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The pressure exerted by the blood against the walls of the blood vessels, especially the arteries. It varies with the strength of the heartbeat, the elasticity of the arterial walls, the volume and viscosity of the blood, and a person's health, age, and physical condition.
In a resting individual the left ventricle of the heart pumps typically 5 litres of blood each minute into the aorta and arteries of the body. Downstream, the small arterioles restrict the outflow of blood from the arteries and are therefore known as the main ‘resistance vessels’. The combined effect of the energy generated by the heart and the outflow restriction results in a distending pressure in the arterial system which is referred to as the blood pressure.
The first report of a direct measurement of arterial blood pressure was by Revd Stephen Hales in 1733. He inserted a tube into an abnormally exposed artery of a horse and observed that a column of blood rose in a glass tube to a vertical height of 8 ft 3 in. This represents the force generated by the heart and transmitted to all the major arteries in the body. We do not now express blood pressure as height in feet and inches of blood. However, we sometimes use centimeters of water, so that the horse's blood pressure would be 250 cm of water or blood. Such a column has obvious practical problems for measuring arterial pressure. But for venous pressures which are much lower, a column of saline connected to a major vein is often used clinically to assess the degree of filling of the circulation. Arterial pressure is usually expressed as millimetres of mercury (mm Hg) because mercury is 13.6 times as dense as water and a mercury column of that height is more practicable. Thus the horse's pressure blood would be 185 mm Hg. An alternative unit for expressing blood pressures, which has not been widely adopted in clinical practice, is the SI unit, the pascal or kilopascal (kPa). One kPa is approximately 7.5 mm Hg.
Blood pressure is not normally expressed as a single figure but rather as two, for example 120/80. This means that the pressure in the arteries varies with each heart beat to a peak, called systolic pressure, of 120 mm Hg, and then declines to a minimum value, called diastolic pressure, of 80 mm Hg just before the next beat. These phasic values of blood pressure can be recorded accurately using modern transducers (electronic measuring devices) connected to catheters (fine tubes) inserted into arteries. However, except for research and measurements during complex investigations in patients, blood pressure is not usually determined by direct puncture of an artery. The most common method is to use the device known as a sphygmomanometer. This is an inflatable cuff which fits round the upper arm and is connected to a mercury manometer. A stethoscope is applied to listen to the artery below the cuff. The cuff is first inflated with a pressure well above systolic and then slowly deflated. The systolic pressure is taken as the pressure in the cuff when the artery just opens and a sound is first heard. The diastolic pressure is that when the sound either becomes muffled or disappears completely.
Blood pressure, like all biological variables, varies widely in different people and, in the same individual, at different times of the day. Typically a normal value for systolic blood pressure would be 120 mm Hg at age 20, increasing perhaps to 140 mm Hg at 60. Diastolic pressure also increases with age but rather less. Estimates of blood pressure in apparently healthy people show values that can be 20 or even 40 mm Hg higher or lower than the average values. This, and the fact that blood pressure varies considerably during the day, particularly in response to stresses such as visiting a doctor, mean that it is very difficult to decide on the basis of a single measurement whether a patient suffers from hypertension (high blood pressure). Definitions of hypertension are constantly changing but, generally, if systolic pressure is consistently greater than 160 mm Hg or diastolic more than 95, a person is considered to be hypertensive.
At rest, each time the heart contracts, it ejects typically 70 ml of blood into the arterial system. This causes a steep increase in arterial pressure, the magnitude of which is dependent both on the volume ejected and on the distensibility of the arteries. Older people have less distensible arteries, which explains why their systolic blood pressure is usually higher than in younger subjects. Because the shape of the arterial pressure pulse is roughly triangular, the mean level of pressure is nearer to the diastolic value.
