Your Arm Position Can Make Blood Pressure Readings Inaccurate

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Think back to the last time you went to the doctor’s office. Chances are at the start of the visit, as a matter of course, they took your blood pressure.

But here’s the question. How did they take it? Turns out there is a right way to get a proper reading on your blood pressure. And often, medical professionals aren’t necessarily following it, which means those all important numbers can be wrong.

Joining me now to talk about this is Dr. Tammy Brady. She’s medical director of the Pediatric Hypertension Program and professor of Pediatrics at Johns Hopkins University in Baltimore. She’s one of the authors of a study on this topic recently published in the journal JAMA Internal Medicine. Welcome to Science Friday.

TAMMY BRADY: Thank you very much.

IRA FLATOW: You’re very, very welcome. So I go to my doctor’s office. An assistant comes in to take my blood pressure. What do the guidelines say? How should they be doing it?

TAMMY BRADY: Right. So a lot of people don’t recognize that there’s a lot more complexity to measuring blood pressure than just putting on a cuff and pushing a button. There are a lot of very important preparatory steps that need to be taken and a lot of important positions that we need to make sure that patients are in in order to get the most accurate blood pressure.

So the first thing that a person should do before measuring their blood pressure is making sure that they have emptied their bladder, make sure that they haven’t recently eaten or had any nicotine or alcohol, because those things can all lead to an elevated blood pressure. They should really rest for three to five minutes before a blood pressure measurement.

And key to an accurate measurement is making sure that you are using the right size cuff for your arm. Oftentimes, I think there is one cuff that is attached to the device in the office, and that’s the one that goes on the arm. But really, what we should be doing is measuring the person’s mid-upper arm circumference, and then selecting a cuff that’s appropriate for them.

And then, of course, there are specific ways in which you should be putting the cuff on the arm. You want it to be snug but not too snug, and you want it to be positioned on the upper arm. And you want to make sure that the arm is supported and positioned in such a way that the middle of the cuff is at mid-heart level, which is essentially the middle of the bony part of your chest.

IRA FLATOW: So your arm should be up then.

TAMMY BRADY: Yes, absolutely. It needs to be resting. And that’s very specific position. Because if it’s not, as we have shown, it can lead to significant inaccuracies.

IRA FLATOW: Those are things that the person giving me the blood pressure reading can do. What am I supposed to be doing while I’m sitting there? Any special positioning?

TAMMY BRADY: Well really need to be in a chair that has back support and allows you to have your feet on the ground. You can make sure that your legs are not crossed. And you really should try to be relaxed, and you should certainly not be looking at your phone.

And you really should not be talking to the person who is measuring your blood pressure. I think we in health care all want to make people feel comfortable and we’ll have a conversation. But those actions, being an active listener or talking, can elevate your blood pressure. So you want to just be nice and quiet.

IRA FLATOW: You know, Dr. Brady, I don’t know one patient who does not sit up on the examination table and have their blood pressure taken, and nobody’s back is against the wall there ever.

TAMMY BRADY: I know, I know. And in fact, I’ve given recently several talks on this. And there’s a picture in the Washington Post of somebody doing home blood pressure monitoring and every single thing is done wrong. Every step is wrong.

They’re sitting at the edge of the couch. They’re holding the device. The cuff isn’t even on their upper arm. It crosses the elbow. I mean, the whole thing is terrible.

[LAUGHING]

So, yeah, yeah.

IRA FLATOW: Let’s talk about your study, specifically. You looked at different arm positions in the same person. And what did that do to the readings? Tell us about what you found.

TAMMY BRADY: So with this study, we were really interested to know how important arm position, like I described earlier is. Because often in clinical practice, people have their arm in their lap or at their side. And really being able to have their arm positioned at a desk with the middle of the cuff at mid-heart level can take a little bit of extra time, and you need to be intentional about it. You need to have the room set up in a certain way. So we wondered, does this even matter.

So we conducted a study to see if there was any over or under estimation of blood pressure when the arms were positioned in the non-guideline specific way. And what we found was that there was actually substantial overestimation of blood pressure when the arm was positioned in the lap. So when the arm was in the lap, it overestimated blood pressure by almost 4 millimeters of mercury systolic– so that’s the top number– and 4 millimeters of mercury diastolic– that’s the bottom number.

But even more striking was when your arm is at the side, your systolic blood pressure is overestimated by almost 7 millimeters of mercury, and the diastolic was almost 4 and 1/2 millimeters of mercury. And so that amount of error can lead to someone being inappropriately diagnosed with hypertension, and potentially started on a medicine they don’t need.

IRA FLATOW: And if the arm position can make the reading really falsely high, are there other factors that can make the reading falsely low?

