Central arterial blood pressure
(What is it, don’t like it. Pay attention it could save your life)
It is a pressure that is measured, almost exclusively, by placing a cuff around one arm – usually the left. The cuff is then inflated to a point whereby all blood flow is stopped. If you have placed a stethoscope over the brachial artery (artery in the arm) you will hear nothing at this point. Because there is no blood flow, and nothing to hear.
As the pressure in the cuff is lowered, a noise will be heard as the blood first starts to squeeze through. This is defined as the systolic blood pressure i.e. the point of highest arterial pressure, just after the heart contracts. A sharp tapping noise would be the best description.
As you lower the pressure in the cuff, the noise changes and muffles. Eventually, there will come a point where the blood is flowing through the brachial artery all the time …the point of ‘lowest’ blood pressure. Once this pressure is reached, the noises in the brachial artery cease. This is defined as the diastolic blood pressure. (The pressure does not reach zero, because the heart pumps once again to boost the pressure again).
All of this means that your blood pressure is presented as two figures. The highest recorded pressure (systolic) over the lowest (diastolic).
Historically, blood pressure measured in millimetres of mercury. Because, in the good old days, blood pressure meant how many millimetres of mercury could be pushed up a tube by the force of the cuff being inflated round the arm. If you used water in the tube, instead of mercury, we would measure in metres, not millimetres.
Despite the fact that mercury has nothing to do with the process any more, the measurement is still called mmHg (millimetres of mercury that can be pushed up a narrow tube). A normal blood pressure is around 120/70. If you are an Olympic weightlifter, the blood pressure can reach over 300mmHg during a lift. Which is pretty high.
In a nutshell that is what your blood pressure is…. but what does it mean? The pressure in your arm is certainly not the same as the pressure in your finger, or your brain. The pressure will be different in the right and left arms, as the blood has further to go before it reaches your right arm. It will be different if the cuff is put in a slightly different place on the arm. It will also be different if you are stressed, if the cuff is a bit small – or slightly too big.
In short, your blood pressure can be all over the place. Which is why a single measurement is not used to define high blood pressure. You need at least three. In fact, this is not nearly enough either. To diagnose someone with high blood pressure you really need to monitor the blood pressure over a twenty four hour period – using ambulatory monitoring. This helps to get rid of ‘white coat’ hypertension (high blood pressure). A phenomenon whereby the act of a healthcare professional wrapping a cuff round your arm sends your blood pressure sky high.
Because of the difficulties of measuring the blood pressure, it is estimated that around twenty five per cent of people diagnosed as having hypertension – do not actually have high blood pressure at all. Which means that they are taking drugs that they do not need. Costing the NHS at least a billion a year, and a great deal more around the world (yes, this truly is an international blog).
The other major problem with measuring the blood pressure at the arm is that it may not reflect the blood pressure just after the blood leaves the heart. The central arterial pressure. This is important, because this is the most critical pressure of all. There are a number of reasons why this is so.
Firstly, the central arterial pressure is the pressure in the aortic arch (the U bend in the aorta (biggest artery in the body)). This represents the pressure that sends blood straight up the carotid arteries and into the brain, which is clearly important with regard to stroke risk. [This where two, critically important, small BP sensing organs sit]
It is also the pressure that has the greatest impact on the kidneys. The renal arteries branch directly from the aorta itself. Therefore the central arterial pressure is closely monitored by the kidneys, which are the primary organs of blood pressure control. In addition, the central pressure has the greatest impact on the aorta itself. A relatively common cause of death is a ‘ballooning’ of the aorta (aortic aneurysm). Such aneurysms can burst, with obviously catastrophic results.
Now, there is no doubt that the pressure at the arm is related to the central arterial pressure. It must be – to a certain degree. And for most people measuring at the arm is probably a good enough estimate of the ‘true’ blood pressure.
However, if your blood pressure measurement is high, or low, or you are on blood pressure medication….then the pressure measured in the arm becomes increasingly unreliable. It can even become misleading i.e. your pressure seems to be going down in the arm – but it is not going down as much centrally1. (It may even be going up.)
‘The results of the Conduit Artery Functional Endpoint (CAFE) study also suggest that the central aortic blood pressure may be more predictive of cardiovascular events, such as stroke and heart attack, than traditional peripheral (brachial) blood pressure measurements. CAFE was the first study to repeatedly measure central aortic pressure in a major clinical outcomes trial and the first to show that central aortic pressure is a plausible mechanism to explain the better clinical outcomes seen in patients treated with amlodipine-based therapy in ASCOT.’
