Chapter 1
The prevalence and causes of hypertension

OVERVIEW

• The distribution of blood pressure in the general population is as a continuous variable forming a roughly normal of 'Gaussian' curve, with no clear dividing lines between low, normal or high readings.
• The dividing lines above which an individual is considered to have hypertension are pragmatic, based on the results of the many placebo-controlled trials of antihypertensive drug therapy.
• From a practical point of view, blood pressures of 140/90 mm Hg or more are considered to be raised and some individuals whose pressures are persistently in this range would be considered to require drug treatment.
• The prevalence of 'clinical' hypertension increases with advancing age. Five to ten percent of teenagers have a blood pressure of 140/90 mm Hg or more at first screening. At the age of 80 years, this figure rises to a 70-75%.
• Average blood pressures and the prevalence of hypertension are higher in people of African origin in Western countries. Hypertension is rapidly becoming commoner in all developing countries.
• In about 5% of all hypertensives, underlying renal or adrenal diseases are identifiable (secondary hypertension). In the remaining 95%, no underlying cause can be found (essential or primary hypertension).
• Essential hypertension runs in families and part of this tendency is related to genetic factors. No single gene is related to essential hypertension; to date, around 27 candidate genes have been investigated, but they only explain a 1-2 mm Hg variation in blood pressure.
• Several lifestyle factors are implicated in the causation of hypertension. These include obesity, heavy alcohol consumption, a low intake of fruit and vegetables and lack of exercise.
• The most important lifestyle factor causing hypertension is a diet with a high salt content as is common in almost all developing countries.

Blood pressure in populations

In the population, blood pressure is a continuous, normally distributed variable. No separate subgroups of people with and without hypertension exist. A consistent continuous gradient exists between usual levels of blood pressure and the risk of coronary heart disease and stroke, and this gradient continues down to blood pressures that are well below the average for the population (Figure 1.1). Above blood pressures of 115/70 mmHg, the risk of developing cardiovascular events doubles for every 20/10mmHg rise in blood pressure. This means that much of the burden of renal disease and cardiovascular disease (CVD) related to blood pressure can be attributed to blood pressures within the so-called 'normotensive' or average range for Western populations. Most cardiovascular events are therefore blood pressure-related rather than hypertension-related.

Figure 1.1 The distribution of diastolic blood pressure in the general population, the risk of cardiovascular disease (CVD) and the number of people who develop CVD.

The main concern for clinicians is what level of blood pressure needs drug treatment. The pragmatic definition of hypertension is the level of blood pressure at which treatment is worthwhile. This level varies from patient to patient and balances the risks of untreated hypertension in different types of patients and the known benefits of reducing blood pressure, while taking into account the disadvantages of taking drugs and the likelihood of side effects.

Hypertension: a disease of quantity not quality

'In an operational sense, hypertension should be defined in terms of a blood pressure level above which investigation and treatment do more good than harm.' Grimley Evans J, Rose G. Hypertension. Br Med Bull1971;27:37-42

Systolic blood pressure continues to rise with advancing age, so the prevalence of hypertension (and its complications) also increases with age. By contrast, diastolic pressures tend to level off at the age of about 50 years and tend to decline thereafter (Figure 1.2).

Figure 1.2 Average systolic and diastolic blood pressures in men and women in the Birmingham Factory Screening Project. This figure excludes 165 patients who were receiving antihypertensive drugs.

Source: Reproduced with permission from Lane, D.A., et al. (2002) Journal of Human Hypertension, 16, 267-273.© Nature Publishing.

Hypertension thus is as much a disorder of populations as of individual people. Globally, high blood pressure and its vascular consequences, heart attack and stroke, account for more deaths than any other common medical condition and is a major burden of disease (Figure 1.3).

Figure 1.3 Worldwide causes of death in 2002 in millions. CKD: chronic kidney disease.

Source: Adapted from Mackay, J., & Mensah, G.A. (2004) The Atlas of Heart Disease and Stroke. World Health Organisation, Geneva.

As hypertension is the most important risk factor for CVD, achievement of a universal target systolic blood pressure of 140 mm Hg or less should produce a reduction of 28-44% in the incidence of stroke and 20-35% of coronary heart disease. This could prevent about 21 400 deaths from stroke and 41 400 deaths from coronary heart disease in the United Kingdom each year. It would also mean about 42 800 fewer fatal and nonfatal strokes and 82 800 fewer coronary heart disease events per year in the United Kingdom alone. Globally, as hypertension is becoming more common, coronary heart disease and stroke correspondingly are becoming common, particularly in developing countries.

A recently published analysis of pooled data from different regions of the world estimated the overall prevalence and absolute burden of hypertension in 2000 and the global burden in 2025. Overall, 26.4% of the adult population in 2000 had hypertension and 29.2% were projected to have this condition by 2025. The estimated total number of adults with hypertension in 2000 was 972 million: 333 million in economically developed countries and 639 million in economically developing countries. The number of adults with hypertension in 2025 is thus predicted to increase by about 60% to a total of 156 billion.

The development of hypertension reflects a complex and dynamic interaction between genetic and environmental factors. In some primitive communities in which obesity is rare and salt intake is low, hypertension is virtually unknown, and blood pressure does not increase with advancing age.

Studies have investigated Japanese people migrating from Japan to the west coast of America. In Japan, high blood pressure is common and the incidence of stroke is high, but coronary heart disease is rare. When Japanese people migrated across the Pacific Ocean to California, a reduction in the prevalence of hypertension and stroke was seen, but the prevalence of coronary heart disease (CHD) increased. These studies strongly suggest that, although racial differences exist in the predisposition to hypertension, environmental factors still play a significant role.

The United Kingdom also has a pronounced north-south gradient in blood pressure, with pressures higher in the north of the country. Studies that compare urban and rural populations in African populations also show clear differences in blood pressure between urban and rural societies with the same genetic composition.

In the United Kingdom, hypertension accounts for approximately 12% of Primary Care consultation episodes and approximately £1 billion in drug costs in 2006. The diagnosis, treatment and follow-up of patients with hypertension is one of the most common interventions in primary care, particularly since the National Service Frameworks for CVD prevention includes routine screening for hypertension.

Prevalence

Depending on age, in up to 5% of people with hypertension in the general population depends on the arbitrary criteria used for its definition, as well as the population studied. In 2853 participants in the Birmingham Factory Screening Project, the odds ratios for being hypertensive after adjustment for age were 1.56 and 2.40 for African-Caribbean men and women, respectively, and 1.31 for South Asian men compared with Europeans (Table 1.1).

Table 1.1 The prevalence of hypertension (=160/95) in three ethnic groups in the Birmingham Factory Screening Project. Insufficient numbers of South Asian women were examined to provide meaningful prevalence rates

The Third National Health and Nutrition Examination Survey 1988-91 (NHANES III) showed that 24% of the adult population in the United States, which represents more than 43 million people, have hypertension (>140/90 mm Hg or receiving treatment for hypertension). The prevalence of hypertension varied from 4% in people aged 18-29 years to 65% in people older than 80 years. Prevalence is higher among men than women, and the prevalence in African-Americans is higher than in...