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New U.S. guidelines mean that millions of people who had normal blood pressure yesterday have high blood pressure today.

A slight definitional change means that 46 per cent of Americans are now classified as hypertensive, compared with 32 per cent previously.

Whether that matters or not depends largely on how physicians and patients react.

The guidelines, a dense 481-page document, are detailed and nuanced but, from a public perspective, the key element is that they say that people with blood pressure of 130/80 millimetres of mercury or more should now be considered hypertensive.

Previously, the cutoff point was 140/90.

What is interesting and potentially positive is that the new guidelines do not say the newly classified should be treated with drugs, unless they have other risk factors such as diabetes.

Rather, they are a loud call for preventing and managing blood pressure with lifestyle modifications.

Practically, that means reducing salt intake, being more physically active, moderating alcohol consumption, not smoking, watch the scales, getting more sleep, eating more fruits and vegetables and all that other healthy stuff that most people don't do.

This has been the approach in Canada for years, and about 20 per cent of adults in this country are considered hypertensive.

In the Hypertension Canada guidelines, the cutoff point for "hypertensive" remains 140/90 for people who are not high-risk.

Normal blood pressure remains 120/80.

(The top number, systolic, measures pressure on the blood vessels when the heart contracts; the bottom number, diastolic, measures pressure when the heart relaxes between beats.)

What is not known is how U.S. physicians – and due to the spillover effect, Canadian doctors – will react to the new guidance. Will they embrace prevention and stick with the healthy living lectures or go straight to prescribing more blood-pressure medication?

There will be a strong inclination to do the latter, especially once someone has that dreaded label "hypertensive" slapped on them.

Yet, there is very little evidence that beginning treatment at 130/80 rather than 140/90 reduces mortality, or actually prevents heart attacks or strokes, especially in younger individuals. (And most of the newly labelled are under 45.)

There is, however, clear evidence of a direct correlation between rising blood pressure and increased risk of illness over time.

In fact, hypertension, while not an illness per se, is a leading cause of a host of illnesses, including heart attack, stroke, heart failure, kidney disease, erectile dysfunction and more.

The good news is that when drugs are used properly, they are relatively effective. First-line blood pressure meds, diuretics, are also cheap, costing just pennies.

As treatment escalates with angiotensin-converting enzyme (ACE) inhibitors, calcium channel blockers (CCBs) and angiotensin receptor blockers (ARBs), so do costs.

Not to mention that every medication that has a positive benefit also has side effects, especially when patients start taking two, three, four or more meds of increasing potency.

Diuretics, for example, can increase blood sugar, a risk for people with diabetes, and older patients. The stronger meds can also cause dizziness, falls and dehydration, which is more problematic for older patients. Intensive blood pressure treatment can also harm the kidneys.

And here's the rub: Under the new guidelines, virtually everyone over the age of 65 will be considered hypertensive. (That's because blood pressure rises with age.)

The real core message of the guidelines though – one that seems to be getting lost in the focus on numbers – is that accurate measurement matters.

Right now, we do a terrible job of measuring blood pressure. That's because blood pressure can fluctuate wildly, especially during a rushed doctor's visit. As a result, a lot of people are overtreated and a lot are undertreated.

The new guidelines speak to the importance of 24-hour, ambulatory blood pressure measurement. But, in Canada, that test is not even covered in most provinces. You have to pay out-of-pocket.

The alternative is self-measurement. Home blood pressure monitoring devices are increasingly affordable, but you have to get a decent one.

Blood pressure is all about numbers – but we have to focus on the right ones.

The new guidelines have led people to focus on how many people have the label "hypertensive." At the end of the day, that's meaningless.

What is far more important is that individuals know their numbers, and get treated – or not – accordingly.