Systolic blood pressure

Elevation of systolic blood pressure predicts the risk of cardiovascular disease better than increases in diastolic blood pressure.¹ Although this was observed more than three decades ago, no attempt was made to translate this evidence into practice until in 1993, when a report of the fifth joint national committee of the United States for the detection, evaluation, and treatment of high blood pressure recognised isolated systolic hypertension as an important target for the control of blood pressure.² Nevertheless it is the elevation in systolic blood pressure that still limits our ability to control blood pressure to the recommended goal of less than 140/90 mm Hg.³

Although associated with more variability in measurement, systolic blood pressure is easier to determine and allows more appropriate risk stratification than diastolic blood pressure. In a recent analysis of the Framingham heart study, knowing only the systolic blood pressure correctly classified the stage of blood pressure in 99% of adults over age 60 whereas knowing the diastolic blood pressure allowed only 66% to be classified correctly.⁴ Isolated systolic hypertension is defined as a systolic blood pressure more than or equal to 140 mm Hg and a diastolic blood pressure less than 90 mm Hg and is the most common form of hypertension.⁴ Its prevalence increases with age occurring in two thirds of people 65 years of age and three quarters of those over 75 years of age.⁵

In people aged up to 50, both diastolic blood pressure and systolic blood pressure are independently associated with cardiovascular risk. At age 50 systolic blood pressure is far more important than the level of diastolic blood pressure in predicting the risk of coronary heart disease, left ventricular hypertrophy, congestive heart failure, renal failure, and mortality in people with hypertension. At age 60 years, however, as vascular compliance is reduced, an increasing systolic blood pressure and a lower diastolic blood pressure increase cardiovascular risk.⁶

Age related physiological changes explain the frequent development of isolated systolic hypertension in older people. Younger people have a highly distensible aorta, which expands during systole and minimises any subsequent rise in blood pressure. Most older people, however, develop progressive stiffening of their arterial tree as they age, which leads to a continuous elevation in systolic blood pressure.⁷ With the diastolic blood pressure remaining normal or decreasing with age, elderly people develop a widening of their pulse pressure (the difference between the systolic and the diastolic blood pressure). The elevation in systolic pressure increases left ventricular work and the risk of left ventricular hypertrophy, whereas the decrease in diastolic blood pressure may compromise coronary blood flow.⁸ This widening of the pulse pressure at specified levels of systolic blood pressure, as assessed in the Framingham heart study, is associated with an increased risk of developing coronary heart disease.⁶ In the absence of trial based evidence that uses pulse pressure narrowing as a target for improving outcome, however, lowering systolic blood pressure to a specific goal continues to be recommended as the major criterion for the management of hypertension, especially among middle aged and older people.⁵

The benefits of treating systolic blood pressure have been well documented. Trials have shown significant reductions in stroke, coronary vascular disease, heart failure, and mortality when treating patients with isolated systolic hypertension (systolic blood pressure more than 150 or 160 mm Hg, diastolic blood pressure less than 90 mm Hg).⁹,¹⁰ When systolic blood pressure was reduced by at least 20 mm Hg and to less than 160 mm Hg or less than 150 mm Hg, a 35-40 % reduction in stroke, a 50% reduction in heart failure, a 16% reduction in coronary events, and a 10-15% reduction in mortality occurred.⁹,¹⁰ The benefits of treating stage 1 isolated systolic hypertension (140 mm Hg or greater with a diastolic blood pressure below 90 mm Hg) have not yet been shown in a clinical trial. Although none of the clinical trials achieved a systolic blood pressure below 140 mm Hg, a consensus statement implies that outcome should improve further when this goal is achieved.¹¹

Systolic blood pressure remains more difficult to control than diastolic blood pressure.³ Nevertheless, doctors should be able to lower systolic blood pressure to less than 140 mm Hg in about 60% of patients. A diuretic and a dihydropyridine calcium antagonist are the only classes of drugs that have been tested as initial treatment in placebo controlled trials on isolated systolic hypertension. If a diuretic is used, potassium concentrations should be kept as close as possible to normal.¹² If not used initially, a thiazide diuretic should be included in most regimens to enhance the efficacy of other blood pressure lowering agents and reduce the risk of ischaemic stroke.^w1 Since two or more agents are often necessary to reach the target of 140 mm Hg, caution should be exercised when lowering diastolic blood pressure to less than 55 mm Hg.^w2

Lifestyle changes are also beneficial in controlling blood pressure in elderly patients. Restricting salt intake to 80 mmol daily reduces systolic blood pressure by 4.3 mm Hg and diastolic blood pressure by 2 mm Hg, and a combination of weight loss and salt restriction reduces blood pressure more than either strategy by itself and decreases the need for antihypertensive treatment.^w3

Isolated systolic hypertension remains the most common form of hypertension and the most difficult to treat.^w4 Substantial evidence supports the value of treating isolated systolic hypertension, and we must better inform doctors and the public about its consequences. It seems appropriate that we continually focus our efforts on more effective control of systolic blood pressure.