What is hypertension?

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Welcome to this Australian Prescriber Podcast. I'm Dr Justin Coleman, a GP on Bathurst Island, a two-day kayak north of Darwin, and today we're talking hypertension. What is it? And I'm speaking with specialist physician Dr Genevieve Gabb, who's a member of the executive committee of the High Blood Pressure Research Council of Australia and a member of the Clinical Committee, Heart Foundation of Australia. Dr Genevieve Gabb, welcome to our all-natural, COVID-free podcast.

Thank you, Justin.

Dr Gabb, you've written an article in the August edition of the Australian Prescriber titled What Is Hypertension? I like the brevity of the title, and it also makes me wonder whether I could honestly have saved you all the trouble because it seems to be another name for high blood pressure. But happily, you do more than answer what is hypertension. You go into some of the detail. And the first thing you do is you look at the definition of hypertension in the guidelines around the world, and you mention that there are two different approaches. What are those?

Historically, the definition of hypertension, which of course is arbitrary, has been around a level of blood pressure where it's clear that the benefits of treatment outweigh the risks. The thing about blood pressure, and I think about a lot of things that we do, is that our knowledge, or the science, I guess, if you like, of things, is based on different sorts of information. So one basic scientific method is that of observation. And the observation in relation to blood pressure is that levels of risk increase from quite low blood pressures, so around 115 mmHG or even lower. But to demonstrate a benefit of treatment, you need an interventional study, and in large part the interventional studies demonstrate benefits of blood pressure lowering when the blood pressure is elevated, so over 140/90.

So if we get to the second or the alternate definition of blood pressure, which has been used in the more recent US guideline where they've adopted a lower level at around 130/80, compared with 140/90 for the treatment-based level of blood pressure, this lower level of blood pressure is based really on a combination of the interventional studies, but also the observational studies of the level of where cardiovascular risk is increased. So there's not the interventional basis to say that necessarily treating at those levels will provide people with benefits. So they're the two basic different ways.

So the Australian and Canadian and UK and a few other countries' guidelines are based on where treatment has been shown to be of benefit, defining high blood pressure as a blood pressure where it'll do you some good going on an anti-hypertensive. Whereas the US-based guidelines have said that, well, just look at who does worse than others, regardless of treatment. And you find there that of course you start to do worse than others at a lower level than necessarily treatment is indicated.

I think the real point about that is if you have an understanding that these definitions are made on a slightly different basis, there actually is not a real conflict.

There may not be a conflict, but I would argue that for GPs and clinicians on the ground, it's far more useful to know when we should be treating someone. So I do think that first approach is superior for clinicians. Certainly on a research basis, the second US approach might be useful. But in terms of us faced with, do we start this patient on a tablet or not, it's very useful to have that treatment-based threshold.

Yeah. And I think that that is the thing. The purpose of the hypertension guideline in Australia is to be of use for treating clinicians. That was something very much that we bore in mind in terms of what we put together is that it should be a useful, informative document for clinicians, and so that's part of our rationale. I think we recognise that.

Indeed. Let's look at the Australian guidelines and look at the grades of hypertension and let's spit out some numbers here. So we have grade one, two and three hypertension. What are we looking at there?

So the point about the relationship between blood pressure and risk is that it's exponential. It's not a linear thing. It's exponential. As the blood pressure goes up, the risk ramps up dramatically in an exponential process. And so the definitions for hypertension in the Australian guidelines reflect that. So we have grade one hypertension as a systolic blood pressure between 140 and 159, diastolic between 90 and 99. Grade two hypertension is between systolic of 160 and 179 or diastolic 100 to 109. And grade three hypertension is 180 systolic or 110 diastolic or above.

Can I just repeat those for auditory learners? So the systolic goes up from 140 to 160 and 180, and the diastolic goes from 90 to 100 to 110 for grades one, two and three.

All right. Now that grading helps to convey a sense that there really is a dramatic change in risk with changing levels of blood pressure. Now, the interesting thing about the US guideline, which had the lower blood pressure definition of 130/80, is it also lost that distinction of the increasing risk at higher levels of blood pressure, because I think it has grade two hypertension as anything over 140/90. It's lost that piece of information that conveys to people that there really is a difference between a blood pressure of 176 and one of 146, which is also I think, clinically, a very useful and important point.

I think one of the influences on the US guidelines was certainly the fact that 9 of the 11 people on the guidelines had very high number of pharmaceutical conflicts of interest. I think they averaged 12 each. There were 60 conflicts of interest on actual paid consultancy advisory boards between those 9 panellists, including the chair. So it was really a panel selected by pharmaceutical companies in some ways, and doctors who'd worked for them for a very long time and were under their employ. So I do think that had something to do possibly with the lowering of the thresholds, because of course once you lower threshold, you get a whole lot more people getting treated at a lower blood pressure.

I liked one of the sentences in your article which was talking about the relative risk reduction remaining fairly constant throughout the spectrum of blood pressure, which of course indicates that the absolute benefit varies markedly according to which patient is sitting in front of you. So for example, someone who has a lowish blood pressure and not many other CV risks may still drop their risk, perhaps, just to pluck a number out of the air, by a quarter if you start them on an antihypertensive. But that quarter, if they've got a 1% risk, that's a quarter of a percent that year of having a heart attack. Whereas if someone has a 10% risk of having a heart attack and you drop it by a quarter, you're doing much more benefit for the patient. So although the relative risk stays fairly constant, the absolute risk varies very much according to the individual.