The importance of blood pressure is that it effectively provides a store of energy, generated by the heart, available to cause blood to flow through the working tissues. It is actually the flow of blood, providing oxygen and nutrients and removing waste products including carbon dioxide, which is really the important factor, but without pressure there would be no flow. Humans, being upright bipedal animals, have a particular problem in supplying blood to all parts of the body. Due just to gravity, pressure in arteries supplying the head is about 100 mm Hg less than that in arteries in the feet. The fact that the brain must have an adequate arterial pressure places a limitation on the range of effective pressures in the upright person.
Control of blood pressure
Mean blood pressure depends on the flow of blood from the heart (cardiac output) and the resistance to flow in the small arteries and microscopic resistance vessels (arterioles).
BP = CO × PVRwhere BP is blood pressure, CO is cardiac output, and PVR is the peripheral vascular resistance or the net resistance to blood flow in all the small arteries and microscopic arterioles.
BP ∝ CO/r4
Some factors which affect blood pressure
Baroreceptors are important for minimizing changes in blood pressure: animal studies have shown that blood pressure is much more variable if the influence of baroreceptors is removed. However, they do not prevent all fluctuations from occurring. Continuous 24-hour recordings have been made in healthy volunteers and have shown variations of 30-80 mm Hg in systolic pressure and of 10-80 mm Hg in diastolic pressure. Blood pressure is particularly low during sleep, and high during physical activity or emotional stress.
Physical exercise causes very major effects on the circulation. Due to the enormously increased blood flow through the exercising muscle, the amount of blood pumped by the heart may increase four-fold, or in elite athletes as much as six-fold. The increased volume of blood ejected at each heart beat causes systolic blood pressure to increase, perhaps to 180 mm Hg. However, because blood flows very rapidly out of the arteries, particularly to the working muscle where the resistance vessels are widely dilated, diastolic pressure remains relatively unchanged or may even decrease. Isometric exercise has quite a different effect. Here there is a much smaller effect on the total amount of blood pumped by the heart, but reflexes, particularly those arising from the contracting muscle itself, cause blood vessels elsewhere to constrict, and consequently both systolic and diastolic blood pressure rise sharply. This response may also be augmented by a straining effect (see below).
Emotional stress can cause quite large increases in blood pressure. Prominent amongst the physiological responses to stress is an increase in activity in the sympathetic nerves. Sympathetic overactivity increases heart rate and force, and constricts resistance blood vessels (Fig. 1). All these effects increase both systolic and diastolic blood pressure and are augmented by increased secretion into the blood of adrenaline and noradrenaline.
Postural changes exert stresses on the cardiovascular system requiring effective reflex responses to constrict arteries and veins and stimulate the heart, to control blood pressure, maintain brain blood flow, and prevent loss of consciousness. The upright position means that blood vessels below the level of the heart are subjected to increased distending pressures due to the effects of gravity. Veins are particularly susceptible to gravitational stress due to their distensibility, and blood ‘pools’ in dependent veins when we stand. Because of this, less blood flows back to the heart and, were it not for effective reflexes, involving baroreceptors, blood pressure would fall catastrophically, particularly in the brain, resulting in insufficient brain blood flow and consequent loss of consciousness. Blood pressure frequently falls transiently when we stand. This is particularly noticeable if we stand suddenly when warm, for example on getting out of a hot bath, because the resistance blood vessels initially will be dilated. In some people blood pressure control may be inadequate to counter the stress of postural changes and the result is that they faint.
Straining (the Valsalva manoeuvre) induces large and complex variations in blood pressure. The sort of stresses that induce these changes include blowing against a resistance, lifting heavy objects, and straining at stool. The effects on the circulation are illustrated in Fig. 2. The primary change is caused by an increase in pressure within the chest (intrathoracic pressure) and within the abdomen. Normally, intrathoracic pressure is lower than atmospheric, due to the tendency of lungs to collapse and their prevention from so doing by the chest wall. This negative intrathoracic pressure aids the flow of blood to the heart from the peripheral veins. Straining causes the pressure in both the chest and the abdomen to become positive. Initially the compression of the heart and large arteries causes an increase in blood pressure. Then, the high pressure in the chest impedes the inflow of blood from peripheral veins (veins in the neck can be seen to distend), so the cardiac output decreases and blood pressure falls. Baroreceptors detect this fall and initiate constriction of blood vessels and an increase in heart rate, so that mean blood pressure is restored. At the end of the strain there is a transient fall in pressure before blood rushes back to the heart, causing an overshoot and often a transient slowing of the heart. In people with some autonomic nerve disorders these responses may be deficient: blood pressure falls continuously, and the overshoot is absent.
