The Nature Of Hypertension

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Hypertension is a raised systemic arterial blood pressure (BP). However, BP is a continuously distributed variable and the numerical boundary between nor-motension and hypertension is arbitrary and is based on the increasing cardiovascular risk, in particular stroke, as BP rises (Fig. 1) [7]. A WHO-based classification of hypertension is shown in Table 1. Considering end-point trials of cardiovascular risk (more specifically, stroke), it is now widely accepted that maintaining BP below 140/90 mm Hg is beneficial and that a BP of >140/ 90 mm Hg is therefore considered "abnormal" [ 10]. However, the level at which pharmacological treatment is used differs between Europe and North America. In North America patients with a diastolic BP of 85 mm Hg or greater are more likely to be given drug treatment to lower BP, but in Europe the criterion for starting antihypertensive drug therapy is approximately 10 mm Hg higher. An isolated numerical definition of hypertension of 140/90 mm Hg or more fails to take into account the normal distribution of BP, its variability, and the contribution of other factors to cardiovascular risk. If rigidly applied it encompasses over 50% of the elderly (> 60-year-olds) population, which is an increasing proportion in developed countries [7], This has significant cost implications, particularly since individual benefit from treatment is so small. Swales [12],

DBP (mmHg)

FIGURE 1. Representation of relative risk of stroke and coronary heart disease (CHD) in relation to diastolic blood pressure (DBP).

using data from the recent MRC trial, has compared the number of patients it is necessary to treat for 1 year to prevent one stroke at each of three ranges of diastolic BP: 105-109 mm Hg, 333; 100-105 mm Hg, 666; 95-100 mm Hg, 2000. Another important issue is how BP should be measured. Initial recordings are often unrepresentative and repeated measurements over several months are necessary. "White coat hypertension" must be excluded, although it may not be as benign a condition as originally thought, and could be a prehyperten-sive state in some individuals. Ambulatory BP recordings are more accurate and reliable, and they correlate better with target organ damage and cardiovascular risk. However, which component of a recording (mean value, variability, diurnal pattern—day versus night) is best, and should be the basis of treatment, is unclear.

Both systolic and diastolic BP are related to cardiovascular risk (Figs. 1 and 2), though the emphasis is mainly on diastolic BP, despite epidemiological data

TABLE 1 WHO-Based Classification of Hypertension (1993)

Systolic blood

Diastolic blood

pressure

pressure

(SBP) (mm Hg)

(DBP) (mm Hg)

Normotension

<140

<90

Mild hypertension (Stage 1/2)

140-180

90-105

Moderate-to-severe hypertension (Stage 3/4)

>180

>105

Isolated systolic hypertension

>140

>90

V 175 g 150

130-139

160-169

V 175 g 150

130-139

160-169

190-300

SBP (mmHg)

FIGURE 2. Risk of cardiovascular disease related to systolic blood pressure (SBP) at a diastolic blood pressure of <95 mm Hg (based on Framingham 20-year follow-up data).

showing a closer correlation between systolic BP and the risk of cardiovascular events (Fig. 2) [1], This is particularly relevant to systolic hypertension in the elderly, which is defined as a systolic BP >160 mm Hg and a diastolic BP <90 mm Hg, and which is found in almost 25% of those over 65 years old. It is now evident that treating this form of hypertension and lowering systolic BP significantly reduces the incidence of stroke and myocardial infarction [6]. What is more, there is no specific target level of BP, since any reduction is beneficial in terms of reduced cardiovascular risk. However, this must be balanced against the common side-effects of treatment, especially postural hypotension in the elderly. Moreover, there is some evidence, though highly controversial, that aggressive treatment of hypertension to lower diastolic BP below 80 mm Hg is associated with an increase in mortality, the so-called "J-shaped" curve

Hypertension per se must be recognized as a cardiovascular risk factor and considered in the context of other associated risk factors for cardiovascular disease in the individual patient. For example, the patient with a BP of 140/100 mm Hg and diabetes mellitus, or who is overweight, a heavy smoker, drinks too much

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