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I know that the upper limits for systotic/diastotic blood pressure are 120/80. What is the mathematics behind these numbers? I am curious because I did not find information regarding how the numbers are arrived at. I am not sure if the numbers are based on statistical study or is there a scientific basis for the same.

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120/80 are not upper limits on blood pressure - many people have blood pressures far in excess of those numbers. Instead, these are numbers beyond which studies have shown connections to negative health outcomes - heart disease, for example.

There is nothing magical about the numbers 120 or 80 besides their divisibility by 10; a blood pressure of 121/81 is no more risky than 120/80 than 122/82 is over 121/81. Because of this, drawing a hard line isn't possible - it's just a suggestion.

Typically, systolic pressures less than 130-135 aren't given any special consideration beyond normal advice to exercise and eat well, unless there are other conditions/risks. Guidelines from the American Heart Association and the UK's NHS are to consider treatment only for diastolic pressures above 140mmHg. There has been some disagreement about these precise limits, with some recommendations for lower targets, for example here. Some of the disagreement seems to be the types of interventions - the guidelines to treat diastolic hypertension only above 140mmHg are mostly considering medications, whereas the guidelines to treat prehypertension are aiming for improved lifestyle: diet, exercise, etc.

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  • $\begingroup$ Thanks. Can you point to a specific study that drew these numbers? I am not familiar with the literature around this. I am curious about the way the sampling is done and the statistical methods that helped that highlighted these number. $\endgroup$
    – randominstanceOfLivingThing
    Commented Oct 21, 2016 at 2:25
  • $\begingroup$ Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted. $\endgroup$
    – MattDMo
    Commented Oct 22, 2016 at 19:40
  • $\begingroup$ Added references; for some of your questions about sampling etc, @randominstanceOfLivingThing, those questions might deserve their own topic $\endgroup$
    – Bryan Krause ♦
    Commented Oct 24, 2016 at 15:34

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