that the long-term risk of AF was significantly increased across categories of systolic and diastolic BP. Multivariable-adjusted hazard ratios for systolic BP categories (<120, 120 to 129, 130 to 139, 140 to 159, and 
160 mm Hg) were 1.0, 1.00 (95% CI, 0.78 to 1.28), 1.28 (95% CI, 1.00 to 1.63), 1.56 (95% CI, 1.22 to 2.01), and 2.74 (95% CI, 1.77 to 4.22) (P for trend <0.0001). Adjusted hazard ratios across baseline diastolic BP categories (<65, 65 to 74, 75 to 84, 85 to 89, 90 to 94, and 95 mm Hg) were 1.0, 1.17 (95% CI, 0.81 to 1.69), 1.18 (95% CI, 0.84 to 1.65), 1.53 (95% CI, 1.05 to 2.23), 1.35 (95% CI, 0.82 to 2.22), and 2.15 (95% CI, 1.21 to 3.84) (P for trend=0.004). When BP changes over time were accounted for in updated models, multivariable-adjusted hazard ratios were 1.0, 1.14 (95% CI, 0.89 to 1.46), 1.37 (95% CI, 1.07 to 1.76), 1.71 (95% CI, 1.33 to 2.21), and 2.21 (95% CI, 1.45 to 3.36) (P for trend <0.0001) for systolic BP categories and 1.0, 1.12 (95% CI, 0.82 to 1.52), 1.13 (95% CI, 0.83 to 1.52), 1.30 (95% CI, 0.89 to 1.88), 1.50 (95% CI, 1.01 to 1.88), and 1.54 (95% CI, 0.75 to 3.14) (P for trend=0.026) for diastolic BP categories.
Conclusions— In this large cohort of initially healthy women, BP was strongly associated with incident AF, and systolic BP was a better predictor than diastolic BP. Systolic BP levels within the nonhypertensive range were independently associated with incident AF even after BP changes over time were taken into account.
Key Words: blood pressure • cardiovascular diseases • atrial fibrillation • epidemiology • hypertension • women
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Introduction |
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Atrial fibrillation (AF) is the most common cardiac arrhythmia, and its prevalence in the general population is increasing rapidly.1–3 The importance of AF as a public health problem is further underscored by its association with stroke, heart failure, death, cognitive dysfunction, and a reduced quality of life.4–8 Because treatment of established AF has limited long-term success rates and significant risks,9,10 characterizing treatable risk factors for AF has substantial clinical relevance.
Clinical Perspective p 2152
Several studies have shown that arterial hypertension is an independent risk factor for incident AF,11–13 but optimal blood pressure (BP) levels for AF prevention have not been established. In addition, few studies have assessed in more detail the association between different BP components or the change in these components over time and subsequent risk of incident AF. Given the nonlinear relationship between age and diastolic BP but not systolic BP,14 alterations in systolic or diastolic BP may also differentially affect the risk of incident AF. Along these lines, a recent study suggested that pulse pressure may be more predictive of AF than either systolic or diastolic BP alone.15 However, these relationships may differ in younger individuals or women, in whom the prevalence, outcome, and underlying comorbidities associated with AF differ.4,5,11,16
To address these issues, we assessed the relationship between systolic and diastolic BP levels and the risk of incident AF in a large cohort of middle-aged women who were free of cardiovascular disease at baseline.
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Methods |
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Participants
All study subjects were participants of the Women’s Health Study, a completed randomized trial evaluating the risks and benefits of low-dose aspirin and vitamin E in the primary prevention of cardiovascular disease and cancer. Details of the study design have been given previously.17–19
Briefly, beginning in 1993, 39 876 female health professionals in the United States who were 45 years of age and free of cardiovascular disease, cancer, or other major illnesses were randomized to receive 100 mg aspirin every other day, 600 IU vitamin E every other day, both agents, or placebo. The trial initially had a beta carotene arm that was terminated early.20 Randomized treatment ended on March 31, 2004, and women were invited to participate in continued observational follow-up, which for the present study was truncated on October 31, 2006. Of the original cohort, 4320 opted out of the observational follow-up. These women were excluded from this analysis because their AF could not be reliably confirmed. However, very similar results were obtained when we repeated our analyses using self-reported AF events among all women as the main outcome variable (data not shown).
We also excluded 813 women with a history of AF at baseline and 465 women with missing information on baseline systolic or diastolic BP. The final study population for the present analysis consisted of 34 221 women with a median follow-up of 12.4 years (interquartile range, 11.8 to 12.8 years). Written informed consent was obtained from all participants. The study was approved by the institutional review board of Brigham and Women’s Hospital, Boston, and was monitored by an external data and safety monitoring board.
