Blood pressure refers to the pressure of blood on the walls of the blood vessels of the body.

It is a fundamental principle of Physics that all fluids when held in a container exert a pressure upon the container walls. This pressure is a hydrostatic pressure.

Blood is no exception to this physical principle and therefore blood pressure is a hydrostatic pressure.

Each blood vessel has it's own blood pressure value, arterial blood pressure, capillary blood pressure, venous blood pressure, left atrial blood pressure, right ventricular blood pressure etc.

The pressure in the systemic blood vessels falls continuously from the aorta until the blood re-enters the heart in the right atrium.

Fig.1 Blood pressure in each section of the systemic and pulmonary circulations

Blood pressure in the pulmonary system is considerably lower than that of the systemic system but there is still a pressure gradient from the blood leaving the right ventricle to the left atrium.

In both systems it is essential to have a pressure gradient so that the blood will flow from the area of highest pressure to the area of lowest pressure thus maintaining the circulation.

In nursing practice reference to blood pressure normally means systemic arterial blood pressure. The use of this as the basic measure is quite appropriate as it is this pressure that ensures an adequate blood flow to the tissues and vital organs. If the pressure falls too far blood flow is reduced, perfusion of the tissues is lowered and shock can occur. Fainting is an example of this, here blood flow to the brain is reduced and loss of consciousness is the result.

There are several mechanisms which exist in order to maintain the blood pressure within normal limits. It is important that the correct blood pressure is maintained if the individual is to maintain a healthy status. As mentioned above low blood pressure leading to shock or fainting.

Abnormal pressure in the blood capillaries can result in abnormal exchange of fluids to and from tissues which can result in conditions such as oedema.

As mentioned already, (in the pages on structure and function of blood vessels) the cardiovascular system is a closed system. As a result the total blood flow leaving and entering the heart (in normal health) will be the same. Thus blood flow is equivalent to the cardiac output.

Pressure difference is the difference between the mean pressure in the aorta and the pressure in the vena cava just before the blood enters the heart (this latter value is almost zero). Since the blood pressure is essentially the same in the aorta and all large arteries, the pressure difference can be said to be equivalent to mean arterial pressure.

Resistance to flow is the total resistance to blood flow. As the majority of the resistance is found in the peripheral vessels, especially the arterioles, it is often described as the total peripheral resistance.

Blood Pressure = Cardiac Output x Total Peripheral Resistance

BP = CO x TPR

This is one of the fundamental equations of cardiovascular physiology. You can see from the equation that blood pressure can be maintained by altering cardiac output and/or total peripheral resistance. The cardiac output itself is changed by altering the heart rate and stroke volume.

Arterial blood pressure fluctuates throughout the cardiac cycle. The contraction of the ventricles ejects blood into the pulmonary and systemic arteries during systole and this additional volume of blood distends the arteries and raises the arterial pressure. When the contraction ends, the stretched elastic arterial walls recoil passively and this continues to drive blood through the arterioles. As the blood leaves the arteries the pressure falls; the arterial pressure never falls to zero because the next ventricular contraction occurs whilst there is still an appreciable amount of blood within the arteries. Thus the pressure in the major arteries rises and falls as the heart contracts and relaxes. The maximum pressure occurs after ventricular systole and is known as the systolic pressure. When the blood pressure in the aorta exceeds that in the ventricle, the aortic valve closes; this accounts for the dicrotic notch.

Fig 2. Wave form showing variation in blood pressure in the main arteries as the heart contracts and relaxes.

Once the aortic valve has closed, the blood pressure in the aorta and large arteries falls as blood flows through the arterioles and capillaries to the veins. The level to which the arterial pressure has fallen before the next ventricular systole, that is the minimum pressure, is known as the diastolic pressure.

The systolic pressure is determined by the amount of blood being forced into the aorta and arteries with each ventricular contraction, i.e. the stroke volume, and also by the force of contraction. An increase in either will increase the systolic pressure. Similarly, if the arterial wall becomes stiffer, as happens in arteriosclerosis, the vessels are not able to distend with the increased blood volume and so the systolic pressure is increased.

Diastolic pressure is also influenced by several factors. The diastolic pressure provides information on the degree of peripheral resistance: if there is increased arteriolar vasoconstriction, this will impede blood flowing out of the arterial system to the capillaries, and the diastolic pressure will rise. Conversely, if the peripheral resistance is reduced by vasodilation, more blood will flow out of the arterial system and thus diastolic pressure will fall. Drugs that modify the degree of arterial vasoconstriction and alter the peripheral resistance will obviously affect the diastolic pressure, and vasodilator drugs, for example hydralazine hydrochloride (Apresoline), minoxidil (Loniten), are sometimes used in the treatment of severe hypertension.