TAMMY BRADY: There are some things that can make the blood pressure low. So some people, if they’re standing when their blood pressure is being measured, their blood pressure can be lower. And then certainly– and sometimes with other people– that can raise their blood pressure, but it definitely can lower blood pressure for certain individuals, as can laying flat. So laying on your back while you’re measuring your blood pressure can lead to a lower blood pressure.

If you had just eaten. So just having had something to eat can lower your blood pressure by 6 millimeters of mercury. And even just having some– if you had an acute alcohol consumption, that can actually lower your blood pressure by almost 24 millimeters of mercury.

IRA FLATOW: No kidding. That’s a big number.

TAMMY BRADY: Big jump.

[LAUGHING]

IRA FLATOW: I know there are lots of people diagnosed with high blood pressure, hypertension in this country. Do you think the cumulative effects of lots of medical professionals measuring incorrectly could be skewing that data?

TAMMY BRADY: That is a great question, and something that I actually wonder myself. I can tell you anecdotally– so I’m a pediatric hypertension specialist, and I have several dedicated clinics for children and adolescents and young adults who have elevated blood pressure in their primary care setting. And many times when they come in and I measure their blood pressure, I measure it three times, I adhere to all of the steps. And I’m very vocal about why I’m doing each thing, why I’m moving them to a certain spot in the room for measurement. And it is really not uncommon for their blood pressure to be stone cold normal when they come into my clinic. And many of them comment that, wow, nobody’s ever done it that way, or, well, the last time I was here, they told me to sit up on the examination table.

So I do think that this could be potentially happening on a grander scale. And since so much of our research these days is relying on what we call big data, taking information from your electronic medical record and using it to estimate prevalences of various disease states. If you use this to estimate hypertension and the blood pressures that are being put in the chart are the ones that are obtained after suboptimal measurement technique, yeah, we could absolutely be having some inaccurate estimates for disease prevalence.

IRA FLATOW: Wow. You know what else? Sometimes I’ve had my blood pressure taken through my shirt.

TAMMY BRADY: Yes.

IRA FLATOW: It sort of drove me crazy when it first happened.

TAMMY BRADY: Yes.

IRA FLATOW: Does it drive you crazy?

TAMMY BRADY: A little bit, because it doesn’t take very much in order to make sure that the arm is bare. Now, I will say I did take a look at some of the literature on blood pressure accuracy when you use a cuff over clothing. And it does seem that it might not be terribly inaccurate if the sleeve that you are covering is thin.

So I think it might be reasonable. It does need to be systematically studied. I do think that we need some better studies to comment on this. But what is published says that might not be so bad.

But the cuffs, they rely on the oscillations in the arms. So the blood flowing through the blood vessels, it relies on sensing that. So if there’s a lot of clothing between the arm and the cuff, that sense might get a little muffled. And there is potential for inaccuracy there.

IRA FLATOW: I mean, should we be proactive in telling who was ever giving us the blood pressure test, do it this way?

TAMMY BRADY: Yes. So one of the things that I really hope that this study is able to do is to empower patients to remind their health care providers about the proper blood pressure measurement steps.

I think we’re all busy. We know that training and certification is essential to make sure that those measuring blood pressure know what they should be doing. But that their skill decay over time, and so that should be really repeated every six months.

And so my hope is that by this study getting attention, patients will feel empowered to remind their providers. So that when they are in the clinic setting, they’re getting the most accurate blood pressure that they can.

IRA FLATOW: Yeah. All right, I’m going to ask you about a really sticky question because I do this myself. My cardiologist gave me a home blood pressure monitor– get a personal monitor, check it themselves. And it actually has a little transmitter that sends it back to his office sometimes. And then I got one myself, and I compared the two, and they were not close many times.

TAMMY BRADY: Yes. So I have a lot of thoughts on this topic.

IRA FLATOW: Yes you do.

TAMMY BRADY: So first of all, home or out of office blood pressure measurement really is essential to hypertension diagnosis. The white coat effect is real.

And what we’re trying to do when we give someone a diagnosis of hypertension is identify a person who’s at risk for heart disease. And so if you get a little stressed out at the doctor’s office and your blood pressure is artificially elevated there, but when you’re home and where you spend 99.9% of the time, your blood pressure is normal, that’s more important for your cardiovascular disease risk than, say, an elevated blood pressure for five minutes when you’re in a doctor’s office. So out of office, blood pressure measurement is key.

However, I will say that essential to accurate blood pressures in the home is making sure that the blood pressure device you’re using has actually undergone testing to ensure that it’s giving accurate results. And what I mean by that is manufacturers have tools called validation protocols that they can use to test that the device is accurate in a population.

