We have discovered that systolic blood pressure (the top number or highest blood pressure when the heart is squeezing and pushing the blood round the body) is more important than diastolic blood pressure (the bottom number or lowest blood pressure between heart beats) because it gives the best idea of your risk of having a stroke or heart attack.
We know, for example, that having a blood pressure of 160/80mmHg is more “risky” than having a blood pressure of 150/90mmHg. Having a raised systolic blood pressure but normal or low diastolic blood pressure is called Isolated Systolic Hypertension (ISH) and carries an increased risk of developing heart attacks or strokes and should be treated.
However, there are some circumstances where diastolic blood pressure may be more important than systolic. For example, some studies suggest that, in people aged younger than 40 years, diastolic blood pressure is a better way of assessing risk. However, younger people are less likely to have a stroke or heart attack, so information on their risk of future problems is limited. It could be that diastolic blood pressure becomes more important when it is very high. There is some evidence to suggest that, for example, a blood pressure of 180/120mmHg gives a greater risk of stroke or heart attack than 180/100mmHg.
The only way to resolve this issue is to obtain data from thousands of patients collected on a systematic basis. The statistical tests to investigate the relative importance of systolic and diastolic blood pressure are immensely complicated. However, current evidence strongly suggests that, over the age of 40, it is systolic pressure that is most important.