Worldwide, hypertension remains a powerful, independent marker of cardiovascular mortality and death from all causes. In 2013, high systolic blood pressure (BP) accounted for more than 10 million deaths globally. In the United States, hypertension caused nearly 397,000 deaths in 2013; an increase of 61.8% since 2000. The highest hypertension-related age-adjusted death rate was seen in African Americans in whom the rate was 44% and 42% higher than in Hispanics and non-Hispanic whites, respectively. Thus, hypertension remains a major contributor to death from stroke, heart failure, kidney failure, and ischemic heart disease in African Americans.
This chapter discusses the epidemiology of hypertension in African Americans as well as the pathophysiological characteristics and strategies for prevention, treatment, and control of hypertension in this population. The magnitude and trends in disparities in care and clinical outcomes are explored and so are opportunities for eliminating these disparities. The role of implementation research and practice-based evidence to inform hypertension treatment and control in African Americans is also addressed. This chapter does not discuss specific forms of hypertension such as pregnancy-related hypertension, white coat hypertension, renovascular hypertension or the strategies for their detection and evaluation which are addressed in other sections of this book.
Epidemiology of Hypertension in African Americans
Hypertension Risk Factors
Important risk factors that predispose to hypertension include advancing age, a strong family history of hypertension, obesity, physical inactivity, high dietary sodium intake, low dietary potassium intake, low vitamin D intake, harmful use of alcohol, psychosocial stress, low socioeconomic status, low educational attainment, and psychological traits such as anger and hostility. These factors are as important in African Americans as they are in other race-ethnic population subgroups. However, they take on additional significance when a greater prevalence of any of them in African Americans is used to explain the greater prevalence of hypertension in this population.
Hypertension Incidence
Hypertension incidence is strongly influenced by age, baseline BP level, the definition of hypertension, and duration of follow-up. It is also influenced by sex, race, ethnicity, family history, obesity, geography, and several psychosocial, environmental, and biomedical risks. Although older studies showed a higher incidence of hypertension in African Americans, more recent, carefully controlled studies of longer duration paint a more nuanced picture. For example, in younger adults who were aged 18 to 30 years when recruited in 1985 to 1986 in the community-based Coronary Artery Risk Development in Young Adults (CARDIA) cohort, hypertension incidence after 20 years of follow-up was significantly higher in African Americans, especially women, even after adjustment for age, race, heart rate, body mass index, smoking, family history, education, uric acid, alcohol use, physical activity, and baseline systolic BP. For example, when the mean age was approximately 45 years, the 20-year incidence was 34.5% in black men, 37.6% in black women, 21.4% in white men, and 12.3% in white women; p < 0.001. Hypertension incidence also varied significantly across urban areas and by race and sex, with higher rates in the southeast and in blacks, especially African-American women. In the Trials of Hypertension Prevention, the incidence of hypertension (defined as BP ≥ 160/95 mm Hg or taking antihypertensive medications) over 7 years of follow-up in middle-aged African Americans and whites was nearly identical (25.7% in African Americans and 25.3% in whites). In the Multi-Ethnic Study of Atherosclerosis, participants aged 45 to 84 years at baseline were followed for a median of 4.8 years for incident hypertension, defined as systolic BP 140 or higher mm Hg, diastolic BP 90 or higher mm Hg, or the initiation of antihypertensive medications. After adjustment for age, sex, and study site, hypertension incidence was higher for African Americans aged 45 to 64 compared with whites but not for those 75 to 84 years of age.
Hypertension Prevalence
Most published studies demonstrate that hypertension prevalence is significantly greater in African Americans compared with other race-ethnic groups in the United States. As shown in Fig. 41.1 , the age-adjusted prevalence in the most recent National Health and Nutrition Examination Survey (2011–2014) was higher in non-Hispanic African-African women (41.5%) and men (40.8%) compared with all other race-ethnic-sex groups. Importantly, in both non-Hispanic African-African women and men, the age-adjusted prevalence has steadily increased in graded fashion over all three national surveys in 1988 to 1994, 1999 to 2006, and 2007 to 2012. Fig. 41.2 shows the extent of the increase in hypertension prevalence in U.S. counties from 2001 to 2009 and the particularly marked increase seen in African-African men and women.
