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(3) High Blood Pressure

Low Blood pressure and High Blood Pressure

Blood pressure is needed to maintain circulation in your body so that oxygen and other nutrients can be transported to your cells. When your blood pressure is too high, it can damage the tissues and cells in your body.

 About blood pressure in general

I don’t think I’d ever thought about blood being under pressure before I was told I had high blood pressure.  Why should blood be under pressure?

The function of blood is to transport materials around the body, mainly to take oxygen and food substances to body cells to keep them alive and well, and to remove their waste products such as carbon dioxide. If your bloods were not under some pressure, it would just stay where it is, stagnant, neither nourishing nor cleansing the cells of your body, so that within about 7 minutes first they and then you would die.

How does blood move around the body?

The circulatory system consists of two circular systems of tubes, circulation through your lungs (pulmonary circulation) and circulation through the rest of your body (systemic circulation). Blood is moved through these two systems by two pumps, the right and left sides of the heart. Although these right and left pumps beat together, the blood in each side is entirely separate and the two pumps can, and do to some extent, function independently. Because pushing blood through your lungs is much easier than pushing it through every other part of your body, the left side of the heart is bigger, more muscular, and carries a much heavier workload than the right.

Obviously blood is under pressure at all points throughout both systems, in arteries, capillaries (the smaller vessels) and veins, otherwise it would not circulate. Pressure in capillaries is not only extremely small, but has to be kept constant, so that conditions for transfer of oxygen and nutrients into, and carbon dioxide and waste products out of, each body cell remain unchanged, despite huge differences in what the rest of your body may be doing. In fact the main reason for variability in your arterial BP is the need to keep capillary BP the same. 

Obviously more pressure is needed to push blood up against gravity than to push it down. The animals that have to cope with the biggest changes in BP are therefore giraffes, whose brain capillary BP has to be kept constant whether their heads are 2 metres or so below their hearts when they drink, or 2 metres above them when they are eating leaves off the trees.

Pressure is generally much higher in arteries than in veins. When doctors talk about ‘blood pressure’, they normally mean arterial pressure. It is important to understand this. If you cough, sneeze, push your car out of a ditch, or strain hard sitting on the toilet, you

go red in the face and you can feel your head filling up with blood.

This is caused by raised BP in your veins (venous pressure), not in your arteries (arterial pressure). Many people imagine that high BP causes similar symptoms, but this is completely wrong.

Arterial pressure is highest close to the heart and diminishes as blood moves further out along the arterial tree. Arterial pressure is most easily measured in the arm just above the elbow (in the brachial artery), so this has long been the international standard way of measuring BP.

The pressure of a fluid flowing through any tube depends on four main variables:

• A the rate at which fluid enters the tube • B the diameter of the tube

• C the friction from its walls

• D the viscosity (stickiness and elasticity) and volume of the fluid.

Likewise in your body:

 A Your heart pumps blood into your arteries at a variable rate, depending on both what you are doing and what you are thinking.

• B Your smaller arteries are of variable diameter, depending on tension in a spiral muscle, encircling Blood pressure and high blood pressure them in much the same way as the spiral wire in a vacuum cleaner hose; this tension in turn depends mainly on fast signals from the brain and slow signals from various circulating chemicals (hormones) released from other organs.

• C Friction along artery walls increases as they get older, rougher, and furred up with waxy plaques made of a mixture of clotted blood and cholesterol (this process of roughening raises BP by increasing resistance to blood flow, and is itself speeded up by raised pressure - a vicious circle).

• D Finally, both the viscosity and the volume of blood vary, depending mainly on salt intake, the efficiency of your kidneys, and the size and shape of red blood cells, which may be much altered by low levels of blood iron or high levels of blood alcohol.

Blood pressure seems to be written down as a fraction, for example 150/85.

What do these figures represent, and what do they mean?

If you stick a vertical glass tube into a large human artery, blood will rise about 9 meters (30 feet) up the tube, to the point where the weight of the atmosphere balances the pressure supporting the column of blood (the Reverend Stephen Hales did this in the 18th century, using a live horse). As 9-12 meters (30-40 foot) tubes of air were inconvenient, they were replaced by tubes of mercury; as puncturing an artery is painful, the pressure required to stop pumping sounds from the heart became used as an indirect way of measuring pressure inside the artery. Since about 1900, BP has been measured in this way in millimeters of mercury (mmHg), with the use of an ‘indirect sphygmomanometer’.