The diastolic pressure also depends on the level of the systolic pressure, the elasticity of the arteries and the viscosity of the blood. Alterations in the heart rate will also affect diastolic pressure: with a slower heart rate, the diastolic pressure will be lower as there is a greater time for blood to flow out of the arteries, and vice versa.

PULSE PRESSURE AND MEAN ARTERIAL PRESSURE. Each ventricular contraction initiates a pulse, or wave, of pressure through the arteries and these pulses can be palpated (felt) wherever an artery passes near the skin and over a bony or firm surface.

Figure. 3 to the left shows the common sites where the pulse is felt.

2. External maxillary (facial) artery at the point of crossing the mandible (lower jaw)

9. Dorsalis pedis artery on the upper front part (anteriosuperior aspect) of the foot

There is one pulse per heart beat, and so the pulse rate is used as an easy method for counting the heart rate. These palpable pulses represent the difference between the systolic and diastolic pressures and this difference is known as the pulse pressure, e.g. the pulse pressure in an individual whose blood pressure is 130/80 mmHg is

Two of the main factors that alter the pulse pressure are the stroke volume and decreased arterial compliance.

It is sometimes useful to have an average, or mean, value for the arterial pressure, rather than maximum and minimum (systolic and diastolic) pressures, as it is the mean pressure that represents the pressure driving blood through the systemic circulation.

The mean arterial pressure is not a simple arithmetical mean; it is estimated by adding one-third of the pulse pressure to the diastolic pressure. So, for a blood pressure of 130/80 mmHg

From the point of view of actual tissue perfusion, it is generally the mean arterial pressure that matters, rather than the precise values of systolic and diastolic pressures.

Blood pressure values. There is no such value as a 'normal' blood pressure for the population as a whole; there is a usual or 'normal' value for any particular individual, but even that value varies from moment to moment under different circumstances and over longer periods of time. Many factors, both physiological and genetic, have an influence on blood pressure and thus it is not surprising that individuals have significantly different, but 'normal', blood pressure values. Therefore it is more appropriate to refer to a normal range of blood pressure than to a single value.

Age (years)	Systolic pressure (mmHg)	Diastolic pressure (mmHg)
New-born	80	46
10	103	70
20	120	80
40	126	84
60	135	89

Table 1. Some 'average' blood pressures relating to age

Parameters such as age, sex, and race influence blood pressure values. In Western societies, blood pressure values tend to increase with advancing age therefore a blood pressure which would be 'normal' for a 70 year old might be considered 'abnormal' for a 40 year old. This is not universal, for example South Pacific Islanders show little, if any, increase in mean blood pressure with increasing age . The elevation in blood pressure with age may be due either to genetic or environmental factors and is likely to be a result of arteriosclerosis.

Men generally have higher blood pressures than women. Race also seems to influence blood pressure levels, e.g. in the USA Afro-Caribbean races tend to have higher blood pressures than whites.

Most authorities agree that a resting diastolic pressure persistently exceeding 90 mmHg or 95 mmHg indicates hypertension, that is, a raised blood pressure; this is an arbitrary definition but proves to be useful for clinical practice. A persistently low blood pressure, hypotension, is relatively rare, although temporary or transient hypotension is more common, e.g. in haemorrhage or fainting.

HYPERTENSION. One of the reasons that clinicians are so concerned about the level of an individual's blood pressure is that there is a significantly increased mortality in those with untreated hypertension when compared with individuals with a 'normal' blood pressure (normotensive): a 35-year-old man with a diastolic pressure of 100 mmHg can expect a 16-year reduction in life expectancy.

It has been estimated that nearly one-quarter of the adult population in the UK has an elevated blood pressure.

Individuals who are hypertensive usually have few, if any, symptoms and often the hypertension is only diagnosed as part of a routine medical screening, for example for insurance purposes. The effects of a raised blood pressure are insidious and develop over many years: the heart has to increase in size (detectable on x-ray) and strength to overcome the increased resistance caused by the increased blood pressure.

The arteries respond to the increased pressure by hypertrophy of the smooth muscle in their walls, so that they are able to withstand exposure to the higher pressures.

Atherosclerosis formation is also potentiated. The blood vessels most commonly affected are the cerebral, coronary and renal vessels; cerebrovascular accidents (strokes) and myocardial infarctions are the commonest clinical manifestations, followed by renal disease.

There has been much research and discussion into the causes of hypertension. In a few instances, hypertension is secondary to renal or endocrine disease, but in the majority of cases the cause of primary or essential hypertension is not fully understood.

The aetiology of essential hypertension is almost certainly multifactorial and it is likely to prove to be a combination of genetic and environmental factors. Mechanisms that seem to be involved include some that affect the extracellular fluid volume and expand the circulating blood volume, e.g. excessive renin secretion and angiotensin production, increased sympathetic activity and excessive dietary salt intake, possibly associated with a low potassium intake.