— Roger Hainsworth
See also autonomic nervous system; baroreceptors; blood circulation; heart.
When the heart contracts (systolic pressure) the normal blood pressure of females is 120 mm mercury at the age of 12 rising to 175 at 70. In males it is 120 rising to 160. When the heart relaxes (diastolic pressure) the normal range for females is 70 rising to 95; males 70 to 85. Diastolic blood pressure above 105 is moderate, and above 115 severe, hypertension.
The pressure exerted by the heart and arteries to push blood around the body. The magnitude of blood pressure is determined by the amount of blood being pumped out of the heart per beat (the stroke volume) and the resistance encountered as it passes through the blood vessels (peripheral resistance). Blood pressure is usually expressed as two measurements: systolic blood pressure, indicating the pressure when the heart is actually pumping; and diastolic blood pressure, the pressure when the heart is filling up with blood. Systolic pressure is always the higher and is expressed first. The pressures are measured in millimetres of mercury. Thus a blood pressure of 130/80, or 130 over 80, refers to a systolic blood pressure which will support a column of mercury 130 mm high, and a diastolic pressure which will support a column 80 mm high. Systolic pressure in children is about 100 and in young adults the value is about 120. It tends to rise with age as arteries thicken. A systolic pressure of 180 is not uncommon and it may be as high as 280. The value varies according to a person's position. It tends to drop when you stand up after lying down; this is called postural hypotensive drop. A typical value for diastolic pressure is 80 mm of mercury. Although it is difficult to define precisely what is ‘normal’ blood pressure, there is general agreement that a desirable blood pressure is less than 140/90. See also hypertension and hypotension.
The pressure exerted on arterial walls by the blood when the heart is in systole (systolic pressure), and the pressure maintained by the elasticity of the arteries when the heart is in diastole (diastolic pressure). A consistent arterial pressure greater than 140/90 is considered abnormally high and suggestive of hypertensive vascular disease.
Blood pressure is a physiological variable—like body temperature, respiratory rate, or heart rate. Blood pressure is not constant throughout the day; each time the heart squeezes and relaxes, there is a new blood pressure. It increases before awakening and declines with sleep. The level of blood pressure is regulated by the kidneys, brain, heart, endocrine glands, and blood vessels. In the United States, the actual level of blood pressure gradually increases from birth to adulthood. Due to difference in diet and activity levels in nonindustrialized countries, however, blood pressure does not increase beyond the age of eighteen.
Whereas temperature is measured with a thermometer, blood pressure is measured with a sphygmomanometer, preferably a mercury sphygmo-manometer, though aneroid and electronic devices are sometimes used.
Blood pressure should be measured after a five-minute period of rest, with the back supported and the legs uncrossed. Constrictive clothing should be removed from around the upper arm, which must be resting on a table at heart level. The blood pressure cuff is evenly and snugly applied around the upper arm above the elbow, and a stethoscope is placed over the crease of the elbow. The cuff is inflated to 15 millimeters of mercury (mmHg) above the point where radial artery pulse (the artery above the thumb at the wrist) disappears. The pressure in the cuff is then slowly released at 2 mmHg per second. The first of two consecutive sounds as cuff pressure decreases is called the systolic blood pressure—the pressure to open the artery occluded with the cuff. The diastolic blood pressure is recorded at the absence of sounds with continued deflation of the blood pressure cuff. Blood pressure is generally recorded to the nearest 2 mmHg. For example, a blood pressure of 142/86 mmHg indicates a systolic blood pressure of 142 mmHg and a diastolic blood pressure of 86 mmHg. Pain and emotional disturbance, as well as caffeine, tobacco, and alcohol, can elevate systolic blood pressure.
