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High blood pressure: it's complicated

A combination of factors should be considered when assessing if blood pressure is too high.

High blood pressure is often referred to as the “silent killer”: it predisposes us to heart attacks and strokes but doesn’t present any symptoms, at least not until it’s too late. We’re encouraged to be afraid of it, but at what point does blood pressure become high? Well, this is where it gets complicated.

Blood pressure refers to the amount of pressure exerted by our beating heart on the walls of our blood vessels. It’s described as two numbers; the first represents the maximum pressure exerted in each heartbeat (systolic pressure), while the second (diastolic pressure), represents the minimum, the pressure exerted when the heart is at rest.

In the UK, the bar for high blood pressure is set at 140/90. It’s the same in the United States, although the American Medical Association has now added a new category, “pre-hypertensive”, for patients between 120/80 and 140/90.

In New Zealand the guidelines for assessing blood pressure, at least in relation to cardiovascular risk, involve a broader and more complex framework; anyone with a systolic blood pressure above 120 should be assessed for their risk of heart attack or stroke, but that assessment needs to look at an array of interacting risk factors. To focus on blood pressure in isolation is often meaningless; the influence of blood pressure is only significant in terms of its interaction with other key risk factors, such as cholesterol levels, age, sex, and whether the person being assessed is diabetic, obese or a smoker.

In other words, it depends. If you are a 50-year-old non-smoking, fit, slim and healthy female, a blood pressure of 150/100 is probably little to worry about. If you are a 60-year-old male diabetic with a generous girth, the same reading may suggest you should be very concerned.

This is because risk factors interact with each other in a way that has a compounding effect. “They work together in an multiplicative way – that is, the risks interact with each other and the effects multiply,” explains Rod Jackson, a University of Auckland epidemiologist whose research (including his development of cardiovascular risk assessment tables) has been a key driver in the development of New Zealand guidelines. “It’s the combination of risk ­factors that is the issue.”

This also explains why many New Zealand specialists and GPs are wary of pointing to a specific number at which blood pressure should be said to be “high”. The risk of a heart attack or stroke rises along with the rise in the various risk factors, but there is no clear point at which blood pressure alone suddenly becomes risky.

We could, most accurately, define “high” blood pressure as a systolic pressure above 120, says Jackson, as that’s the point at which risk begins to increase – but by that definition, around 75% of New Zealanders would have high blood pressure, “which is just silly”.

Stewart Mann, head of the Department of Medicine at the University of Otago, Wellington, agrees. “It is ultimately an arbitrary line, and different authorities will draw lines at different places.” Mann recently delivered a talk at a medical conference in Australia in which he outlined the different guidelines for management of hypertension in different countries. “At one end, you had the US, which concentrated almost entirely on blood pressure and hardly mentioned other risk factors. New Zealand was at the opposite end, in that we emphasised the interaction of all risk factors.”

America, says Mann, is inclined to a “hypertensive-centric” view of things, with its hypertension specialists and hypertension societies and people who “for medico-political reasons tend to push their own focus”. Which explains its fondness for concepts such as “pre-hypertensive”, which, Mann suggests, is as emotionally charged as it is biased. “It’s almost saying, ‘Yeah, you’re destined to have hypertension.’ It really is moving in on the area where there are a lot of people who are at low risk overall, and probably shouldn’t be treated.”

Despite all that, the general consensus is that lower blood pressure is better than higher blood pressure, and anything above 140/90 might merit some attention. Excluding pharmacological intervention, we all know the drill. Cut down on the booze – one or two wines a day might be good for the heart (which is debatable) but more than three is bad for it. Give up the smokes, get plenty of exercise and, whenever possible, cut down on salt.

Blood pressure does matter, but it also needs to be put in context. “It is a contributor [to cardiovascular disease] and a strong one,” says Mann. “But it’s not the be-all-and-end-all.”


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