BP Ascertainment
Information on baseline variables was collected through mailed questionnaires. Follow-up questionnaires asking participants about study outcomes and other information were sent every 6 months during the first year and every 12 months thereafter. Systolic and diastolic BP was self-reported at randomization and again at 12, 48, 120, and 132 months of follow-up. At randomization and at 12 months, women were asked to categorize their BP levels into 9 different categories for systolic BP and 7 different categories for diastolic BP. Beginning at 48 months, women were asked to directly report their current systolic and diastolic BP. Such self-reported BP measurements among female health professionals have proved to be highly accurate in prior studies.21–23
Other covariates of interest that were assessed at study entry and at various points of follow-up included age, smoking, diabetes, history of hypercholesterolemia (self-reported cholesterol of at least 240 mg/dL [6.22 mmol/L]), body mass index (weight in kilograms divided by the square of height in meters), exercise, alcohol consumption, and highest education level achieved.
Ascertainment of Incident AF
Women were asked to report diagnoses of incident AF at baseline, at 48 months, and then annually thereafter. Beginning on September 19, 2006, women enrolled in the continued observational follow-up who reported an incident AF event on at least 1 yearly questionnaire were sent an additional questionnaire to confirm the episode and to collect additional information. They were also asked for permission to review their medical records, particularly available ECGs, rhythm strips, 24-hour ECGs, and information on cardiac structure and function. For all deceased participants who reported AF during the trial and extended follow-up period, we contacted family members to obtain consent and additional relevant information. An end-point committee of physicians reviewed medical records for reported events according to predefined criteria. An incident AF event was confirmed if there was ECG evidence of AF or if a medical report clearly indicated a personal history of AF. The earliest date in the medical records when documentation was believed to have occurred was set as the date of onset of AF. Only confirmed events are included in the present report.
Statistical Analysis
The primary analysis examined the relationship between self-reported BP at baseline and the subsequent development of AF over the course of the study. Continuous BP values were assigned to each participant by taking the midpoint value for the reported BP category. Participants with systolic BP <110 or 180 mm Hg were assigned a value of 95 or 190 mm Hg, respectively; those with diastolic BP <65 or 105 mm Hg were assigned a value of 55 or 110 mm Hg, respectively. Because of the high correlation between baseline and 1-year BP in this cohort,22 the average of the 2 measurements was used as baseline BP to reduce BP variability and potential reporting errors. Women were then classified in the following prespecified clinically relevant baseline BP categories: <120, 120 to 129, 130 to 139, 140 to 159, and 160 mm Hg for systolic BP and <65, 65 to 74, 75 to 84, 85 to 89, 90 to 94, and 95 mm Hg for diastolic BP.
Subsequently, Cox proportional-hazards models were constructed to calculate hazard ratios and 95% CIs across BP categories. Multivariable models were adjusted for age, body mass index, history of diabetes, smoking, history of hypercholesterolemia, exercise, alcohol consumption, education, and randomized treatment assignments (aspirin, vitamin E, beta carotene). In a first step, we performed separate analyses for systolic and diastolic BP. We then assessed the joint contribution of systolic and diastolic BPs by including both BP variables in the same models. The –2 log likelihood was used to compare the fit of nested BP models.
Because the rounding of BP into categories may differentially influence the strength of the association between systolic or diastolic BP and incident AF, we performed a sensitivity analysis examining the relationship between the directly reported BP values obtained at 48 months and subsequent risk of AF. In addition, because most participants indicated a baseline diastolic BP between 75 and 84 mm Hg (Table 1), we performed an additional secondary analysis that subdivided the range between 75 to 84 mm Hg into 2 equally spaced categories at 48 months (80 to 84 mm Hg and 85 to 89 mm Hg).
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To incorporate the effect of BP changes over time on the risk of incident AF, we used a modified Kaplan-Meier method that estimated cumulative probabilities of incident AF according to the most recent BP level using time-updated BP categories.24 We then constructed multivariable-adjusted Cox models in which BP categories were similarly updated at 12, 48, 120, and 132 months of follow-up, and the most recent BP measurement before the event was used to estimate risk. For example, the 12-month BP measurement was used for the 12-month through 48-month follow-up period, the 48-month measurement was used for the 48-month to 120-month follow-up period, and so on. Similarly, the effect of antihypertensive treatment over the course of the study was assessed by including an indicator for antihypertensive treatment as a time-varying covariate in these models. We obtained a rough estimate of the potential benefit of lowering systolic BP <120 mm Hg among women with higher BP levels by using the following formula: [1–(1/HR)]x100, where HR is the hazard ratio.
