What is Blood Pressure (BP)?
We all have “blood pressure”. Blood pressure (BP) is the pressure of blood within the major arteries. It is expressed as a two-figure quantity – say 120/80. The higher figure is the systolic pressure, which is the peak pressure reached when blood is pumped out from the heart to the main arteries. The arterial pressure then falls to the lower figure, which is the diastolic pressure, the lowest level reached before it is pushed up again by the next contraction of the heart.
How is BP measured?
Using a “sphygmomanometer”. A pressure measuring cuff is wrapped around the upper arm and inflated and then the air is allowed to escape gradually. Listening to the artery in the bend of the elbow reveals the pressure at which the blood starts to flow and this is the systolic pressure. The noise of flow ceases at the diastolic pressure . Nowadays the sounds are usually detected electronically rather than through a stethoscope.
The unit of measurement of BP is millimetres of mercury (mmHg). This a strangely antique unit came into being because blood pressure was always measured with a mercury ‘sphygmomanometer’ (a BP-measuring instrument) which uses the height of a column of mercury as the measure of pressure. Modern sphygmomanometers no longer use mercury, so it would be more logical to use the Système International (SI) unit of pressure, the Pascal. However, the long-established mmHg is so ingrained into doctors’ psyche that they have clung on to this archaic unit, rather as we continue to measure distance in the UK in miles rather than kilometres.
High BP – “hypertension”.
High blood pressure is classified as a disease – though to my mind it is more a risk factor for disease – mainly for coronary disease and stroke.
The distribution of blood pressure (BP) in the population follows the usual distribution of most human characteristics – the bell-shaped curve. As with height, for instance, there are a few people at the lower end of the range and a few at the upper end, but most are somewhere in between.
Range of systolic blood pressure in the population, taking the Gaussian or bell-shaped distribution
For the majority, high blood pressure is not a disease – just in the upper range of a continuous scale. For a few, high BP is due to some other condition, particularly kidney disease. Someone deemed by the medical profession to have high blood pressure is labelled as having ‘hypertension’. It is a very difficult condition to define – just where is the cut-off between acceptable upper level of blood pressure and unacceptable hypertension? It could be the level above which the complications of raised blood pressure kick in – but unfortunately the ill effects develop insidiously and the higher your pressure the higher the risk. Wallis Simpson said that ‘you can never be too thin or too rich’ and to that I would add ‘nor have too low a blood pressure’.
A better definition of hypertension would be the level above which lowering the blood pressure reduces risk. Here we should be on firmer ground, but unfortunately even this soil is somewhat boggy. For decades it has been assumed that when a mildly raised pressure is treated with the appropriate medication to reach a more pleasing figure, risk is automatically reduced. However, there has been no evidence for reducing the level below 140 systolic until a recent study from the US, the SPRINT study, which has indicated that getting the pressure down to 120 systolic may have substantial advantages over the higher target of 140. There is less certainty about appropriate BP levels in people over the age of 80. Enthusiastic lowering of BP in this group carries the risk of overdoing it and causing postural hypotension – low BP when standing. This may cause faintness, dizziness and possibly dangerous falls in the elderly. It may also aggravate cognitive decline in older people.
Another confusion in the assessment of blood pressure is that it is not a fixed figure; it is a fiction of the moment. It varies with time of day, pressure of work, timing of meals and contact with other people – particularly doctors. We are all aware of ‘white coat’ hypertension, which is the condition in which blood pressure is markedly higher when measured by a doctor. If you want to know what your blood pressure really is, take it yourself. There are plenty of reliable automatic home blood-pressure machines which will give you a much more accurate picture of your BP, at whatever time of day you wish, than does the occasional snapshot BP taken in the surgery. The alternative is the 24-hour blood pressure recorder which is available to most GP surgeries.
However it is measured, raised blood pressure is common in Western societies and it continues to rise steadily with age. A recent American study found the prevalence of hypertension to be 9 per cent for those aged 18–44, 40 per cent for those aged 45–64 and 75 per cent for those aged 65 and over.
I will tell you about the ill effects of hypertension, how it is treated and particularly the role of exercise in preventing and treating this very common condition.
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