Hypertension Severity
In addition to their greater prevalence of hypertension, African Americans (in comparisons with whites) develop hypertension at an earlier age ; have higher average BP levels; and higher average nondipping nocturnal BP and greater 24-hour BP variability on ambulatory monitoring. Additionally, African Americans are more likely to experience accelerated conversion from prehypertension to hypertension. As a result, severe hypertension is more common in African Americans compared with whites and is often more likely to be associated with a greater prevalence of target organ damage. However, there is little, if any, evidence that hypertension is a different disease or is “more severe” in African Americans. Thus race, per se, does not cause more severe hypertension. As Schmieder et al. demonstrated in a matched-pair analysis of early target organ damage that also controlled for confounding factors such as age, sex, body weight, and BP level, race per se does not predict hypertension severity or extent of target organ damage.
Awareness, Treatment, and Control
Over the last three decades, awareness and treatment of hypertension in African Americans have improved significantly as it has in the general population ( Fig. 41.3 ). In fact, hypertension awareness has been higher in non-Hispanic blacks compared with the total U.S. population or in non-Hispanic whites and Hispanics in most years of the survey ( Fig. 41.3 ). In 2011 to 2012, hypertension treatment rates were similar among non-Hispanic blacks (76.5%), and non-Hispanic whites (75.8%) but lower in Mexican Americans (69.6%).
Although hypertension control has also improved steadily over the last three decades, the most recent control rate in African Americans (49.4%) is lower than that in non-Hispanic whites (54.3%) and also lower than achievable in an integrated health system model that uses implementation, dissemination, and performance feedback strategies in chronic disease care. For example, the Kaiser Permanente Southern California health care system was able to improve hypertension control in a multiethnic population from 54% to 86% in the total population and achieved a control rate of 80% or more in African Americans and other population subgroups, regardless of preferred language or type of health insurance plan.
Mortality and Morbidity
The age-adjusted hypertension-related mortality rate in non-Hispanic blacks is nearly double the rate seen in non-Hispanic whites and Hispanics ( Fig. 41.4 ). The disparity is even starker when examined by sex. For example, in 2013, the death rates per 100,000 population were 51.6 for non-Hispanic black males but 18.9 for non-Hispanic white males, and 20.0 for Hispanic males. The corresponding rates for women were 36.5 for non-Hispanic black females, 15.8 for non-Hispanic white females, and 15.3 for Hispanic females. Hypertension is also an important contributor to stroke, myocardial infarction (MI), heart failure, kidney failure, and other morbid events and reduced quality of life in African Americans. The greater prevalence of hypertension, onset at an earlier age, and lower control rates in African Americans, compared with whites, contribute to the greater prevalence of hypertensive target organ damage in the heart, brain, kidney, and arterial vasculature with resulting chronic organ failure and reduced quality of life.
Pathophysiology
The pathophysiological mechanisms that initiate and maintain chronic hypertension are complex, interrelated, dynamic, and have multiple feedback loops that, to a large extent, contribute to the marked heterogeneity seen in the phenotypic expression of chronic hypertension at the population level. Among the most studied of these mechanisms are increased sympathetic nervous system (SNS) activity; alterations in the renin-angiotensin-aldosterone axis; other neurohormonal influences; alterations in the circadian control of BP; exaggerated BP responses to various stimuli; increased sodium sensitivity; excess intake of dietary sodium; impaired renal handling of sodium; endothelial dysfunction; and other chronic alterations in vascular structure and function. These mechanisms are discussed in detail in Chapter XX of this book.
In light of this complexity and the fact that African Americans are not a biologically monolithic population, a definitive pathophysiological basis for their greater prevalence of hypertension remains speculative. Most likely, all of these mechanisms play some role in the long-term maintenance of hypertension in African Americans but the literature suggests that some may play a greater role than others in contributing to the higher prevalence of hypertension in this population. In this section, the current evidence on mechanisms that likely contribute to the pathophysiological basis for hypertension in African Americans is discussed.