If you put an ear (or, more conveniently, a stethoscope) over a large artery in the crook of your elbow, you will hear nothing; but, if you squeeze the artery with an inflatable cuff until it is completely blocked above the listening point, and then very slowly release it, you will first hear clear, regular tapping sounds, then these will disappear. The level of pressure at which these sounds are first heard is ‘systolic pressure’, the pressure at which blood is first pushed out of the heart into the arteries. If you keep listening, while slowly dropping the pressure in the cuff, the tapping disappears. Then, at a pressure about 50-100 mmHg lower, you will hear regular but much softer whooshing sounds, which will also disappear after a further fall of 5-10 mmHg. The point at which these soft sounds disappear is ‘diastolic pressure’, the pressure of blood in the arteries between heart beats. At one time doctors disagreed about whether to define diastolic pressure as appearance or disappearance of the whooshing sound, but there is now international agreement to accept disappearance.

BP is normally recorded as systolic/diastolic, for example 105/54 mmHg (an unusually low pressure), 125/70 mmHg (an average’s pressure), 164/95 mmHg (a highs pressure), 182/106 mmHg (a definitely high pressure) or 235/140 mmHg (a dangerously high pressure).

Should I know what my own figures are?

In our opinion, yes, and we think that you should know them as actual figures rather than as general statements like ‘high’, ‘normal’ or ‘low’. Other doctors will ask you about your BP and, without some figures, your answers will be as meaningless to them as to you. You should also know what your overall risk is of suffering a stroke or heart attack). You should appreciate that your BP reading will vary.

How and why does BP vary so much?

 Measurements were made very accurately through a polythene catheter pushed into an artery in his arm, following a local anaesthetic to ensure a painless procedure, which would not itself affect BP.

As you see, his BP varied throughout the day, with a sustained fall during sleep, a marked rise when somebody pushed a pin into his skin (marked P on the chart), another when he made love (marked C), and another sustained rise during the first half of the morning. This pattern of rises and falls is typical not only of people with normal BP, but also of those with high BP.

Everyone, whether their BP is high or low, has mechanisms for distributing varying amounts of blood to different parts of the body, depending on what they are doing. For example, if you are thinking hard, your brain needs a larger blood flow than when you are asleep; after a large meal you need a much larger blood flow to your gut; and, if you are running, blood flow to all your large muscles is enormously increased.

In a normal man or woman, total blood flow at rest is about 6 litres (about 10.5 pints) a minute, with about 13% going to your brain, 24% to your gut, 21% to your large muscles, 19% to your kidneys, and 4% to your heart muscle. If you run as hard as you can, blood flow to your brain remains exactly the same, but total flow through your body as a whole rises more than four-fold to 25 litres (95 pints) a minute. Flow through your heart muscle

rises four-fold, and through other large muscles ten-fold, but falls to four times less than at rest in your kidneys, and five times less. These changes begin even if you just think about running; your body prepares for action by redistributing blood flow and a substantial rise in BP is part of this process. In this way BP responds quickly, though usually for only a short time, to emotional states such as fear, embarrassment, anger, sexual interest, and even simple curiosity.

Between this high-pressure network of arteries conveying blood from the lungs and heart, and the low-pressure network of veins taking it back again, lies a vast mesh of croscopically small capillaries, their walls made of extremely thin cells through which molecules of oxygen, nutrients and cellular waste products can be easily exchanged. For these delicate but vital transactions to take place, pressure within the capillaries must remain constant, within very narrow limits. Given these huge shifts in blood flow between organs, this constancy can be maintained only by precise control of flow through small arteries and arterioles.

This control is mainly exerted through changes in tension in the spiral muscle surrounding the smallest arteries (arterioles), which result in big changes in arterial BP.

My BP seems to change throughout the day. Is this normal?

As you can see, there is a ‘diurnal’ pattern to normal BP (in other words, BP changes throughout the day). In addition, there are many other different types of patterns of high BP that vary between different people. ‘White coat hypertension’ will be discussed later -this occurs as a response to BP measurement itself. Isolated systolic hypertension occurs more commonly in elderly people. The other patterns shown in the figure will also require BP-lowering drug treatment. Most often, detailed 24-hour readings are not necessary and the ‘prognostic’ significance between these different types of BP readings (whether one type of  ambulatory reading is more harmful than another type of reading in terms of risk of stroke or heart attack) over a 24-hour period is not substantial.

Low blood pressure

Presumably, if there are people with high BP, then there can be also be people with low BP. Does low BP cause symptoms?