Hypertension
An abnormal blood pressure requires confirmation on two subsequent days. An optimal blood pressure is less than 120/80 mmHg. High blood pressure, or hypertension, is defined as either a systolic blood pressure greater than 140 mmHg or a diastolic blood pressure greater than 90 mmHg. Systolic blood pressure is a more powerful predictor of cardiovascular events than diastolic blood pressure. With increasing age, the diastolic blood pressure may actually decrease while systolic blood pressure increases; this indicates increased stiffening of the arteries throughout the body.
Hypertension is not a nervous disorder or an anxiety state, but rather a disease of the blood vessels that increases blood vessel constriction of the small arteries. It particularly damages the blood vessels inside the brain, heart, kidneys, eyes, and the largest artery, the aorta. Damaged arteries may rupture, thicken, or harden and narrow—resulting in strokes, heart attacks, kidney failure, visual impairment, or tearing or rupture of the aorta. Also, the left heart chamber thickens as a consequence of increased blood pressure. When the heart can no longer thicken or enlarge to overcome the increased pressure in the blood vessels, the squeezing function of the heart decreases, resulting in congestive heart failure.
Causes of Hypertension
Fifty million Americans (about one-fifth of the U.S. population) have hypertension. Over 90 percent of the causes of hypertension remain unknown. Four groups are predisposed to developing hypertension: the obese, the elderly, diabetics, and African Americans. Certain drugs are known to elevate blood pressure, including most arthritis medications (except acetaminophen and aspirin), many cold remedies, nose sprays, weight-reducing pills, and alcohol. Increased heart rate, anemia, excessive thyroid hormone, or stiff
(nondistendible) arteries can increase systolic blood pressure. Blocked arteries to the kidney, kidney failure, and decreased production of thyroid hormone are common causes of hypertension. Other rare causes include tumors of the adrenal gland.
Treatment of Hypertension
Nondrug treatment of hypertension should include weight loss, salt restriction, smoking cessation, and alcohol restriction. A reduced saturatedand total-fat diet that is rich in fruits, vegetables, and low-fat dairy products lowers blood pressure in some individuals, avoiding the need for drug treatment. The treatment goal for uncomplicated hypertensives is below 140/90 mmHg. To achieve that goal consistently, most individuals will need to be treated with more than one drug. Treatment has been proven to decrease heart attacks, strokes, and heart failure, and is usually required throughout life.
(SEE ALSO: Atherosclerosis;
Bibliography "The Sixth Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure." (1997). Arch Intern Med 157:2413–2446.
— L. MICHAEL PRISANT
For more information on blood pressure, visit Britannica.com.
The force exerted by blood against a unit area of blond vessel. A blood pressure gradient from the arteries leaving the left ventricle to the veins entering the right atrium enables blond to circulate the body. The magnitude of the blond pressure is determined by the amount blood pumped out of the heart per beat (stroke volume) and the resistance the blond encounters as it passes through the blond vessels (peripheral resistance). Usually two measurements of blood pressure are made: systolic and diastolic pressures, traditionally expressed as two figures in millimetres of mercury (mmHg), e.g. 130/80 mmHg (or 130 over 80). The first figure is always the highest. It refers to the systolic blood pressure, obtained when the blond is ejected into the arteries from the heart. In children, systolic pressure is about 100 mmHg; in young adults, 120 mmHg; thereafter it tends to rise with age as arterial walls thicken. The second figure is the diastolic pressure. It is obtained when the blond drains from the arteries. Blood pressure varies according to the subject's body position (see postural hypotension drop). It is difficult to define precisely what is ‘normal’ blood pressure, but there is general agreement that a desirable blond pressure is less than 140/90 mmHg. See also hypertension.