An association between BP and incident AF may be caused by intercurrent cardiovascular events, given the strong relationship between BP and cardiovascular disease.22,23 We therefore refitted all Cox models after censoring women at the date of their first cardiovascular event. A cardiovascular event was defined as myocardial infarction, stroke, or coronary revascularization.19 Finally, we performed analyses excluding 4621 women (13.5%) who took antihypertensive treatment at baseline.
In all regression models, categorical variables were entered using binary indicator variables, and tests for linear trend were performed using integer scores across categories. We tested for deviation from linearity by including a quadratic term in the models. Effect modification was assessed using multiplicative interaction terms. The proportional-hazards assumption was examined for all models by including a BP category by logarithm of time interaction into the model.25 No violation of this assumption was detected. All analyses were carried out with SAS version 9 (SAS Institute Inc, Cary, NC). A 2-tailed value of P<0.05 was considered to indicate statistical significance.
The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
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Results |
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During a median follow-up of 12.4 years, 644 of 34 221 women had at least 1 confirmed episode of incident AF. Baseline characteristics of the study population are shown in Table 1. Mean age was 55±7 years; 16.8% of participants had a systolic BP
140 mm Hg, and 4.8% had a diastolic BP 90 mm Hg.
There was a highly significant increase in risk of incident AF across increasing categories of systolic and diastolic BP. Although multivariable adjustment attenuated these trends somewhat, both BP components remained strongly associated with incident AF (Table 2). Even women with systolic BP between 130 and 139 mm Hg or diastolic BP between 85 and 89 mm Hg, ie, below the current threshold for diagnosing and treating hypertension,26,27 had a significantly increased risk of incident AF (hazard ratio, 1.28 [95% CI, 1.00 to 1.63; P=0.05] and 1.53 [95% CI, 1.05 to 2.23; P=0.03], respectively). When both BP components were entered in a joint multivariable model, we found a persistent risk gradient across systolic but not diastolic BP categories. Accordingly, adding diastolic BP to the multivariable systolic BP model did not improve model fit (likelihood ratio, 
2=2.23, P=0.82).
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When we assessed baseline systolic and diastolic BP as continuous variables, we found an adjusted hazard ratio of 1.16 (95% CI, 1.09 to 1.23; P<0.0001) per 10-mm Hg increase in systolic BP and 1.17 (95% CI, 1.05 to 1.29; P=0.003) per 10-mm Hg increase in diastolic BP. In a combined model, systolic but not diastolic BP remained a significant predictor of incident AF (hazard ratio per 10-mm Hg increase, 1.17 [95% CI, 1.08 to 1.27; P<0.0001] for systolic BP and 0.98 [95% CI, 0.86 to 1.12; P=0.74] for diastolic BP).
When we examined the continuous relationship between directly reported BP measurements ascertained at 48 months and subsequent risk of AF, systolic but not diastolic BP was associated with AF (hazard ratio per 10-mm Hg increase, 1.12 [95% CI, 1.05 to 1.19; P=0.001] for systolic BP and 1.00 [95% CI, 0.90 to 1.12; P=0.94] for diastolic BP). In a combined model, systolic BP remained significantly related to incident AF (hazard ratio, 1.16; 95% CI, 1.08 to 1.25; P=0.0001), and an inverse association between diastolic BP and incident AF began to emerge (hazard ratio, 0.88; 95% CI, 0.78 to 1.00; P=0.05).
Importantly, women with high normal systolic BP (130 to 139 mm Hg) at 48 months continued to be at increased risk of subsequent AF (hazard ratio, 1.50; 95% CI, 1.12 to 2.02; P=0.006). At 48 months, there was an increased risk even among women with systolic BP between 120 and 129 mm Hg (hazard ratio, 1.35; 95% CI, 1.01 to 1.82; P=0.04). When we subdivided diastolic BP at 48 months into 1 additional category (<65, 65 to 74, 75 to 80, 80 to 84, 85 to 89, 90 to 94, and >94 mm Hg), we obtained the following adjusted hazard ratios: 1.0, 0.97 (95% CI, 0.69 to 1.35), 0.65 (95% CI, 0.43 to 1.00), 0.92 (95% CI, 0.66 to 1.28), 0.91 (95% CI, 0.59 to 1.40), 1.01 (95% CI, 0.65 to 1.57), and 1.38 (95% CI, 0.62 to 3.07). When diastolic BP categories were entered into the systolic BP model, a U-shaped relationship between diastolic BP and AF emerged (hazard ratios, 1.0, 0.84 (95% CI, 0.60 to 1.18), 0.52 (95% CI, 0.33 to 0.80), 0.67 (95% CI, 0.46 to 0.96), 0.62 (95% CI, 0.39 to 0.99), 0.67 (95% CI, 0.41 to 1.08), and 0.86 (95% CI, 0.37 to 2.00)), associated with a borderline significant improvement in model fit (likelihood ratio, 12.58; 6 df; P=0.05). The addition of a quadratic diastolic BP term to this model provided a significant result (P=0.031), again suggesting a nonlinear relationship.