You are correct. There are people who have low BP. Most people have no symptoms from their low BP. However, if BP in your neck arteries is not high enough to supply the oxygen and glucose needed to support the full function of your brain cells, you lose consciousness – that is, you faint. In teenagers (particularly girls) this happens easily and often, because their BP is generally very low (systolic pressures under 100 mmHg are common), and often less stable than in mature adults. The same thing will happen if your BP is brought down too low by overtreatment.

There is no evidence that ‘low BP’ justifies prescription of drugs such as non-steroidal anti-inflammatory drugs (NSAIDs) to raise BP, although this is quite commonly done in Germany. In the UK there has been a professional consensus against this practice.

 High blood pressure

What is high blood pressure?

I have heard my doctor use the word ‘hypertension’. Is this the same as high BP?

Yes, there is no difference: these words are used interchangeably by doctors, but have the same meaning.

I always thought that ‘hypertension’ implied a main cause (stress or tension) and therefore conveys more meaning than ‘high BP’. Is it not therefore a better name?

No. The word ‘hypertension’ comes from translating the French tension arterielle. This originally referred not to psychological or physical tension in the mind, but to tension (stretching) of artery walls. Tension in the mind is one possible cause of high BP in

some people, but is certainly not the main cause in everybody.

My husband has high BP, but he seems to be just as well as everyone else in our family. So is high BP really an illness?

With the exception of ‘malignant’ high BP, high BP is not an illness that you either do or do not have. Rather it is a risk marker for other illnesses that you would wish to avoid. These include stroke, heart attacks, kidney problems and other problems affecting the circulatory system. Most people (like your husband) have high BP but do not have any symptoms. However, not having symptoms does not imply that he is not at risk of having any of the potential harmful consequences of having high BP. His risk of suffering complications can be reduced by non-drug or drug treatment and he also will need to address other risk factors, for example smoking, that increase his overall risk.

Is there a dividing line between normal and high BP?

Research has shown that at every level of BP, higher BP means a higher risk of stroke, heart attacks and other circulatory problems – all these are considered as ‘cardiovascular’ risks. So there is no conventional level below which BP is ‘normal’. Risk of any of these problems is related specifically to your level of BP. Added to your level of BP are other important contributory factors to your overall level of risk of stroke, heart disease and other cardiovascular problems. These include your age, sex, cholesterol level, smoking status, height and weight . It is for this reason that your overall risk of cardiovascular illness  will be estimated.

Assuming that you have had enough careful measurements to measure your average BP correctly, the truth is that ‘normal’ BP becomes high at the point where the advantages of being supervised by your doctor (and probably eventual medication) outweigh the disadvantages. This will also depend on your overall level of cardiovascular risk and on whether you are prepared to take BP-lowering drugs or not.

I haven’t had any symptoms despite my BP being high.

Why does it need to be treated?

The only reason anyone needs treatment for high BP is to prevent its likely consequences – what we call ‘cardiovascular risk’. Many serious sorts of organ damage are eventually caused by high BP, including all forms of stroke, coronary heart attacks, heart failure, retinal damage in your eyes, obstruction of blood flow in leg arteries, stretching (‘aneurysm’) of the aorta with consequent high risk of bursting (‘rupture’) of the aorta, and damage to the kidneys leading eventually to kidney (‘renal’) failure.

All these problems happen more often in people with uncontrolled high BP, and the higher the pressure is, the more likely they are to occur. After a few years of treatment,

these risks become roughly proportional to the level of BP after treatment, not to the level of BP before treatment – so careful treatment works.

High BP is not the only cause of these kinds of organ damage, so reducing BP cannot wholly prevent them. Smoking is another very important cause of all of them, and so are high blood cholesterol levels and diabetes, which are often associated with being overweight and doing insufficient regular exercise.

Prevention is more effective if these factors are addressed.

Are there different kinds of high BP?

Indeed there are, and probably more than we know of at the present time.

High BP was traditionally classified into two main groups:

• Rare cases for which causes were known (‘secondary hypertension’ – in other words, where high BP is secondary to some other condition), and

• Common cases where no cause was known (‘essential hypertension’). ‘Essential’ did not mean that the high BP was necessary, but that it was ‘of the essence’: in other words, you had it because you had it, for reasons unknown.

This word is now beginning to give way to the more commonsense terms of ‘primary hypertension’, or ‘primary high BP’.