Blood pressure is strongest in the aorta, where the blood leaves the heart. It diminishes progressively in the smaller blood vessels and reaches its lowest point in the veins (see circulatory system). Blood pressure manifests itself dramatically when an artery is severed or pierced and the blood (under pressure) ejects in spurts.
Since blood pressure varies in different arteries, the pressure in the brachial artery of the forearm serves as a standard. A sphygmomanometer measures blood pressure in millimeters of mercury; blood pressure gauges that do not use mercury also produce readings that are expressed in terms of millimeters of mercury. Normal blood pressure readings for healthy young people should be below 120 mm for systolic pressure and 80 mm for diastolic pressure, commonly written as 120/80 and read as “one-twenty over eighty.” With age, and the constriction of the small arteries and then the larger ones, blood pressure increases, so that at 50 years, a person may typically have a systolic pressure between 140 and 150, and a diastolic pressure of about 90.
Factors other than heart action and the condition of the arteries also influence blood pressure. Temporary high blood pressure usually occurs during or following physical activity, nervous strain, and periods of rage or fear. Therapy for persistent high blood pressure, sometimes called hypertension, consists of sufficient rest, a diet low in salt and alcohol, reduction in weight where there is obesity, and increased exercise. Drug therapy may include diuretics, beta-blockers, calcium-channel blockers, or ACE inhibitors. Low blood pressure (hypotension) has not been studied as extensively as high blood pressure. If not caused by disease or injury, it is generally considered to be a benign or even advantageous condition; however, studies have linked hypotension with feelings of tiredness or faintness and minor psychiatric conditions in some people.
Bibliography
See N. H. Naqvi and M. D. Blaufox, Blood Pressure Measurement: An Illustrated History (1998).
The pressure of the blood against the walls of the blood vessels, especially the arteries. It is expressed in two figures, said to be one “over” the other: the systolic pressure, which is the pressure when the left ventricle of the
The pressure of the blood in the blood vessels. The term usually refers to the pressure of the blood within the arteries, or arterial blood pressure. This pressure is determined by several interrelated factors, including the pumping action of the heart, the resistance to the flow of blood in the arterioles, the elasticity of the walls of the main arteries, the blood volume and extracellular fluid volume, and the blood's viscosity, or thickness.
Relatively simple Doppler instruments can provide accurate blood pressure measurements in dogs and cats. The systolic pressure in dogs is 132±22 mmHg; in cats it is 108±23 mmHg. Thoroughbreds have been shown to be 112/77 mmHg. Indwelling catheters can be used in dogs to monitor central venous pressure.
Blood pressure (strictly speaking: vascular pressure) refers to the force exerted by circulating blood on the walls of blood vessels, and constitutes one of the principal vital signs. The pressure of the circulating blood decreases as blood moves through arteries, arterioles, capillaries, and veins; the term blood pressure generally refers to arterial pressure, i.e., the pressure in the larger arteries, arteries being the blood vessels which take blood away from the heart. Arterial pressure is most commonly measured via a sphygmomanometer, which uses the height of a column of mercury to reflect the circulating pressure (see Non-invasive measurement). Although many modern vascular pressure devices no longer use mercury, vascular pressure values are still universally reported in millimetres of mercury (mmHg).
The systolic arterial pressure is defined as the peak pressure in the arteries, which occurs near the beginning of the cardiac cycle; the diastolic arterial pressure is the lowest pressure (at the resting phase of the cardiac cycle). The average pressure throughout the cardiac cycle is reported as mean arterial pressure; the pulse pressure reflects the difference between the maximum and minimum pressures measured.