Some of the treatments prescribed for hypertension relate to these mechanisms, i.e. diuretics (e.g. a thiazide) to increase sodium and water loss; methyldopa, B-adrenoreceptor blocking drugs (e.g. propranolol), and relaxation techniques to reduce sympathetic activity; restriction of salt intake. One drug, captopril, inhibits angiotensin converting enzyme in the lungs and reduces the production of angiotensin II.

Raised peripheral resistance is linked with hypertension and so drugs that produce vasodilation are useful.

There are many risk factors associated with the development of hypertension, including obesity, high alcohol and salt intakes and some drugs (e.g. oral contraceptives, corticosteroids, monoamine oxidase inhibitors). There is also often a positive family history of hypertension: if both parents are hypertensive, there is a significantly greater risk that their children will also develop high blood pressure.

If hypertension is diagnosed and effectively treated, usually by drug therapy, much of the cardiovascular-related disease can be prevented.

Measurement of arterial blood pressure. The first documented measurement of blood pressure dates back to the eighteenth century. In 1773, Stephen Hales, an English theologian and scientist, directly measured mean blood pressure in an unanaesthetized horse by inserting an open-ended tube directly into the animal's neck.

The blood entered the tube and rose upwards (to a height of 2.5 m) towards the tube opening until the weight of the column of blood was equal to the pressure in the circulatory system of the horse. This is the basis of a simple pressure manometer which is still used for measuring blood pressure. It is the basis too for measuring cerebrospinal fluid pressures during a lumbar puncture.

Catheters can be inserted directly into an artery (the radial artery is often used) to give direct arterial pressure measurements. The indwelling catheter is now usually attached to small electronic transducers, and pressures can be monitored continuously.

However, in most instances it is not desirable or practicable to use invasive techniques to measure arterial pressures. In the eighteenth century, an Italian physiologist, Scipione Riva Rocci, invented the sphygmomanometer (sphygmo = pulse and manometer = pressure meter hence the meaning is "pulse pressure meter") which enabled a non-invasive measurement of systolic pressure.

A rubber inflatable cuff is placed over the brachial artery and the pressure in the cuff is raised until the cuff pressure exceeds that of the blood in the artery.

At this point the artery collapses and no radial pulse can be felt as blood is not able to flow through the brachial artery. The pressure in the cuff is then slowly released and the radial pulse reappears. The pressure at which the pulse reappears corresponds to the systolic pressure as it is the point at which the peak pressure (i.e. the systolic) in the brachial artery exceeds the occluding pressure in the cuff.

The mercury sphygmomanometer is used as the standard reference for measuring blood pressure and it still forms the basis for our present-day indirect method of assessing arterial pressure although it is now somewhat more sophisticated and uses electronic output.

Traditionally, blood pressure is measured in millimetres of mercury (written as mmHg); this means that if the blood pressure is 100 mmHg, the pressure exerted by the blood is sufficient to push a column of mercury up to a height of 100 mm. (The SI unit for pressure is the Pascal (Pa) or kilopascal (kPa) and so sometimes blood pressure may be written as, say, 13.3kPa instead of 100 mmHg.)

The method was developed further a few years after Riva Rocci by a Russian surgeon, Dr Nicolai Korotkov. Korotkov reported a method for measuring both systolic and diastolic pressures by auscultation, that is, by listening, using a stethoscope placed over the brachial artery and the sphygmomanometer. Various sounds were audible and Korotkov classified the sounds into five phases, which are now known simply as the Korotkov sounds.

Phase	Sound	Approx. Pressure in mmHg
1	Sharp, clear	120-110
2	Blowing or swishing	109-100
3	Sharp but softer than in 1	99-88
4	Muffled, fading	82-87
5	No sound	< 81

The precise origin of the various Korotkov sounds is not fully understood, but they are due primarily to turbulent flow and vibratory phenomena in the brachial artery as it opens and closes with each beat and as the blood flows through the semi-occluded vessel. When the pressure in the blood pressure cuff is greater than that in the artery, the vessel is completely occluded and there is no blood flow and no turbulence, and hence no sound.

There is considerable controversy as to whether phase 4, the muffling of the sounds, or phase 5, the disappearance of the sounds, is the best measure of the diastolic pressure. In the UK, phase 4 is favoured in clinical practice, whereas in the USA, phase 5 is used.

Some researchers suggest that both phases 4 and 5 should be recorded, for example 120/72/64 mmHg. Phase 5 correlates better with direct arterial measurement of blood pressure and there is also often better agreement among observers when using the disappearance of sounds rather than muffling. The main problem with using phase 5 is that in some individuals, especially when the cardiac output is high, the sounds do not disappear (although they do muffle), and sometimes persist right down to zero. However, in most people muffling and disappearance of the sounds usually occur within 10 mmHg of each other and may even occur together. When transferring between hospitals, nurses should always check on local policy regarding this.

