Key epidemiological factors in hypertension, the definition of hypertension, the at-risk population, and the causes and impact, are reviewed in this lecture.

Hypertension is a sustained level of blood pressure above which treatment is more beneficial than harmful. The guidelines should be evidence-based, and reflect the experimental data. Many factors must be considered to establish the specific numbers to define hypertension. These factors include what is considered sustained blood pressure, whether systolic or diastolic or both blood pressures should be considered, the position of the patient when the blood pressure is measured (supine, sitting, or standing), the location where the blood pressure is measured (clinic or home), the number of blood pressure readings performed, and the method of measurement (oscillometric, sphygmomanometric). Currently, hypertension is defined by the major treatment guidelines as a blood pressure above 140/95 mmHg.

Person at risk of Hypertension
 

The prevalence of essential hypertension increases steadily with age. In older age, more women than men develop essential hypertension, and there are more women in that age group. Blacks are more prone to develop hypertension than Caucasians. Abdominal obesity is a large contributing factor in Black men and women (with an odds ratio of 1.58 for men and 1.39 for women) to the probability of developing hypertension, whereas this has little effect in Caucasians (with an odds ratio of 0.96 for men and 1.08 for women). Age has an odds ratio of 1.05 for Black and Caucasian men, 1.08 for Black women and 1.07 for Caucasian women. Alcohol is a more important contributing factor in Caucasian men and women (with an odds ratio of 1.15 and 1.22, respectively) than in Black men and women (with an odds ratio of 1.00 and 0.82, respectively). [NHANES III, Okosun et al., 2001].
Cohort data from the Framingham Heart Study show in both men and women that systolic blood pressure increases with age, whereas diastolic blood pressure flattens out at about age 50-60 years and then begins to decrease. This gives rise to the fact that very old people have a high pulse pressure. From menarche to menopause, blood pressure increases steadily in women, and after menopause there are more women with hypertension than men in the same age group.
As systolic blood pressure rises, the associated risk rises (Figure 1). The death rate is 45 per 1000 persons per year for a systolic blood pressure of 130-139 mmHg in the age group of 60-69 years, which increases to 70 deaths per 1000 for a systolic blood pressure of 150-159 mmHg, and to 80 deaths per 1000 for a blood pressure greater than 160 mmHg.

For men 40-59 years, systolic and diastolic blood pressure increases steadily with age. Importantly, it must be remembered that diastolic blood pressure is more important than systolic blood pressure up to the age of 40 years and perhaps up to age 50 years.
Hypertension occurs in all industrialized societies, and does not appear to be related to race or ethnicity. As countries in Africa and Asia have become industrialized, hypertension has occurred. Factors that influence this incidence of hypertension is increased age, increased sedentary lifestyle and decreased exercise, and increases in tobacco smoking, alcohol intake, weight and body mass index, salt intake, saturated fat intake, and stress.

Genetics, diet, obesity, smoking and alcohol contribute to the development of hypertension.

Genetic data. In identical twin studies, systolic blood pressure differs by about 5 mmHg, whereas in fraternal twins blood pressure differs about 8 mmHg, showing that genetic factors do play a role in the incidence of hypertension.

Salt intake. Data from 1961 that plotted daily salt intake against the percentage of persons with hypertension showed a linear increase in hypertension as salt intake increased (Figure 2). In Pacific Marshall Islanders whose daily salt intake was about 8 gm, about 8% of the population had hypertension, compared to about 38% of the Japanese population in Northern Japan where the daily salt intake was 25 gm.

Exercise. A meta-analysis by Fagard in 2000 showed that exercise was associated with a reduction of 7/5 mmHg in blood pressure in persons with hypertension. This exercise-induced blood pressure reduction is independent of weight loss-induced blood pressure reduction (1 kg weight loss lowers systolic blood pressure by about 2 mmHg).

Stress. Using a Scandinavian model of stress accounting for the amount of work and the degree of control by the subject, a study in the London Civil Service found little sustained effect on blood pressure, despite some acute effects on blood pressure. However, there appears to be a relationship between higher levels of stress and high sodium intake and increased blood pressure, as shown by a study by Staessen and colleagues (Figure 4). For example, a person in the highest tertiles for stress and sodium intake had a systolic blood pressure of 140 mmHg, compared to a systolic blood pressure of 133 mmHg for a person in the lowest tertile of sodium and highest tertile of stress.

Alcohol. Data from Klatsky and colleagues in North America show that at about 28 units of alcohol intake per week there is an increase in blood pressure independent of obesity.The contraceptive pill. The currently available low dose oral contraceptive pills do not increase blood pressure, in contrast to the older, higher dosage pills. Hormone replacement therapy also has no effect on blood pressure.

Intra-uterine factors. The higher the birth weight of a baby, the lower the blood pressure in later life.

Drug treatment should be tailored to the geographic region. In the US, where the rate of ischemic heart disease deaths is 4.6-fold that of stroke deaths, the use of drugs shown to have a maximum benefit in preventing ischemic heart disease should be used. In Japan, where the rate of stroke deaths is higher, drugs shown to maximally prevent stroke should be used.

Dietary measures that includes increased intake of protein, fiber, fruits and vegetables and decreased salt intake. Weight loss should also be a goal.
The attributable risk is greatest in the persons with borderline hypertension, who comprise the majority of the population who suffer strokes and coronary heart disease. Cardiovascular risk increases markedly at very high levels of blood pressure. But it should not be overlooked by clinicians that risk also increases at medium levels of blood pressure, e.g., 80-85 mmHg compared to lower levels of blood pressure. Fewer patients have very high levels of blood overlooked by clinicians that risk also increases at medium levels of blood pressure, e.g., 80-85 mmHg compared to lower levels of blood pressure. Fewer patients have very high levels of blood pressure, while many people have the medium levels of blood pressure and are thus at risk. Thus, the high risk population must be identified, but the people with medium levels of blood pressure should comprise the major focus of treatment efforts, including lifestyle modifications, and thus the population approach, to lower everbody's pressure to a small extent, has a majore role to play.

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