Of course, central arterial blood pressure is somewhat difficult to measure. Up till fairly recently you had to insert a catheter, with a measuring device, into to the femoral artery, and push it up to the aortic arch. This would not be highly practical during a consultation with a GP. So central BP is very rarely measured. But it would be best if it could…
The anomalies of blood pressure trials
Now to introduce another thread to this discussion. Which is the fact that, if you choose to look at the clinical trials on blood pressure lowering with an objective eye, there is almost no correlation between the amount the blood pressure is lowered (at the arm) – and any clinical outcomes. By which I mean that the rate of heart attacks and strokes do not relate to the degree of blood pressure lowering.
To quote a series of bullet point in the European Journal of Cardiology entitled ‘There is a non-linear relationship between mortality and blood pressure’:
- Drugs that lower the blood pressure by about the same amount have very different effects on outcomes
- Cardiovascular benefits of ACE-inhibitors (Angiotensin Converting Enzyme – Inhibitors), independent of blood pressure, are not observed with calcium antagonists, despite the latter having more pronounced effects on blood pressure.
- HOPE (Heart Outcomes Prevention Evaluation study) demonstrated that ACE inhibitors provided diverse and profound cardiovascular benefits, with only trivial differences in blood pressure between the treatment and control groups
- ALLHAT (Antihypertensive and Lipid Lowering treatment to prevent Heart Attack Trial) showed a dramatic difference in cardiovascular risk between alpha blockers and diuretics, with essentially no difference in their effect on blood pressure. The investigators of ALLHAT concluded ‘blood pressure lowering is an inadequate surrogate marker for health benefits in hypertension.’
This is extremely important, because for many years, most of the ‘evidence’ on blood pressure treatment has been based on a statistical model known as the ‘log-linear’ model. This model states that ‘the relation of blood pressure to risk of death is continuous, graded, and strong, and there is no evidence of a threshold.’ (Stamler). The model itself, and that statement, were almost entirely based on evidence from the Framingham Heart Study. The study that your doctors will use to calculate your risk of dying of heart disease.
Essentially, according the log-linear model, the lower your blood pressure (measured at your arm), the better. And the more that drugs lower it, the better. At least this is the thinking that is currently used.
However, thirty years ago Ancel Keys (yes, him) concluded that the linear model, in terms of the relationship of overall and coronary heart disease death to blood pressure was ‘unjustified’. Ten years ago, the authors of the article ‘There is a non-linear relationship between mortality and blood pressure’ further concluded (after reviewing the Framingham data – the data upon which your doctor will determine your future risk of dying of CVD)…the following:
‘Shockingly, we have found that the Framingham data in no way supported the current paradigm to which they gave birth. In fact, these data actually statistically rejected the linear model. This fact has major consequences. Statistical theory now tells us that the paradigm MUST be false ….’ (Their italics and capital letters).
In short, the blood pressure model that is used worldwide is simply, plain damned wrong. The reality is that the amount the blood pressure is lowered in the arm bears little, or no, relationship to any benefit on heart attacks and stroke. How can this be? Well, there are two major reasons for this. One of which I am covering in this article. The other, later. Now to introduce another thread.
How do blood pressure lowering drugs work?
I am going to avoid being too technical here – which is tricky. I am also, only focussing on the four most commonly used blood pressure lowering agents/classes.
1: Diuretics. These drugs make you pass more urine, by blocking sodium re-absorption by nephrons in the kidney. This means that you pass a lot of urine (diuresis). This puts you into a state of mild dehydration, thus reducing blood volume. Exactly why this lowers your blood pressure is a moot point. (You may think you know. However, it is almost certainly far more complicated that what you are thinking – I certainly don’t understand it)
2: Beta-blockers: These, effectively, slow your heart rate and also decrease the pumping force of your heart. An unwanted effect is that they also cause peripheral blood vessel constriction.
3: Calcium channel blockers: These reduce the force of contraction of the heart, dilate blood vessels (arteries not veins), and slow the heart rate at bit. All of which lowers the blood pressure.
