Why is 120/80 Considered Normal Blood Pressure? What's the logic behind 120 and 80? Is that value normal at any age, or does "normal" change?

 

It does change, and there is nothing “normal” about it.

120/80 mmHg is considered normal because a panel of medical professionals has said so.

We call this a consensus statement.

These statements are formulated and promoted by medical organizations like the American Heart Association, the American College of Cardiology, or their European equivalents.

These consensus statements are typically translated into treatment guidelines, and doctors are encouraged to follow those guidelines.

Yet these values (120/80 mmHg) do not represent a biological threshold.

The motivation to formulate such thresholds comes from the motivation to prevent cardiovascular disease (CVD, e.g. angina, heart attack, stroke, vascular dementia, kidney and heart failure).

So, researchers look at how blood pressure and cardiovascular disease correlate, ideally in very large groups of people.

Many of such studies have been done in many different populations and the general finding was that hypertension (high blood pressure, above “normal”) and CVD correlate with each other.

Now there are two problems that are not often talked about in the popular media.

First, correlation can never prove causation.

And, second, has that “normal” value been derived using the correct statistic?

Neither of these two points supports the 120/80 “normal” threshold.

But here it is in brief:

Blood pressure increases as we age. Hence, what is “normal” for a 25-year old is not normal for a 65-year old.

If we were to base our recommendation of what is normal on the association between blood pressure and CVD, the following equation describes reality much better [1]:

For men: Systolic BP = 120 + (2/3 x age)

For women: Systolic BP = 114 + (5/6 x age)

These equations come from a re-examination of the data that had prompted the “120/80” guideline in the first place.

The re-examination, however, used the correct statistic (as I also explained in my answer to the other Quora question).

Coming back to the correlation-causation conflict. If higher blood pressure causes disease, then lowering blood pressure should reduce the incidents of disease.

But that’s not happening. At least not in the range of blood pressure elevation that covers most hypertensives.

A meta-analysis of studies which investigated the outcomes in treated vs. untreated hypertension (140-160 mmHg systolic & 90-100 mmHg diastolic) concluded that there was no benefit from treatment for heart disease, stroke or total cardiovascular events.

In fact, almost 10% of treated patients discontinued treatment, because of negative side-effects.

This meta-analysis was performed by the non-profit Cochrane Collaboration.

I mention “non-profit” because the annual market for antihypertensive drugs is worth US $ 20” Billion.

A large number of the studies that are used to develop medical guidelines are financed by the industry.

Let’s look at the association between positive or critical study results and their authors’ ties to the industry.

In the case of the antihypertensive calcium-channel blocker, 96% of the authors who supported the use of the drug had received financial benefits from pharma, whereas only 37% of critical authors had such industry relationships.

In the case of the anti-diabetes drug rosiglitazone authors recommending the use of this drug were 4 times more likely than critical authors to have received financial benefits from the manufacturer.

The real numbers might be even worse, because underreporting of financial interests is still rampant, affecting more than 90% of drug related guidelines.

That’s why you shouldn’t consider 120/80 mmHg as “normal” but simply as “business as usual”.

SOURCE: QUORA