Norman Swan: Hello, and welcome to this week's Health Report with me, Norman Swan. Today, good news for parents of children with asthma. Dyslexia in a digital world. The media and cancer reporting. And a caution for people with high blood pressure. Don't ignore the bottom number of your pressure, what's called your diastolic.
Last November on the Health Report we covered a study called the SPRINT trial which found that for people at high risk of a heart attack or stroke, going on to high blood pressure treatment which aimed for a top number or systolic blood pressure of 120 or less was associated with significant benefits compared to the usual target of 140. This has made many countries wonder whether they should tell doctors to lower blood pressure targets in their patients at high risk of heart disease.
But you'll notice I have only talked about the top number, the systolic pressure which is the pressure generated by the heart beating and pumping blood through your arteries. The bottom number, the diastolic pressure, is the background pressure between heartbeats and has a critical role in keeping blood going to organs like the brain and heart. A group led by researchers at Johns Hopkins University in Baltimore was concerned that there might be a risk in people whose diastolic pressure falls too far with more aggressive blood pressure treatment. It's a story consumers need to know about. Bill McEvoy was the lead author, and I spoke to him earlier today.
Bill McEvoy: Patients who have very stiff blood vessels, which is a risk factor for heart disease and stroke, the diastolic pressure, which is the pressure you have between heartbeats, tends to be lower. And that means that there can be less profusion of the organ during diastole, which is for all intents and purposes approximately half of the heartbeat.
Norman Swan: And just to capture what you said there, essentially a low diastolic blood pressure could be a reflection of very stiff arteries in somebody who is already considerably at risk.
Bill McEvoy: Yes, and to add to that, most patients with stiff arteries tend to have higher systolic blood pressures, and that means that the pulse pressure, which is the difference between the two, systolic minus diastolic, is larger. So those patients with a higher pulse pressure, which is the higher systolic and the lower diastolic, might be due to stiff arteries. If they are going to be treated to this target of 120, they may end up with a non-treatment diastolic blood pressure that is lower than we would like and that has theoretical reasons for concern, specifically with respect to the coronary blood flow.
The coronary arteries, as you know, are the arteries that supply the heart muscle itself, and those coronary arteries, because they go through the heart muscle, when that the heart squeezes in cystoid, which is the time when the heart is pumping blood out, those vessels are constricted and there is no flow. So the only time that there is actually flow to the heart muscle through those coronary arteries is during diastole, and for that particular reason it could be that in diastole when the diastolic blood pressure is too low that you are getting actually reduced blood flow to the heart muscle which can cause damage.
Norman Swan: So you studied 11,500 people, average age 57, both men and women, to look at the relationship between whether or not you could measure heart damage by a chemical called troponin, related to their diastolic blood pressure. Were those people being treated for their blood pressure or this was their natural diastolic blood pressure?
Bill McEvoy: It was a mix. This was a fairly healthy population in their 50s, and specifically did not have a prior history of coronary artery disease. So most of the patients were not on treatments. We found that even when we stratified the analysis based on hypertension treatment status, that is we looked both at the group of patients or adults in the study who were getting treatment for blood pressure and we looked at the group who were not on any treatment, we saw that the results were the same. That is, that the diastolic blood pressure, the lower that blood pressure was, the more likely that we saw an association with heart damage by this troponin.
Norman Swan: What's the significance of that raised troponin? Was it high enough to be registered as a heart attack or is it just chronic damage that could lead to heart failure, for example?
Bill McEvoy: More the latter than the former. We know from prior studies that individuals with troponin levels this high are far more likely to have adverse outcomes, specifically adverse coronary heart disease, cardiac death and most particularly adverse heart failure outcomes. So I see this as individuals who are ambulating, walking around their communities, likely mostly asymptomatic, who have detectable levels of heart muscle injury relating to the lower diastolic blood pressure and the consequent reduction in the coronary blood flow. And again, our take-home message is that the SPRINT study is important, there are individuals who will benefit from a lower systolic blood pressure, I think that can't be underestimated, but a real simple point in this analysis was that physicians really need to look at both.
Norman Swan: And you're cut-off that you thought was the one that was significant was 70, that the bottom number shouldn't fall below 70.
Bill McEvoy: Yes, we certainly saw a trend for certain outcomes for heart disease and for the troponin outcome we saw a significantly elevated association in the individuals who have diastolic blood pressure less than 70, and particularly less than 60 would be of concern.
Norman Swan: So what's a general practitioner to do, because you can't treat diastolic by itself, you've only got one blood pressure pill or one lifestyle measure, what are you to do?
Bill McEvoy: I think in an individual who for instance has an elevated systolic blood pressure and in whom the general practitioner is considering blood pressure treatment, I think they need to assess what the diastolic blood pressure is, understand that particularly for patients who have known coronary artery disease, who have had prior bypass surgery, prior heart attacks, that particularly in those individuals who have that history who have a wide pulse pressure, I think there needs to be some caution in pushing the systolic blood pressure too low.
Norman Swan: But the SPRINT people would say, well, we did a randomised controlled trial and we properly studied it in a randomised way, all you've done is looked at a group of people, which is not as good evidence.
Bill McEvoy: So they haven't reported the outcomes based on diastolic blood pressure, so I would say that's an average result, and therefore it's fair to say that on average if you take a given patient and take them at random and you treat them, if they have the criteria for SPRINT to a systolic of 120, they are likely going to benefit. But for the individual who has a low diastolic blood pressure I would advocate that those patients not be treated as aggressively, particularly if they have a history of coronary artery disease. I think this is just something that puts a little bit more emphasis on the individual rather than on the average. It doesn't contradict the SPRINT study in any way, it just really highlights the point that diastolic blood pressure shouldn't be ignored and that for individuals who start off with that low diastolic blood pressure there needs to be some caution of dropping it lower, particularly if they have known coronary artery disease.
Norman Swan: Bill McEvoy, thanks for joining us on the Health Report.
Bill McEvoy: Thank you.
Norman Swan: Bill McEvoy is in the cardiology department at Johns Hopkins University School of Medicine in Baltimore.
And you're listening to the Health Report here on RN, at ABC News Radio and CBC Radio across Canada. I'm Norman Swan.