Of course we now have electronic sphygmomanometers which remove some of this ambiguity, however the ausculatory method is still a valuable nursing skill.

Nurses should ensure that blood pressure measurements are taken under standardised conditions and using the correct technique. Blood pressure values vary according to the situation that the individual is in and many physiological variables influence them. The individual should rest for at least 5 minutes before measurement, and should avoid exertion and not eat or smoke for 30 minutes beforehand. Both systolic and diastolic pressures are reported to rise by 10-33 mmHg within 15-45 minutes after the subject has eaten a meal. Blood pressure also rises as the bladder fills.

The emotional state of the patient, e.g. whether anxious or in pain, will affect blood pressure values, but this is often difficult to avoid in clinical situations. The simple arrival of doctors at the bedside of patients can induce an immediate rise in blood pressure (and heart rate), with mean values increasing by approximately 27 mmHg for systolic and 15 mmHg for diastolic above the pre visit values. The fact that anxiety influences blood pressure readings should be remembered, especially when intending to use observations taken at the time of admission to hospital as a baseline for subsequent observations.

There are also many potential sources for error in the actual measurement technique, for instance, the arm should be at heart level or, more specifically, the arm should be horizontal with the fourth intercostal space at the sternum. The observer should also support the patient's arm, otherwise the patient will have to perform isometric muscle contractions which can increase the diastolic pressure. Raising or lowering the arm away from heart level causes significant changes in blood pressures, the error can be as large as 10 mmHg. The same arm should be used each time as in some individuals there are differences in the right and left brachial artery pressures. An appropriate size cuff should also be used; ideally the bladder of the cuff should encircle the arm, but if this is not feasible the centre of the bladder must be placed directly over the brachial artery. Tight or constricting sleeves of clothes pushed up to allow application of the cuff will also give false readings.

Observer bias, especially by looking at previously charted values and expectations of individuals' values, e.g. older people having higher blood pressures, is also a potential source of inaccuracy. The observer should also be at eye level with the mercury manometer scale when reading off the values. For some reason observers show a strong preference for the terminal digits 0 and 5, e.g. 125/75 mmHg, even though a 5-mmHg mark does not appear on many scales!

It is advisable to record an approximate value for systolic pressure by palpation, before auscultation, because in some people the Korotkov sounds appear normally giving the systolic pressure, but then disappear for a short time before returning above the diastolic pressure. This period of silence is known as the auscultatory gap and, although nothing can be heard, the pulse can be felt.

On many occasions it may not be possible to obtain all the optimum conditions, and if this is the case the qualifying factor(s) should be recorded on the chart, e.g. '150/94 mmHg - patient in severe pain'.

Sphygmomanometer:Height, Upright scale,maintenance, clogged vent,level of mercury
Nurse: Training, Observer Bias, Preferred digit,Lack of concentration, Sight, Hearing,Distance from sphygmomanometer,Diastolic dilemma		State of patient: Anxiety, Pain, Fear, Recent exercise, Full bladder, Food, Tobacco, Alcohol, Obesity, Arrhytmias.
Cuff: Correct application, Dimensions of bladder, Positioning of bladder.		Patient: Position, Right Arm, Left Arm, Support to arm.
Environment: Temperature, Noise, Distractions

If it is not possible to use the arms for blood pressure readings, it is possible, using special large leg cuffs, to record the blood pressure using the Korotkov sounds from the popliteal artery in the popliteal fossa (at the back of the knee). The technique is more cumbersome but is useful in some instances, e.g. for patients with suspected coarctation of the aorta or with arm injuries. Pressure in the arteries of the legs is normally the same as that in the arms.

Aneroid sphygmomanometers that work on a bellows system rather than on a mercury column, and also semiautomated systems that detect Korotkov sounds using a microphone or detect arterial blood flow using ultrasound, may be used clinically to measure blood pressure. However, the values obtained do not exactly correlate with direct arterial measurements.

A crude value for mean arterial pressure can be obtained using the method described earlier to demonstrate reactive hyperaemia. The mean arterial pressure is the pressure level when the arm flushes bright red as blood returns to the arm. This is described as the 'flush method' and is sometimes used in children or in shocked patients when other methods are not possible.

This page authored by John Ross. Please e-mail any comments or queries to johnross@cwcom.net or leave a message in the guest book.

Blood Vessels

Fig.1 Blood pressure in each section of the systemic and pulmonary circulations

Blood Pressure = Cardiac Output x Total Peripheral Resistance

BP = CO x TPR

Figure. 3 to the left shows the common sites where the pulse is felt.

Table 1. Some 'average' blood pressures relating to age