High Systolic but Normal Diastolic Blood Pressure: Isolated Systolic Hypertension

A healthy aorta works like an elastic reservoir. During the heart's contraction it stretches and stores part of the blood, and during relaxation it releases it to the periphery, smoothing the pressure wave. With age, and also with hypertension, diabetes or kidney disease, elastin fibres break down and the vessel wall stiffens. A stiff aorta does not 'cushion' the pulse wave: systolic pressure rises quickly, and during diastole it falls lower because the vessel no longer releases the stored energy. The result is isolated systolic hypertension and a wide pressure amplitude. What is more, a stiff aorta transmits a large pulsation to organs with low vascular resistance, such as the brain and kidneys, which explains the link with stroke, dementia and decline in kidney function.

How the role of the different components of blood pressure changes with age was shown by the classic Framingham study, which followed more than 6,500 people for 20 years.

The practical conclusion: after age 50 you must not be reassured by a good 'bottom number'. What matters most is the systolic value and the amplitude of the pressure. At the same time, a very low diastolic pressure in an older person with a high systolic is a sign of a very stiff aorta and calls for caution when intensifying treatment, as we discuss in the practical section.

A different situation concerns people between 18 and 40, mainly tall, slim men, in whom the clinic shows, for example, 145/75 mmHg. Here aortic stiffness is usually not the main cause. One hypothesis points to increased amplification of the pulse wave on its way from the aorta to the brachial artery (pulse pressure amplification): central pressure, that is pressure in the aorta, is then normal while pressure in the brachial artery is higher. A second points to increased sympathetic activity and a hyperkinetic state, with a fast heart rate and large stroke volume, often intensified by an alarm reaction to the doctor's visit. Subsequent research has shown that ISH in the young is a heterogeneous phenomenon.

Since ISH in the young is heterogeneous, it is crucial to confirm the diagnosis outside the clinic: with home measurements or 24-hour blood pressure monitoring (ABPM). If the average home or 24-hour pressure is normal, you are dealing with white-coat or spurious hypertension; if not, ISH is real and worth assessing further. We write about the difference between white-coat and true hypertension in a separate article.

All these studies are observational, so they show an association and do not prove that treating ISH in the young improves prognosis. It is also worth remembering that among people with ISH in these cohorts smoking, obesity and hypercholesterolaemia were more common, so part of the risk comes from accompanying factors that can be modified.

In people over 60 the situation is far better documented. Two classic randomized, placebo-controlled trials established the standard of treatment for isolated systolic hypertension.

Two interpretive remarks. First, both trials enrolled people with systolic pressure of at least 160 mmHg, so for the 140-159 mmHg range we rely on later trials and guidelines that broadened the indications, especially in people at higher risk. Second, in these trials all-cause mortality did not fall significantly; the documented benefit was a reduction in strokes and cardiovascular events. This does not diminish the value of treatment, but it lets you realistically expect its effects.

Stiffening of the arteries is the most common but not the only cause of a wide pressure amplitude. The classic secondary causes that a doctor considers, especially in younger people or when the picture is atypical, are: an overactive thyroid (increased cardiac output), aortic valve regurgitation (part of the blood flows back into the ventricle during diastole, so diastolic pressure falls), severe anaemia, arteriovenous fistulas, and hyperkinetic states associated with fever, pregnancy or certain drugs and stimulants. In such cases treating the cause often normalizes the pressure, which is why sensible diagnostics start with simple laboratory tests and a clinical examination of the heart.

Factors that favour ISH in adults also include excess weight and insulin resistance, too much salt in the diet, a sedentary lifestyle and smoking. How salt affects blood pressure is covered in our separate article on salt sensitivity, and the thyroid in our knowledge-base entry on TSH and thyroid hormones. All of these factors can be partly modified.

Recommendations on the DASH diet and on reducing salt are described more fully in our knowledge-base entry on the DASH diet. In people with ISH who are treated with drugs, the doctor usually aims to bring systolic pressure down to target values while carefully watching diastolic pressure and tolerance of treatment. Detailed thresholds and targets depend on the guidelines and the risk profile (in the 2024 ESC guidelines the general direction is to achieve a systolic pressure of about 120-129 mmHg if the drug is well tolerated), so they are set by the doctor, not the patient.

Limitations. The data on young adults are purely observational and do not settle whether drug treatment of ISH in this group improves prognosis. The SHEP and Syst-Eur trials enrolled people aged at least 60 with systolic pressure of at least 160 mmHg, so their results cannot be mechanically transferred to younger patients or to lower blood pressure values. In the cohorts the relative risk is moderate and the absolute risk at a young age low. Diagnostic thresholds differ between guidelines (140/90 mmHg in the European ones, 130/80 mmHg in the American ones from 2017). Finally, the ESC recommendations are subject to updating, and some of them rest on expert opinion.

A high systolic with a normal diastolic pressure is not a 'milder' version of hypertension. In people over 60 it is a well-studied risk factor for stroke that responds well to treatment. In young adults it is a signal to first check that it is not a clinic artefact, and then - if the ISH persists - to take it seriously and deal with the factors you can influence. One thing is common to both groups: base the decision on reliable measurements outside the clinic, not on a single reading.