Secondary high BP is mostly caused by various sorts of kidney disease, or occasionally by malformations of the aorta (‘coarctation’), overproduction of some BP-raising hormones by tumours of the pituitary gland, adrenal glands or kidneys, or by disorders involving compression of the brain or brain stem. These ‘classical’ secondary causes altogether used to account for less than 1% of all treated cases of high BP. If we exclude high BP caused by the oral contraceptive pill, this remains true today. As ‘essential’ high BP was by definition of unknown cause, the category has inevitably become more and more unreal, as more interacting causes are found. We now have the situation that many important causes are known, which, if dealt with early, can lead to a fall in BP, but these have not yet been reclassified as ‘secondary hypertension’. Examples are being overweight and drinking too much alcohol in young men, mostly those with a family history of high BP and therefore genetically susceptible to these causes. As more real causes are discovered, even primary hypertension will eventually have to be recognized as a diverse group.

If doctors know all this, then why do they still use the terms like ‘essential hypertension’ or ‘primary high BP’?

The category remains useful because, although causes are diverse, consequences of uncontrolled high BP, and methods of controlling it, are not. Whatever the cause of high BP, risks of stroke, heart failure, coronary disease and various other sorts of organ damage are increased. Perhaps even more importantly, no convincing or consistent evidence has yet been found that different causes would benefit much from different medication.

In practice, the aim of treatment is usually not to find causes and treat them, but to find medication that works and is well tolerated, and then to keep on prescribing it. In some ways this is a realistic acceptance of the limitations of current medical knowledge, but this attitude also leads to very widespread neglect of other measures (such as weight reduction and alcohol restriction, which can be equally effective, particularly in young people), and great wealth for pharmaceutical companies.

I heard the words ‘malignant hypertension’ mentioned in the papers recently? Does it have anything to do with cancer?

No, despite the name, it has nothing to do with cancer, but it is the most serious form of high BP and can cause immense damage (and even death) in a very short time. Malignant high BP is a medical emergency. If it is not recognized and treated urgently, irreversible damage of the kidney, retina and brain are likely in a very short time – delays before starting treatment should be measured in hours rather than days. Before effective treatment of high BP became available in the 1950s, death rates from malignant high BP were normally 100% within 2 years, mostly from heart failure, kidney failure or massive stroke. Many people became blind or paralysed long before this.

As more and more people with high BP are picked up early by routine measurements of BP, it is becoming rare. Malignant high BP does not occur unless either very high BP has persisted for a Blood pressure and high blood pressure long time, usually several years, or BP rises very fast indeed, with no time for artery walls to thicken and resist this pressure.

Although many people with malignant high BP get severe headache, others may have severe kidney damage and very high pressures without any symptoms at all. Anyone with a severe headache should have their BP measured immediately, although high BP is rarely the cause. Another common early symptom is blurred vision or patchy loss of vision starting in one eye. Anyone with a diastolic pressure over 120 mmHg might have malignant high BP, so tests will include a urine test for protein and a retinal examination with an ophthalmoscope, either in a dark room or after putting drops into the eyes to dilate the pupils. Today, if malignant high BP is found, you would normally be sent into hospital urgently, and BP will brought down gradually over the next 2–3 days. Subsequent treatment is usually the same as for all other cases of high BP, but will be maintained for the rest of your life.

So what exactly is happening in malignant high BP?

If BP remains very high for weeks, months or years, with a sustained diastolic pressure of at least 120 mmHg (usually much more), the walls of the smallest arteries (arterioles) begin to crumble. Blood then leaks out of them, interrupting the supply of arterial blood wherever they happen to be. This usually begins in the kidney, where damage leads to release of hormones, which push BP up even higher, thus setting up a vicious circle of acceleration, in which already very high BP pushes itself higher still.

The next site of arteriolar damage is usually the retina, causing leaks of blood (‘retinal haemorrhages’) and leaks of plasma (‘retinal exudates’). Finally, there is arteriolar damage to the brain, causing first swelling of the head of the optic nerve, then fits, and finally small strokes, unless the whole sequence is interrupted by heart failure.

 The scale of the problem

How many people have high BP?

High BP is the commonest major disorder seen and tackled by doctors. BP high enough to require some kind of medical treatment and continuing supervision affects between 10 and 30% of the adult population, depending on age, and ethnic or social background. This compares with 2–8% of the adult population for diabetes, a disorder of comparable significance for health.

Is it a growing or diminishing problem?