In models in which BP was updated over time, systolic BP remained a strong predictor of AF, and women with systolic BP between 130 and 139 mm Hg continued to have significantly elevated risks of AF compared with women with systolic BP <120 mm Hg (hazard ratio, 1.37; 95% CI, 1.07 to 1.76; P=0.01; Table 2 and the Figure). These analyses also suggest that lowering systolic BP to <120 mm Hg in women with systolic BP of 120 to 129 mm Hg, 130 to 139 mm Hg, 140 to 159 mm Hg, and 160 mm Hg may be associated with reductions in incident AF of 12%, 27%, 42%, and 55%, respectively.
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Diastolic BP also was associated with incident AF in the updated model (P for linear trend=0.026). In an updated combined model, systolic BP remained strongly and positively associated with incident AF, even among women with BP values between 130 and 139 mm Hg (hazard ratio, 1.43; 95% CI, 1.09 to 1.87; P=0.009), but the relationship between diastolic BP and AF became nonsignificant.
Findings were similar when we took into account the effect of antihypertensive treatment during follow-up. In these models, multivariable-adjusted hazard ratios were 1.0, 1.10 (95% CI, 0.86 to 1.41), 1.28 (95% CI, 0.99 to 1.64), 1.54 (95% CI, 1.18 to 2.00), and 1.93 (95% CI, 1.26 to 2.97) for increasing systolic BP categories and 1.0, 1.10 (95% CI, 0.81 to 1.51), 1.08 (95% CI, 0.79 to 1.46), 1.18 (95% CI, 0.81 to 1.71), 1.32 (95% CI, 0.88 to 1.96), and 1.32 (95% CI, 0.64 to 1.70) for increasing diastolic BP categories. Treatment by systolic or diastolic BP interaction terms were not statistically significant (P=0.80 and P=0.31, respectively).
In total, 47 women had a cardiovascular event before the development of new-onset AF. Censoring these women at the date of the cardiovascular event attenuated but did not offset the association between incident AF and baseline BP (Table 2), and linear trends across BP categories remained significant for systolic BP (P for linear trend=0.0001) and diastolic BP (P for linear trend=0.02). Results also did not significantly change if we excluded the 13.5% of women who took antihypertensive treatment at baseline (data not shown). Analysis for effect modification revealed that the risk of AF according to both systolic and diastolic BPs was similar for women <65 and those 65 years of age (P for interaction=0.32 and 0.39 for systolic and diastolic BP, respectively) and similar for those who took antihypertensive treatment at baseline and those who did not (P for interaction=0.24 and 0.75 for systolic and diastolic BP, respectively).
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Discussion |
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The present study demonstrates that BP is a strong and independent predictor of incident AF in initially healthy, middle-aged women. This study also found that BP values below the current threshold for the diagnosis of arterial hypertension are significantly associated with the risk of incident AF, extending prior studies that have consistently shown a strong and independent relationship between hypertension and incident AF.11–13 Women with high-normal systolic (130 to 139 mm Hg27) or diastolic (85 to 89 mm Hg27) BP at baseline had a 28% and 53% increased risk of incident AF compared with women with systolic BP <120 mm Hg or diastolic BP <65 mm Hg, respectively. Thus, in our study, even slightly elevated BP levels at baseline imposed some degree of increased risk. These results parallel findings from prior studies that demonstrated continuous relationships between BP and other cardiovascular disease outcomes.22,23 However, further studies in individuals with low BP levels are needed to confirm the absence of a BP threshold below which the risk of incident AF is not increased.
To the best of our knowledge, this is one of the first studies examining the influence of BP changes over time and subsequent risk of AF. Not surprisingly, we found that women with a systolic BP 140 mm Hg during follow-up had a significantly increased risk of incident AF compared with those with systolic BP values <140 mm Hg. Of note, even women with systolic BP values between 130 and 139 mm Hg during follow-up had a significantly increased risk of subsequent AF. Although no firm recommendations can be made on the basis of these observational findings, they nevertheless suggest that tight BP control may help to reduce the burden of AF among women.