Increased Sympathetic Nervous System Activity
Increased SNS activity and an exaggerated adrenergic response to stress are important contributors to acute and chronic BP elevation. Increased SNS activity directly contributes to the initiation as well as chronic maintenance of hypertension through its effect on cardiac output, peripheral vascular resistance, and renal fluid and sodium retention. Several studies demonstrate a greater prevalence of increased SNS activity in African Americans compared with whites. For example, in African American men and women in the CARDIA study, systolic blood pressure (SBP) hyper-responsivity to two laboratory-induced psychological stressors was associated with a higher SBP at 3 years of follow-up. Chronic repeated exaggerated SNS responses to various stressors may be important mechanisms for the increased prevalence of hypertension and target organ damage in African Americans. In fact, it has been suggested that SNS over-reactivity in young adulthood may be an important explanations for both the high incidence of obesity-related hypertension in African-American women and the disproportionately high incidence of hypertension in lean African-American men.
Increased Dietary Salt Intake and Salt Sensitivity
Increased intake of dietary salt, especially in the setting of increased salt sensitivity, has been suggested as an important contributor to the increased prevalence of hypertension in African Americans. Although widely varying methodologies and criteria have been used to diagnose or define salt sensitivity, the phenomenon is generally considered to be present when mean arterial BP increases by at least 5% in normotensive and borderline hypertensive individuals and greater than 10% in hypertensive patients in response to sodium loading. Other definitions require a 10 mm Hg absolute increase or a 10% relative increase between mean arterial pressure on low- versus high-salt diets.
In general, most studies show a greater prevalence of salt-sensitivity in African American hypertensive patients often associated with increased forearm vascular resistance, decreased venous compliance, suppressed plasma renin activity, and reduced circulating aldosterone concentration. The myriad factors that can contribute to salt-sensitivity in African Americans include reduced dietary potassium intake, decreased urinary kallikrein excretion, upregulation of sodium channel activity, and alterations in atrial natriuretic peptide production. Importantly, salt sensitivity has also been associated with increased prevalence of target organ damage and excess mortality, especially from cardiovascular and renal causes independent of the BP rise. These findings provide a firm foundation for dietary salt reduction and the clinical use of thiazide-type diuretic antihypertensive medications as essential components of multidrug therapy for BP control in African American patients.
Impaired Renal Handling of Sodium and Expanded Extracellular Plasma Volume
Abnormalities in the renal handling of sodium excretion, expanded extracellular plasma volume, and impaired tubuloglomerular feedback have been suggested as important factors in the greater prevalence of hypertension and hypertensive renal damage in African Americans. These abnormalities are not present in all African Americans or even the majority of African Americans; nevertheless, their greater prevalence in African Americans compared with whites may contribute to the known racial disparities in hypertension and hypertension-related renal damage. These mechanisms are likely to take on even greater importance when dietary sodium intake is increased in salt-sensitive individuals. For example, in a recent study that examined the association of dietary sodium and potassium intakes with blood pressure separately by race/ethnicity, age, and sex among 1568 participants, Bartley et al. noted that African-American and Hispanic males aged 50 years and younger consumed considerably more sodium and less potassium compared with their white counterparts. Weinberger et al. demonstrated more than three decades ago, the importance of sodium in blood pressure regulation, especially in individuals predisposed to avid sodium conservation.
Renin-Angiotensin-Aldosterone System Activation
Activation of the renin-angiotensin-aldosterone system (RAAS) is one of the primary pathophysiological mechanisms in acute and chronic regulation of systemic BP level as well as a major modulator of cardiovascular structure and function and hypertension-related target organ damage. Not surprisingly, some of the most powerful antihypertensive medications target this system using angiotensin-converting enzyme (ACE) inhibitors, angiotensin II receptor blockers (ARBs), renin inhibitors, and mineralocorticoid receptor blockers. African-American hypertensive patients, especially those who are salt sensitive, have a high dietary sodium intake, and therefore have suppressed circulating plasma renin activity, invoking an activated RAAS seems counterintuitive and paradoxical. However, Michel et al. recently demonstrated in a community sample of African ancestry participants that in the presence of high-sodium, low-potassium diets, and suppressed renin release, RAAS system activation downstream from renin and its impact on BP are maintained in part by circulating angiotensinogen concentrations. In fact, the study further demonstrated a positive relationship between angiotensinogen and serum aldosterone concentrations and SBP, independently of confounders in the setting of high dietary sodium intake. It is therefore reasonable to conclude that the RAAS may play an important role in hypertension in African Americans and the greater prevalence of severe target organ damage.
