Central Blood Pressure Measurement: Any Added Value?
Henry R. Black, MD: I am Dr Henry Black, adjunct professor of medicine at the Langone New York University School of Medicine. I'm here today with Dr Ray Townsend.
Raymond R. Townsend, MD: I am Dr Ray Townsend, professor of medicine at the University of Pennsylvania Perelman School of Medicine and director of the hypertension program.
Dr Black: I would like to ask you about central blood pressure measurement. If you're not the world's expert, you are at least one expert in this technique. What is it?
Dr Townsend: Central pressure measurement began with the notion that if you look at clinical trials in general, you can clearly show the benefit of blood pressure reduction with the drugs that we commonly use, but some patients just don't seem to derive benefit. The question has arisen as to whether the blood pressure reduction that we see in the brachial artery (standard blood pressure measurement) is equal to the blood pressure reduction in the central aortic region. That is where all of the important "business" is—the brain, the kidneys, the legs, and the heart and its connection to the aorta.
The idea of measuring central pressures grew from the finding that when you do a heart catheterization and you pass that wire up to the heart, you notice that, counterintuitively, the systolic blood pressure declines as you approach the heart. The mean pressure doesn't change, nor does the diastolic pressure, but the systolic pressure declines. That fascinating phenomenon, which we can now measure noninvasively, is what is driving this field.
Dr Black: You say "noninvasively." How does that work?
Dr Townsend: Given that necessity is the mother of invention, several ways have been discovered to do this. Some involve recording the pulse wave with a wand that has a little tonometer built into the end of it, which is calibrated according to the brachial pressure. We can measure it with that wand and a transfer function that says, "Here's what the radial pulse looks like, and knowing the blood pressure, here is what the central arterial pressure looks like." That is one of several popular ways.
The other popular way is to put a cuff on the brachial artery, pump it up, and let the oscillometric device in the cuff measure the waveform. Then you don't have the issue of calibration, nor do you have the issue of operator dependency—trying to get that wand over that 1-mm radial artery. The oscillometric technique is growing in popularity. There are tonometric techniques and oscillometric techniques for post-wave analysis.
Dr Black: How do you interpret a central blood pressure measurement regardless of the method of measurement?
Dr Townsend: We look at a couple of things. We make sure that the quality of the waveform is good, because "garbage in, garbage out" is still a problem in the field. You have to know a little bit about what to expect.
That aside, what you want to see is how well the brachial blood pressure and the central pressure profiles correlate. The difference can be as little as 2 mm Hg of systolic pressure difference, or as much as 30-35 mm Hg of systolic blood pressure difference. With the algorithm that is used on the waveform shape itself, you can estimate the central systolic pressure, and therefore the central pulse pressure and the degree of amplification of the systolic pressure as it travels from the heart to, for example, the arm where the blood pressure is measured.
Part of our interpretive process is to look at the magnitude of amplification, but more recently we have been very interested in the effects of the returning wave on the heart. The velocity that the pressure wave travels in the circulation is such that to a variable degree, depending on that velocity, that wave can return like an echo to the heart before the aortic valve finishes closing, and when that adds a substantial late end-systole load to the heart, that is a forerunner of eventual heart failure.
Dr Black: What does this add to what we learn with standard measurements?
Dr Townsend: That's a great question, because there is currently much skepticism in the field. Some people say that most of what you learn from a central pressure profile you already knew from taking the brachial blood pressure. In our experience, 80%-84% of what you see in the central circulation is predicated upon knowing the brachial blood pressure.
That aside, that still leaves 15%-20% unexplained. In my mind, that is where we will find the information that can help us understand why someone has target organ damage who shouldn't, because his or her blood pressure is well controlled. One of the uses is to predict, or at least look back, through a ceilometer scope instead of the retrospectoscope.
What is going on centrally now that I know the brachial pressure? Sometimes it is surprising how different the two are; sometimes it's very clear that the central pressures are very similar to what you see in the brachial pressure. In that case, it doesn't add anything.
Other times it adds huge late systolic waves that we think are important in saying, "This person is at risk for a heart failure event. This is the kind of patient for whom I may want to look further into therapies that might have a predominantly late systolic effect to lower that load." If we can reduce the pulse wave velocity a little bit but lighten the load on the left ventricle as it contracts, we may be able to prevent heart failure. What we lack, and this is an acknowledged shortcoming in the field, are good intervention-based outcome trials using this kind of technology.
We have a grant at the National Institutes of Health right now to do this type of study, but it is still considered innovative and it doesn't have full traction with everyone who deals with these phenomena.
Dr Black: When you talk about therapy, are there therapies that have different effects on late systolic action?
Dr Townsend: Yes, and that is what has piqued the interest in the field. There was an add-on to the ASCOT study called the CAFE trial. That trial showed that there were differences in the approach taken to lower blood pressure. There were equivalent degrees of brachial blood pressure reduction in the beta-blocker diuretic group vs the calcium channel blocker angiotensin-converting enzyme (ACE) inhibitor group. A 4-mm Hg difference in systolic pressure was sustained over the 5 years of follow-up. It is apparent that some drugs have a greater systolic pressure reduction in the aorta compared with what we measure in the arm.