There is no evidence that average BP in the general population has risen over the 70 or so years over which measurements are available. There is some evidence from the USA that it may have diminished, and that this reduction is due to the increasing use of BP- lowering drugs over the last 30 years. Whatever definition of high BP is used (and this has varied a good deal, since all definitions are arbitrary), the proportion of people with high BP is directly related to average pressure throughout the population. Standardized and accurate BP measurements for large representative populations have been available in the UK, Scandinavia and the USA only since the 1950s, and more recently for other countries. There is some evidence from countries whose national diet has shifted from very high to much lower sodium intakes, notably Japan, Portugal and Belgium, that average BP in the general population has fallen, probably for this reason. These reductions in sodium intake reflect shifts in methods of food preservation from salting, smoking and pickling, to refrigeration and rapid transport of fresh food. As these changes have occurred in all economically developed societies, BP has probably fallen everywhere, compared with average levels in the 19th century.

This view is supported by trends in death rates from stroke, which are known to depend more on average BP than on any other factor. In every country that collects complete and reliable data on deaths by medically certified cause, stroke rates have been falling, probably since the 1920s, certainly since the 1950s.

Are you more likely to get high BP if you’re rich rather than if you’re poor?

No, it’s the other way around. Research has shown higher average BPs in poorer people. Although these BP differences in different social classes are not large, there are also differences in other risk factors for heart disease. These differences translate into differences in rates of stroke and heart disease according to different social class.

Are there differences in BP between races, or between different sorts of societies?

High BP does not seem to exist in the few uncontaminated primitive societies that still exist, for example South American rainforest Indians on the upper Amazon, and Papua-New Guinea Highlanders. In the rest of the economically undeveloped countries, high BP is a much more serious problem than in developed economies, with very high stroke rates, particularly in rural areas, and corresponding burdens of care for these populations. In the economically less developed parts of southern and central Europe, death rates from stroke are still more than twice as great as death rates from coronary heart disease, whereas in developed European economies death rates from coronary disease are about three times as great as those from stroke.

High BP is a serious problem in the African - Caribbean black population, occurring in up to 40% of adults of this ethnic background. Amongst this group, high BP tends to be more severe and is associated with a higher risk of complications, particularly heart and kidney problems. British South Asians (from the Indian sub-continent) also have a risk of developing high BP. They are also more likely to suffer from diabetes, which acts in tandem with their high BP to increase the risk of heart attacks and strokes substantially. Well-fed people from the Indian subcontinent have very high rates for diabetes, affecting up to 12% of the middle-aged population in some immigrant communities in the UK, compared with 2–4% for ethnic Europeans. As diabetes is itself a major risk factor for stroke, populations of Indian descent have much larger health problems related to BP control. A multiple approach, including stopping smoking, and taking cholesterol-lowering drugs and aspirin is often recommended, in order to reduce overall risk of stroke and heart attack.

It seems impossible these days to talk about anything to do with people’s health without talking about money incurred by the National Health System (NHS) – so what are the economics of treating high BP?

This depends first on how much treatment is rational, i.e. there is good evidence that it is preventing stroke and other problems of uncontrolled high BP, and how much treatment is driven by other incentives and pressures: in private practice, these include fees;  in competitive NHS practice, they include satisfying the expectations of people (real or assumed), and for almost everyone they include pressure from the pharmaceutical industry (which, like any other industry, wishes to increase its sales and profits). Secondly, it depends on whether we see benefits in terms of health gains for people or cash relief for taxpayers.

In the UK, roughly one-fifth of the people with high BP who would benefit from treatment are not identified, one-fifth of those with known high BP are not having treatment, and half of those treated do not have their BP properly controlled. This failure to deliver care is socially distributed: those in the greatest need in the poorest sections of society often get the least care.

It is most likely that the greatest gain from the least investment would be not to look for more undiscovered and untreated people with high BP, but to reorganize the care of those already known and treated. General practices need to organize follow-up clinics so that they have lists of people who need to attend, check whether they have actually done so, and if not, ask themselves why they haven’t. Research in both the USA and UK has shown that organized review and re-call clinics in the community are highly effective in helping people with high BP reach their ‘goal’ of reducing their BP.

A more recent change is that management of high BP is now seen in the context of overall risk of having a stroke, heart attack etc. GPs are now being encouraged to treat high BP by dealing with other major risk factors for stroke and coronary heart disease, such as lowering cholesterol and stopping smoking.

High blood pressure, is a risk factor for heart disease and stroke. Having high blood  pressure generally causes no symptoms for most people. It is only in the very rare situation of ‘malignant’ high blood pressure that symptoms may occur, usually in the form of headaches.

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