Circadian Biology and Nocturnal Blood Pressure Levels
Abnormal circadian regulation of BP manifests as an absence or a blunted nocturnal “dipping” of BP, higher average sleep BP, and exaggerated morning BP surge seen during ambulatory monitoring. These derangements have been reported to be associated with increased prevalence of hypertension and hypertension-related target organ damage. Most studies suggest a greater prevalence of blunted nocturnal dipping in African Americans that may contribute to greater prevalence of hypertension and hypertension-related target organ damage. Impaired renal handling of sodium has been suggested as a likely explanation for nocturnal nondipping of BP ; however, many other factors such as physical activity, salt sensitivity, dietary electrolyte intake, sex, body size, socioeconomic status, age, psychological factors, stressful life circumstances, perceived racism, and neighborhood environment all influence the pattern of ambulatory BP variation and therefore confound unadjusted racial comparisons.
Psychosocial Stress
The weight of the evidence suggests that several categories of chronic psychosocial stress including occupational stress, job strain, housing instability, social isolation, and perceived racism and hostility contribute to the onset and maintenance of chronic hypertension. These factors occur more often in African Americans than in whites and have been considered contributory to the greater prevalence of hypertension in African Americans. Although the definitive underlying mechanisms remain incompletely understood, prominent roles have been described for the sympathetic nervous system, neuroendocrine system, renal handling of sodium, endothelial function, and gene-environment interactions.
Strategies for Hypertension Treatment and Control
In the African-American patient with hypertension, an effective strategy for the treatment and control of hypertension must begin with the establishment of a trusting patient-provider relationship and a commitment to follow through on an action plan. The initial clinical history and physical examination help establish a diagnosis and stage of primary hypertension at the same time as clues for secondary hypertension, masked hypertension, or white coat hypertension are explored and excluded. An assessment for the presence and extent of hypertension-related target organ damage, comorbid clinical diagnoses, and determination of short-term and long-term total cardiovascular risk is essential. The initial laboratory tests will be invaluable in the calculation of cardiovascular risk. Additionally, an assessment of the patient’s health literacy, educational level, social support, and self-management skills is necessary. Collectively, these initial assessments and their findings help match the intensity of hypertension treatment strategy to the stage and level of cardiovascular risk of the patient.
Behavioral and Lifestyle Interventions
Behavioral and lifestyle interventions are as important in the African-American patient as they are in other patients. These include changes in diet, physical activity, sleep duration and pattern, weight management, alcohol consumption, and psychosocial stress. Although cigarette smoking does not directly contribute to long-term BP elevation, it contributes to total cardiovascular risk and it is therefore included as an important part of behavioral and lifestyle changes.
Dietary Interventions
A diet that is rich in fruits and vegetables and low in sodium is important in the management of hypertension. This dietary pattern, as used in the Dietary Approaches to Stop Hypertension (DASH) trial, leads to BP reduction in hypertensive patients, an effect that persists as long as the recommended dietary pattern is maintained. This phenomenon has been demonstrated in many adult patient populations including African Americans. For example, in the DASH trial, a combination diet rich in fruits and vegetables and low in saturated fat, total fat, and cholesterol reduced SBP in African Americans (−6.8 mm Hg) and whites (−3.0 mm Hg) and was particularly effective in patients with hypertension, lowering systolic BP by −11.5 mm Hg. In fact, the dietary pattern’s effect on BP was independent of changes in body weight and sodium intake and is considered to be of a magnitude sufficient to prevent progression from prehypertension to hypertension and serve as an important strategy in the nonpharmacologic treatment of hypertension or as a supplement in drug therapy.