But just because you sell a device– there’s a device on Amazon, let’s say, or some other marketplace– just because it’s for sale does not mean it’s ever been tested. And in fact, 80% of devices that are on the market have never been tested. So you really need to make sure–

IRA FLATOW: So how do I know?

TAMMY BRADY: That that first step has been–

IRA FLATOW: How do I know?

TAMMY BRADY: That you really need to make sure that it’s been tested to be accurate before you even start using it for yourself.

IRA FLATOW: So I have to just trust them that this has been calibrated and tested?

TAMMY BRADY: Well, so I’m very passionate about blood pressure device validation. And I serve as co-chair for the American Medical Association Validated Device Listing Committee. And what this committee does is it takes a look at the validation testing for various devices. Manufacturers have to proactively submit their documentation to us.

And devices that have undergone the rigorous testing without any protocol violations, they are listed on our site. So if you as a patient or you as a health provider want to be able to find a device that’s been tested, you can go to validatebp.org and you can find a variety of devices there that have undergone this testing. So that for me is the first step to make sure you get accurate, out-of-office blood pressure measurement.

IRA FLATOW: validatebp– like in blood pressure– validatebp.org.

TAMMY BRADY: Yes, validatebp.org That’s right.

IRA FLATOW: OK. Now when I get home and I bring this home blood pressure monitor home, I’ve got to make sure I’m doing the same things correctly at home that would be happening in the doctor’s office, right?

TAMMY BRADY: That’s right. So that is absolutely right. And if we as health care providers can’t consistently measure blood pressure properly, we need to do a better job. Also, making sure that patients who we are counseling to do these home blood pressures know how to do this.

So I think that’s really important. I can tell you anecdotally that many of my patients, they wake up and they sit at the edge of their bed and they put the cuff on and push a button. But they have a full bladder. They’re not resting their backs against the wall. There’s many things that are happening that are not consistent with the protocol or the practice guidelines.

IRA FLATOW: I plead guilty to one of those. I’m not going to tell you which one. But now that I know, I will do that.

Well, what about the kiosks you see in grocery stores or pharmacies, the machines where you can check yourself?

TAMMY BRADY: Right. So, I also have a lot of thoughts. So the kiosks that are in public places, I think this offers a lot of opportunities to people who want an out-of-office blood pressure measurement, but can’t afford to get a home blood pressure device for use in their home. I think that this can increase equity, and I think it can increase access. So I think that there’s a lot of benefits from these devices.

But just like I said, home blood pressure devices need to be tested for accuracy, so do these kiosk devices. And in fact, one of the more common devices that– kiosk devices, that is– that is used in the United States has never been validated. So it can be really hard for a consumer, a patient, to know whether or not the machine that’s sitting there in their pharmacy has undergone this testing.

IRA FLATOW: Do you have the name of that on your website, too?

TAMMY BRADY: So, there’s one kiosk that is on the validatebp.org site. But the one that I’m thinking of is Higi. There’s over 10,000 kiosks in stores in the United States. And this, at least as of my last check, which was about a year ago, had not been tested for accuracy. It had been cleared by the FDA, but it had not been tested to see if it was accurate.

IRA FLATOW: Well, you are a fount of knowledge on this. I’m very happy we’re talking.

And to wrap it up, what can people be doing to advocate for themselves when they go to the doctor’s office? Should they be watching out for the things that you do and being an advocate– meaning, speaking up if they don’t see it being done correctly?

TAMMY BRADY: Yes. So I tell all of my patients– and again, all of the patients that I take care of, I verbalize what I’m doing, why I’m moving them, why I’m measuring their arm, why I’m selecting a certain cuff, why they need to be sitting in a specific chair that I put them in. And then after I’m done, I tell them that they should tell any health care provider who is measuring their blood pressure that they need to make sure to do all of these things.

And so I absolutely think patients should feel empowered to do this. And if it’s hard to remember all of the steps, the American Heart Association has some beautiful resources on there. The American College of Cardiology has also some beautiful resources online. So I would encourage you, these are things that you can print out and you can bring with you to your health provider as a gentle reminder that these are the ways I would like to have my blood pressure measured.

IRA FLATOW: Well, Dr. Brady, this has really been helpful. I want to thank you for taking time to be with us today. I think our audience age group really will benefit from this advice.

TAMMY BRADY: Great. Well, I’m really glad that you’re interested in the study. And I’m very, very glad that I can share some of this information with you.

IRA FLATOW: You’re welcome. Dr. Tammy Brady, medical director of the Pediatric Hypertension Program and professor of Pediatrics at Johns Hopkins University in Baltimore, Maryland.

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