It turns out that ACE inhibitors and angiotensin-receptor blockers tend to achieve a little bit more reduction of central compared with brachial blood pressure. Calcium channel blockers and diuretics tend to be similar and beta-blockers don't achieve as much central pressure reduction as they do brachial blood pressure reduction. The real unexplored country here is the role of other therapies, especially nitrate-based therapies, which have very large effects on returning wave and are just crying out to be studied in intervention trials to examine their value in preventing target organ damage based on the central pressure profile.
Dr Black: What is the cost of doing this? Is it reimbursed?
Dr Townsend: In March, the American Medical Association approved a level 1 code called 9300X1. Whether that will be paid, I don't know. It's in a category with a code. It used to have a level 3 code (0311T). With a level 3 code, you can do it—they will log it in, but you are not usually paid for it.
Occasionally you will find an insurance company that will pay something. The typical reimbursement for interpretation ranges from $50 to $100. Who knows what it will be once a real payer like Centers for Medicare & Medicaid Services (CMS) gets hold of it. In general, the front-end cost is buying the equipment.
It costs $20,000-$30,000 to buy an office space device that does these sorts of procedures. At $50 each, it will take some time to make that up, along with paying someone to do the measurements. We don't have a good pricing structure yet. It only takes a couple of minutes to turn the computer on, put the data in, and wrap the cuff on if you use the oscillometric approach. Within 5 minutes you have your answer. The key is spending a minute or two thinking about what the information means. Therein lies the potential costs of the ultimate reimbursement.
Dr Black: In my experience, it was very easy to train a good nurse or medical technician to do this.
Dr Townsend: In general, if you use the tonometric approach, you need someone who has the ability to use their hand, capture the radio waveform, and know whether what they are looking at is of good quality. With the oscillometric-based waveforms (where the field tends to be moving right now), it's a little more operator independent. Then it's just a matter of making sure that the pressure profile that you see is actually a good one.
Dr Black: Thank you very much. We are moving into a whole new area of how we measure blood pressure, which we have been doing the same since the 19th century. Who knows what the future will bring.
Dr Townsend: It's an exciting time. There is a lot of work to do. A position paper is being drafted by the American Society of Hypertension that I hope will add a little bit of knowledge to this database. It will be written in such a way as to explain what these technologies offer to a clinical practitioner.
Central Blood Pressure: What Is It?
Henry R. Black, MD: I am Dr Henry Black, adjunct professor of medicine at the Langone New York University School of Medicine. I'm here today with Dr Ray Townsend.
Raymond R. Townsend, MD: I am Dr Ray Townsend, professor of medicine at the University of Pennsylvania Perelman School of Medicine and director of the hypertension program.
Dr Black: I would like to ask you about central blood pressure measurement. If you're not the world's expert, you are at least one expert in this technique. What is it?
Dr Townsend: Central pressure measurement began with the notion that if you look at clinical trials in general, you can clearly show the benefit of blood pressure reduction with the drugs that we commonly use, but some patients just don't seem to derive benefit. The question has arisen as to whether the blood pressure reduction that we see in the brachial artery (standard blood pressure measurement) is equal to the blood pressure reduction in the central aortic region. That is where all of the important "business" is—the brain, the kidneys, the legs, and the heart and its connection to the aorta.
The idea of measuring central pressures grew from the finding that when you do a heart catheterization and you pass that wire up to the heart, you notice that, counterintuitively, the systolic blood pressure declines as you approach the heart. The mean pressure doesn't change, nor does the diastolic pressure, but the systolic pressure declines. That fascinating phenomenon, which we can now measure noninvasively, is what is driving this field.
Noninvasive Methods of Central Blood Pressure Measurement
Dr Black: You say "noninvasively." How does that work?
Dr Townsend: Given that necessity is the mother of invention, several ways have been discovered to do this. Some involve recording the pulse wave with a wand that has a little tonometer built into the end of it, which is calibrated according to the brachial pressure. We can measure it with that wand and a transfer function that says, "Here's what the radial pulse looks like, and knowing the blood pressure, here is what the central arterial pressure looks like." That is one of several popular ways.
The other popular way is to put a cuff on the brachial artery, pump it up, and let the oscillometric device in the cuff measure the waveform. Then you don't have the issue of calibration, nor do you have the issue of operator dependency—trying to get that wand over that 1-mm radial artery. The oscillometric technique is growing in popularity. There are tonometric techniques and oscillometric techniques for post-wave analysis.
Interpreting Central Blood Pressure
Dr Black: How do you interpret a central blood pressure measurement regardless of the method of measurement?
Dr Townsend: We look at a couple of things. We make sure that the quality of the waveform is good, because "garbage in, garbage out" is still a problem in the field. You have to know a little bit about what to expect.