Physical Activity Interventions
The independent beneficial impact of regular physical exercise on BP control in hypertensive subjects has been well demonstrated. A recent narrative review of 27 randomized controlled trials of regular medium-to-high-intensity aerobic activity demonstrated mean BP reductions of 11/5 mm Hg in hypertensive persons. Staffileno et al. have demonstrated that tailored interventions that incorporate lifestyle-compatible physical activity in young, hypertension-prone African-American women result in significant reductions in SBP and diastolic BP (DBP) and greater reductions in nocturnal BP load compared with women in the control “No Exercise” group.
Comprehensive Multifaceted Lifestyle Interventions
Ideally, simultaneous implementation of multifaceted interventions that are considered culturally acceptable, affordable, and can be sustained long-term have the greatest potential for most benefit in hypertension control. These interventions include increased physical activity; weight loss or ideal weight maintenance; reduced alcohol intake in those who drink alcohol; strategies to reduce or address psychosocial stress; dietary sodium reduction; increased fruit and vegetable intake; and other dietary approaches to lower BP. The PREMIER trial, whose participants included 34% African Americans and 62% women, was an example of such multifaceted interventions. It demonstrated the feasibility of comprehensive multifaceted interventions and their beneficial effects on BP control in hypertensive patients not on medical therapy as well as in the prevention of hypertension in at-risk subjects with above-optimal BP.
Sleep-Disordered Breathing and Sleep Apnea
Sleep-disordered breathing, manifesting as apneic or hypopneic episodes during sleep, together with reduced duration and quality of sleep have been associated with the development of hypertension. African Americans, compared with whites, have a disproportionately greater risk of having poor sleep quality and duration and, thus, may be at greater risk of sleep-related hypertension. For example, the CARDIA study reported that objectively measured average sleep duration was 6.7 and 6.1 hours for white women and men, respectively, but 5.9 and 5.1 hours for African-American women and men, respectively and that the race-sex differences remained significant ( p < 0.001) after adjustment for socioeconomic, employment, household, and lifestyle factors and for apnea risk. In addition, African Americans are at greater risk of living in environments with a greater exposure to environmental factors that impair sleep duration and quality. Interventions to address adverse sleep habits and the use of continuous positive airway pressure (CPAP) to treat sleep apnea when present can be important strategies in a comprehensive approach to the treatment and control of hypertension.
Drug Treatment
The primary objective in the drug treatment of hypertension is to use safe, effective, and affordable medications to reach goal BP and reduce mortality and morbidity in patients already using behavioral and lifestyle interventions in the long-term control of hypertension. Ideally, drug treatment should be informed by published guidelines that meet national or international standards for trustworthiness. It is not enough for patients to be simply started on antihypertensive medications; every effort must be used to reach goal BP as safely as possible and as tolerated by the patient. Although the threshold BP for initiating drug treatment in hypertension and the goal BP to be attained remain controversial, all recent major guidelines remain consistent in the selection of medications for treating African Americans. In addition, there is now compelling evidence that systemic implementation of specific strategies, such as the use of an evidence-based treatment algorithm and a multidisciplinary approach using community health workers, medical assistants, nurses, and pharmacists as key stakeholders, can result in a similar level of good BP control as seen in patients of other race and ethnicities.
In African-American patients with stage 1 hypertension, including those with diabetes, there is moderate evidence to support initiating treatment with a calcium channel blocker (CCB) or thiazide-type diuretic. There is also moderate evidence to recommend the use of an ACE inhibitor or ARB as initial or add-on drug therapy in the presence of chronic kidney disease (CKD) to improve kidney outcomes. When stage 1 hypertension is complicated by the presence of chronic heart failure, coronary artery disease, or stroke, drug selection from an appropriate drug class is recommended based on the compelling indication. However, the use of an ACE inhibitor is not recommended as monotherapy in African Americans. This is supported by several recent guidelines and findings from recent clinical trials suggesting that hypertensive African Americans have higher risk of cardiovascular events when treated with an ACE inhibitor-based regimen compared with CCBs or thiazide diuretics. For example, in one cohort study of patients using data from a clinical data warehouse of 434,646 patients from January 2004 to December 2009, a propensity score-matched comparisons, ACE inhibitors were associated with a higher risk of primary outcome, MI, stroke, and heart failure when compared with CCBs or thiazide-type diuretic ( Fig. 41.5 ).