Yes. Really in a sense what this is about, it's really about, I think, something that we aspire to in practice, and that is to provide holistic care to whoever is in front of you. So considerations of risk have been part of the Australian blood pressure guidelines for a very long time, since at least 1979, I think. And in that sense, the Australian blood pressure guideline, I think, is a bit of a leader in that it hasn't always been incorporated into other international guidelines. So the recommendation is very much that a patient's risk should be considered together with accurate blood pressure assessment and clinical assessment of the patient for evidence of end-organ damage. Of course, if it's possible to identify the things that are contributing to that risk, so in some populations it's things like smoking, then they need to be addressed in their own right. But it's really just part of providing holistic care.

Indeed. And you're happily talking to an audience of listeners who are very much into providing holistic care. When you say accurate measurement, there are various ways of getting blood pressure measurements. What we're used to in the clinic is of course you get a person in, measure their blood pressure, if it's high you perhaps get them back a couple of times over the next few days or few weeks, which has worked well historically, and I think is reasonably accurate, but there are other ways of getting blood pressure measurements.

Yes, well, it is a vexed issue. I completed my training and concluded that there was only one person in the world who knew how to measure blood pressure accurately, and that was my boss. So it is actually a very difficult thing. And I think if clinicians recognise that and understand that it is difficult, I think that's useful for them. I think one point to make with all the arguments about blood pressure measurement, of which there are many, is that the majority of the clinical trials which have been done to establish the benefits of blood pressure lowering have been done using clinic blood pressures. So much as people might like to malign the clinic blood pressure or say there are problems with it, which there are, it is the basis for a lot of the evidence underpinning therapy. So before we're too ready to throw out the clinic blood pressure, I think people need to remember that.

So what you're saying there, for example, is that even if a 24-hour ambulatory monitoring or home blood pressure monitoring is shown to be more reliable and accurate, interestingly, currently the blood pressure guidelines are based on what we pretty much do, which is, as a GP, for example, measure it two or three times.

Yes. So I mean, the thing is to think about the circumstances of how it's measured: that the patient's appropriately positioned, that they're calm, they're sitting there relaxed having a period of rest. The real key is taking multiple measurements. I mean, I've done this in the clinic myself. I took multiple measures of blood pressure, and for some people you can see it decrease by 18 to 20 mmHg if you take four or five measures over several minutes. But really the key to getting a robust reading is several measures. And then there are these other techniques. Ambulatory blood pressures, which can record the blood pressure over 24 hours. And I think interestingly within, again, the context of the coronavirus pandemic, the value of home blood pressures is something that's being increasingly recognised.

And you mentioned in the article an interesting concept which the Canadians apparently use a lot, you say, which is instead of a home blood pressure monitoring, getting an automated blood pressure monitor. But instead of expecting each patient to buy one, they use one in the clinic but use a separate room, a quiet room where the person presses it a few times over a period of time. Is that the way they do it?

Yeah. So I mean, this is a blood pressure measurement technique called automated office blood pressure. The thing to understand is that there's a very strong and clear focus around hypertension in Canada, and part of this has included a focus on measurement. And so this technique of automated office blood pressure is something that is not used universally, but is used in a substantial number of settings in Canadian practice, maybe 50% of primary care. But it's something that they've worked on over 20 years, so it's not something that's come in overnight.

Now, it's just using a machine similar or the same to machines which are in Australia in general practice, but it's the way that you use it. The patient comes in, they have a period of rest, maybe five minutes of rest. They have a cuff put on. The button is pressed for the machine to start itself after a period of five minutes. It then takes several readings, maybe between three and five readings with a minute between, and then within the machine, those readings are averaged.

I don't think that that's something that's being done routinely in general practice. And I think in the Canadian setting, they probably have a space in the clinic set up for this to be able to happen before people come in and see the doctor. And the thing about those readings is that they're very similar to the result that you would get for the daytime blood pressures from an ambulatory blood pressure monitor and they're lower than clinic readings. So they may be 10 to 15 mmHg lower than what you would get doing a clinic reading by the doctor in the office.

Yeah, that's really interesting. It's possible Canadian doctors are scarier than Australian doctors so that the blood pressure goes up a little bit there, but it's certainly something worth thinking about. And I can imagine some of Australian general practice could incorporate that if it was felt to be worthwhile.

Yeah. So the Australian blood pressure guideline, I saw some discussion about various blood pressure and guideline-related things on that authoritative source of Twitter. So there was a comment made about the Australian guideline by one of the very senior Canadian blood pressure guideline people. And he referred to the Australian blood pressure guideline as being the most coherent and I was quite...

Bully for you.

It was a very nice thing to see.

Well, it's very nice to have had one of the authors of those guidelines on the podcast with us today. So Genevieve Gabb, I think that's all we've got time for today, but thank you very much on behalf of Australian Prescriber for giving us your time and also writing that article, which I encourage people to look up in the August edition called What Is Hypertension? Thanks for coming along.

Thank you.

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My guests' views are their own and don't represent Australian Prescriber, and my views are certainly all mine.