DEFINITION/DIFFERENTIAL DIAGNOSIS
Hypertension is a leading cause of mortality worldwide. Although the association between blood pressure and cardiovascular (CV) risk exists at every level of systolic and diastolic blood pressure, the diagnosis of hypertension is established by exceeding a blood pressure threshold of 140/90 mm Hg on two occasions (
1). Repeated measures of blood pressure will determine whether initial elevations persist and require immediate attention or return to normal values and require only surveillance. The 140/90 mm Hg cut point correlates to an acceleration in CV risk that has been established from natural history studies (the Framingham Heart Study) (
2). However, the amount of CV risk that is acceptable in a population may vary from country to country. For example, historically Canada and Europe accept higher blood pressure cut points to confirm the diagnosis of hypertension (
3). Over the decades, the United States has modified the threshold of elevated blood pressure that confers the diagnosis of hypertension, from 160/90 mm Hg to a current value of 140/90 mm Hg, thereby increasing the prevalence of hypertension from approximately 14.5% of the population to 23% (
Table 12.1) (
4).
The classification of blood pressure in the adult population is presented in
Table12.2. The reclassification of blood pressure according to the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC-7) is based on new data on lifetime risk of hypertension and its associated risk for cardiovascular complications. This new description of blood pressure levels has been categorized as normal blood pressure, prehypertension, stage I, and stage II hypertension.
Prehypertension is not a disease category. Rather, it is a designation chosen to identify individuals at high risk of developing hypertension, so that both patient and clinician are alerted to this risk and encouraged to intervene and prevent or delay the disease from developing (
5). Patients identified as prehypertensive are not candidates for drug therapy on the basis of their level of blood pressure, but are encouraged to practice lifestyle modifications as an effort to reduce their risk of developing hypertension in the future. However, prehypertensive patients with risk factors of diabetes and kidney disease for whom lifestyle modifications have failed to reduce their blood pressure to 130/80 mm Hg or less are candidates for immediate pharmacologic treatment (
5). In addition to classifying stages of hypertension on the basis of average blood pressure levels, clinicians should specify presence or absence of target-organ disease and additional risk factors. Patients identified as having stage I and II hypertension should be treated to a BP goal of 140/90 or less (
5). Lifestyle modification should be relegated to adjunctive therapy for all patients recommended for pharmacologic therapy. Risk stratification and treatment of prehypertensive and hypertensive patients are shown in
Table 12.3.
Several current national guidelines advocate even lower levels of blood pressure in the subpopulations of hypertensive patients at higher risk for cardiovascular disease
(diabetes and renal insufficiency), targets below 130/80 mm Hg (
6,
7). However, the recommendations for aggressive targets for blood pressure control are predicated on a paucity of clinical trial evidence demonstrating greater benefit than that seen with achieving the more conventional target of 140/90 mm Hg. Prospective trials are needed to provide the evidence from which clinicians could make changes in patient management for more aggressive blood pressure targets (
8). The existing data available on aggressive blood pressure control are reviewed in this chapter (Management of Hypertension).
Natural history surveys indicate that both SBP and diastolic blood pressure (DBP) confer risk for CVD; however, national guidelines prior to 1997 placed emphasis on the DBP for the purpose of defining hypertension. Current guidelines use both systolic and diastolic BP in the diagnosis and staging of hypertension. Moreover, SBP level may predict CV risk better than the DBP level (
6). Isolated systolic hypertension, a consequence of aging and a major risk for cardiovascular disease, is defined as SBP of 140 mm Hg or higher and DBP less than 90 mm Hg. Data from the Framingham Heart Study show that most uncontrolled blood pressure involves isolated SBP with ensuing excess development of coronary heart disease, stroke, cardiac failure, and peripheral artery disease (
9).
Considerable evidence indicates that pulse pressure (the difference between SBP and DBP) may provide even greater prognostic information on CV risk than either systolic or diastolic. In the Framingham Heart Study, middle-aged and elderly persons with SBP of 120 had increased CVD risk as the DBP decreased. This suggests that higher pulse pressure is an important component of risk. Ultimately, pulse pressure can be reduced or controlled only by targeting the SBP (
6).
Recently the measure of central blood pressure, as estimated from tonometric devices placed on the radial pulse, has emerged as an important blood pressure parameter. These noninvasive measures of arterial wave contour allow calculation of peripheral compliance and an estimate of aortic pressure. In the Anglo-Scandinavian Cardiac Outcomes Trial (ASCOT) trial, the antihypertensive therapy that was effective at reducing both peripheral and aortic blood pressure reduced cardiovascular risk greater than just reducing peripheral blood pressure alone (
10). These results suggest additional risk prediction from measuring central blood pressure; however, more evidence from prospective trials is needed before adopting this strategy. Ultimately, all measures of blood pressure (systolic, diastolic, pulse, and central pressure) are important for estimating cardiovascular risk. However, it remains clinically prudent to focus special attention on reducing SBP for reducing global cardiovascular risk.
Gender and ethnicity have a significant effect on the prevalence of hypertension in the United States. The estimated prevalence rate of hypertension is approximately 23% in the general population but as high as 36% among African Americans (
Table 12.1). Hypertension occurs most frequently in the fifth and sixth decades of life. African Americans have higher incidence rates of hypertension occurring at earlier ages (
7).