4: ACE-inhibitors: (a bit more explanation is required to explain how they work). The kidneys are the primary organs that control blood pressure. If they pick up that the BP is too low, they release a substance called renin. Renin triggers a whole series of other hormones into action. Ending up with increased angiotensin II levels.
This hormone has multiple effects. It reduces urine production, by increasing sodium absorption. It causes constriction of arteries, and stimulates the pituitary gland to produce anti-diuretic hormone (ADH) – thus reducing urine output. It does several other things too, all of which result in the blood pressure going up.
As is the complex way of the body, the kidney doesn’t actually produce angiotensinogen II when the blood pressure drops(which would seem logical). The kidney produces renin, the liver produces angiotensinogen. When these two hormones met, angiotensinogen is converted into angiotensin I. Then angiotensin I is further converted to angiotensin II by an enzyme called Angiotensin Converting Enzyme (ACE). (There will be an exam later)
All of this means that angiotensin II is the main, active, substance. Once it has been produced, angiotensin II goes off to do all its blood pressure raising things. If, however, you give an ACE-inhibiting drug (ACE-inhibitor), angiotensin II production is blocked, and the blood pressure will fall.
Anyway, as I hope has now become clear, the blood pressure lowering classes of drugs all work though very different mechanism. They all lower the blood pressure at the arm, but what else are they doing?
Beta blockers tend to constrict peripheral blood vessels. Calcium channel blockers and ACE-inhibitors tend to dilate them. Diuretics are mainly neutral on blood vessel diameter. ACE-inhibitors also do something else that is extremely important. They stimulate Nitric Oxide synthesis in the blood vessels themselves, which both dilates arteries, and increases blood vessel flexibility.
In short, the effect on central and peripheral blood pressure of various BP lowering medications will be very different.
Bringing these thoughts together
You may think, why now? Why is he quoting articles, and research, from many years ago? Well, you have to bear in mind that it is a long time since anyone did a placebo controlled blood pressure lowering study. It would be considered unethical to do so now (such are the alleged enormous benefits of BP lowering). So, there isn’t really any fresh information. Just the monitoring of one drug vs. another, and assuming benefit based on the degree of BP lowering – using the log-linear scale.
However, it has now become possible to measure central blood pressure by simply using a cuff placed round the arm. I have had this process explained to me many times, and cannot really understand how it is done. But the results are repeatable, and accurately reflect central blood pressure. Which is all that really matters.
When you do this, you can also measure the velocity of the pulse wave, which is an accurate indicator of arterial flexibility – and thus arterial health. If your arteries are stiff this is a worrying sign, and reflects poor arterial health. The more flexible your arteries are, the better.
At last, hoorah, instead of wrapping a simple cuff round people’s arms, we can use a complicated cuff to look at two more, really important things. The central blood pressure, and arterial flexibility. This gives us far more information.
Perhaps most importantly, we can monitor the effects that different blood pressure lowering medications have, beyond their impact on the BP measured at the arm. We can see if central pressure is increased, or decreased, or if arterial compliance (flexibility) is improved.
I think that this is a major breakthrough in medical practice. So much so, that I have acquired a machine for myself, and will be using it on a regular basis. I fear it will take the wider medical profession about twenty years or so for this to become an accepted way of measuring blood pressure. This is about the normal lead time for new ideas to become standard practice.
It may, of course, take longer. Or never happen at all. At the risk of going off on a major tangent, I remember looking at pictures of roads Sweden in 1967. This was when they switched from driving on the left, to driving on the right. 3rd Sept 1967
As you can see from the picture, a bit of a mess. But imagine if any country tried to do it now, with the extra number of cars and lorries, and roads, and signs. This would probably be just too difficult.
How about changing the way we look at measuring blood pressure. We have always measured blood pressure using a simple cuff on the arm. All the clinical studies on BP lowering were done using this technique. All the data, all the guidelines….everything, is now based on doing BP measurement in this way.
Just imagine what happens if someone now says. Hold on, this is not good enough. The measurement is inaccurate and potentially confusing, and it doesn’t’ really tell us what we need to know. Let us start again. Let us drive on the right, not the left.
In the meantime, whilst the medical world grapples (or chooses not to grapple) with a trillion dollar problem, you can do yourself a favour and get your blood pressure measured centrally.
2: Port S, et al: ‘There is a non-linear relationship between mortality and blood pressure.’ European Heart Journal (2000) 21, pp. 1635 – 1638