That aside, what you want to see is how well the brachial blood pressure and the central pressure profiles correlate. The difference can be as little as 2 mm Hg of systolic pressure difference, or as much as 30-35 mm Hg of systolic blood pressure difference. With the algorithm that is used on the waveform shape itself, you can estimate the central systolic pressure, and therefore the central pulse pressure and the degree of amplification of the systolic pressure as it travels from the heart to, for example, the arm where the blood pressure is measured.
Part of our interpretive process is to look at the magnitude of amplification, but more recently we have been very interested in the effects of the returning wave on the heart. The velocity that the pressure wave travels in the circulation is such that to a variable degree, depending on that velocity, that wave can return like an echo to the heart before the aortic valve finishes closing, and when that adds a substantial late end-systole load to the heart, that is a forerunner of eventual heart failure.
Dr Black: What does this add to what we learn with standard measurements?
Dr Townsend: That's a great question, because there is currently much skepticism in the field. Some people say that most of what you learn from a central pressure profile you already knew from taking the brachial blood pressure. In our experience, 80%-84% of what you see in the central circulation is predicated upon knowing the brachial blood pressure.
That aside, that still leaves 15%-20% unexplained. In my mind, that is where we will find the information that can help us understand why someone has target organ damage who shouldn't, because his or her blood pressure is well controlled. One of the uses is to predict, or at least look back, through a ceilometer scope instead of the retrospectoscope.
What is going on centrally now that I know the brachial pressure? Sometimes it is surprising how different the two are; sometimes it's very clear that the central pressures are very similar to what you see in the brachial pressure. In that case, it doesn't add anything.
Other times it adds huge late systolic waves that we think are important in saying, "This person is at risk for a heart failure event. This is the kind of patient for whom I may want to look further into therapies that might have a predominantly late systolic effect to lower that load." If we can reduce the pulse wave velocity a little bit but lighten the load on the left ventricle as it contracts, we may be able to prevent heart failure. What we lack, and this is an acknowledged shortcoming in the field, are good intervention-based outcome trials using this kind of technology.
We have a grant at the National Institutes of Health right now to do this type of study, but it is still considered innovative and it doesn't have full traction with everyone who deals with these phenomena.
Treatment of Central Blood Pressure Elevation
Dr Black: When you talk about therapy, are there therapies that have different effects on late systolic action?
Dr Townsend: Yes, and that is what has piqued the interest in the field. There was an add-on to the ASCOT study called the CAFE trial. That trial showed that there were differences in the approach taken to lower blood pressure. There were equivalent degrees of brachial blood pressure reduction in the beta-blocker diuretic group vs the calcium channel blocker angiotensin-converting enzyme (ACE) inhibitor group. A 4-mm Hg difference in systolic pressure was sustained over the 5 years of follow-up. It is apparent that some drugs have a greater systolic pressure reduction in the aorta compared with what we measure in the arm.
It turns out that ACE inhibitors and angiotensin-receptor blockers tend to achieve a little bit more reduction of central compared with brachial blood pressure. Calcium channel blockers and diuretics tend to be similar and beta-blockers don't achieve as much central pressure reduction as they do brachial blood pressure reduction. The real unexplored country here is the role of other therapies, especially nitrate-based therapies, which have very large effects on returning wave and are just crying out to be studied in intervention trials to examine their value in preventing target organ damage based on the central pressure profile.
Cost and Reimbursement Issues
Dr Black: What is the cost of doing this? Is it reimbursed?
Dr Townsend: In March, the American Medical Association approved a level 1 code called 9300X1. Whether that will be paid, I don't know. It's in a category with a code. It used to have a level 3 code (0311T). With a level 3 code, you can do it—they will log it in, but you are not usually paid for it.
Occasionally you will find an insurance company that will pay something. The typical reimbursement for interpretation ranges from $50 to $100. Who knows what it will be once a real payer like Centers for Medicare & Medicaid Services (CMS) gets hold of it. In general, the front-end cost is buying the equipment.
It costs $20,000-$30,000 to buy an office space device that does these sorts of procedures. At $50 each, it will take some time to make that up, along with paying someone to do the measurements. We don't have a good pricing structure yet. It only takes a couple of minutes to turn the computer on, put the data in, and wrap the cuff on if you use the oscillometric approach. Within 5 minutes you have your answer. The key is spending a minute or two thinking about what the information means. Therein lies the potential costs of the ultimate reimbursement.
Dr Black: In my experience, it was very easy to train a good nurse or medical technician to do this.
Dr Townsend: In general, if you use the tonometric approach, you need someone who has the ability to use their hand, capture the radio waveform, and know whether what they are looking at is of good quality. With the oscillometric-based waveforms (where the field tends to be moving right now), it's a little more operator independent. Then it's just a matter of making sure that the pressure profile that you see is actually a good one.
Dr Black: Thank you very much. We are moving into a whole new area of how we measure blood pressure, which we have been doing the same since the 19th century. Who knows what the future will bring.
Dr Townsend: It's an exciting time. There is a lot of work to do. A position paper is being drafted by the American Society of Hypertension that I hope will add a little bit of knowledge to this database. It will be written in such a way as to explain what these technologies offer to a clinical practitioner.