We believe that our study also has other potential implications. First, guidelines for the management of arterial hypertension do not currently classify AF as a risk factor, target organ damage, or associated clinical condition.26,27 However, the tight association between BP, incident AF, and subsequent cardiovascular events4–8 suggests that future hypertension guidelines may assign a more important role to AF for cardiovascular risk stratification in patients with hypertension. Second, current guidelines do not assign a lower BP treatment target for patients suffering from AF.26,27 Given the strong, continuous relationship between systolic BP and incident AF found in this study, individuals with AF may also benefit from a lower BP treatment threshold of <140/90 mm Hg. Our time-updated analyses suggest that substantial risk reductions may be obtained if systolic BP levels are maintained at or lowered to levels <120 mm Hg (Table 2). However, optimal BP targets should ideally be evaluated in a randomized trial.
Prior work indicated that pulse pressure is an important risk factor for AF in a middle-aged to elderly community sample. In this prior study, diastolic BP provided significant additional information when added to a model containing systolic BP in this population.15 Accordingly, the investigators suggested that aortic stiffness might be an important factor in the pathogenesis of AF. In the present study of middle-aged women, we found that systolic BP was a stronger predictor of incident AF than diastolic BP. However, additional analyses using directly reported continuous BP measures at 48 months suggested that after systolic BP was taken into account, a U-shaped relationship between diastolic BP and incident AF emerged. These models indicated that women with a diastolic BP <65 mm Hg had the highest risk of developing AF, providing some evidence that elevated pulse pressure and aortic stiffness may also play a role in this population.
U-shaped relationships for diastolic BP have been described for other cardiovascular disease outcomes, but mainly in the elderly. For example, in elderly individuals with coronary disease, there was a J-shaped association between diastolic BP and cardiovascular events.28 Similar relationships have been reported among apparently healthy elderly individuals.29 In contrast, among middle-aged civil servants participating in the Whitehall study, both systolic and diastolic BP were significantly related to the incidence of coronary heart disease, but only the association with systolic BP remained significant in a combined BP model.30 Further studies are needed to gain more insights into the relationships between different BP components and risk of AF in different population groups.
There are several other potential mechanisms besides arterial stiffness that could underlie the relationship between BP and incident AF. For example, elevated systolic BP may be associated with an increase in left atrial fibrosis,31,32 which in turn is related to prevalent AF.33 Left ventricular hypertrophy and increased left atrial size are other important mediators of the relationship between BP and incident AF.34,35 However, it is important to emphasize that prior studies showed independent BP effects even after adjustment for echocardiographic variables.15
Study Strengths and Limitations
Strengths of the present study include its prospective design, sample size, and long-term follow-up with a large number of confirmed events. Potential study limitations also require discussion. First, the study population consisted of predominantly white, middle-aged female health professionals, and our findings may not be generalizable to men or other populations of women. Second, BP was self-reported. However, the prognostic value of self-reported BP in cohort studies involving US health professionals is similar to directly measured BP values in participants of other cohort studies.23 Furthermore, the validity of this approach has been examined in the Nurses’ Health Study, in which 99% of the women who reported high BP had their diagnosis confirmed by medical record review.21 Moreover, in the Women’s Health Study, self-reported BP, total cholesterol, and body mass index are strong predictors of cardiovascular risk, with relative risks consistent in magnitude with those observed in other major studies.22,36–38 However, it is possible that the rounding of baseline BP into categories may have differentially affected the relationship between systolic or diastolic BP and incident AF. Our sensitivity analyses at 48 months suggest that in middle-aged women, there might be a U-shaped association between diastolic BP and incident AF. Further studies are needed to confirm these preliminary findings.
Conclusions
In middle-aged, initially healthy women, BP is a powerful predictor of incident AF, and systolic BP is a better predictor than diastolic BP. Our study also revealed that even participants with BP values currently considered normal have an increased risk of developing AF. Taken together, our findings indicate that tight BP control may help to reduce the growing burden of AF in the community.
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Acknowledgments |
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Sources of Funding
Dr Conen was supported by grant PASMA 118586/1 from the Swiss National Science Foundation. The Women’s Health Study was supported by grants HL-043851, HL-080467, and CA-047988 from the National Heart, Lung and Blood Institute and the National Cancer Institute.
Disclosures
None.
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Clinical trial registration information—URL: http://clinicaltrials.gov. Unique identifier: NCT00000479.
Guest Editor for this article was Paolo Verdecchia, MD.
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