Typical values for a resting, healthy adult human are approximately 120 mmHg (16 kPa) systolic and 80 mmHg (11 kPa) diastolic (written as 120/80 mmHg, and spoken as "one twenty over eighty"), with large individual variations. These measures of arterial pressure are not static, but undergo natural variations from one heartbeat to another and throughout the day (in a circadian rhythm); they also change in response to stress, nutritional factors, drugs, or disease. Hypertension refers to arterial pressure being abnormally high, as opposed to hypotension, when it is abnormally low.
Arterial pressures can be measured invasively (by penetrating the skin and measuring inside the blood vessels) or non-invasively. The former is usually restricted to a hospital setting.
The non-invasive auscultatory (from the Latin for listening) and oscillometric measurements are simpler and quicker than invasive measurements, require less expertise in fitting, have virtually no complications, and are less unpleasant and painful for the patient. However, non-invasive measures may yield somewhat lower accuracy and small systematic differences in numerical results. Non-invasive measurement methods are more commonly used for routine examinations and monitoring.
The auscultatory method uses a stethoscope and a sphygmomanometer. This comprises an inflatable (Riva-Rocci) cuff placed around the upper arm at roughly the same vertical height as the heart, attached to a mercury or aneroid manometer. The mercury manometer, considered to be the gold standard for arterial pressure measurement, measures the height of a column of mercury, giving an absolute result without need for calibration, and consequently not subject to the errors and drift of calibration which affect other methods. The use of mercury manometers is often required in clinical trials and for the clinical measurement of hypertension in high risk patients, including pregnant women.
A cuff of appropriate size is fitted and inflated manually by repeatedly squeezing a rubber bulb until the artery is completely occluded. Listening with the stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When blood just starts to flow in the artery, the turbulent flow creates a "whooshing" or pounding sound (first Korotkoff sounds). The pressure at which this sound is first heard is the systolic blood pressure. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound), at the diastolic arterial pressure. Sometimes, the pressure is palpated (felt by hand) to get an estimate before auscultation. With a mercury manometer this is simple technology which gives accurate pressure readings without issues of calibration.
Oscillometric methods are sometimes used in the long-term measurement and sometimes in general practice. The equipment is functionally similar to that of the auscultatory method, but with an electronic pressure sensor (transducer) fitted in to detect blood flow, instead of using the stethoscope and the expert's ear. In practice, the pressure sensor is a calibrated electronic device with a numerical readout of blood pressure. To maintain accuracy, calibration must be checked periodically, unlike the inherently accurate mercury manometer. In most cases the cuff is inflated and released by an electrically operated pump and valve, which may be fitted on the wrist (elevated to heart height), although the upper arm is preferred. They vary widely in accuracy, and should be checked at specified intervals and if necessary recalibrated.
Oscillometric measurement requires less skill than the auscultatory technique, and may be suitable for use by untrained staff and for automated patient home monitoring.
The cuff is inflated to a pressure initially in excess of the systolic arterial pressure, and then reduces to below diastolic pressure over a period of about 30 seconds. When blood flow is nil (cuff pressure exceeding systolic pressure) or unimpeded (cuff pressure below diastolic pressure), cuff pressure will be essentially constant. It is essential that the cuff size is correct: undersized cuffs may yield too high a pressure, whereas oversized cuffs yields too low a pressure. When blood flow is present, but restricted, the cuff pressure, which is monitored by the pressure sensor, will vary periodically in synchrony with the cyclic expansion and contraction of the brachial artery, i.e., it will oscillate. The values of systolic and diastolic pressure are computed, not actually measured from the raw data, using an algorithm; the computed results are displayed.
Oscillometric monitors may produce inaccurate readings in patients with heart and circulation problems, that include arterial sclerosis, arrhythmia, preeclampsia, pulsus alternans, and pulsus paradoxus.
In practice the different methods do not give identical results; an algorithm and experimentally obtained coefficients are used to adjust the oscillometric results to give readings which match the auscultatory as well as possible.[1] Some equipment uses computer-aided analysis of the instantaneous arterial pressure waveform to determine the systolic, mean, and diastolic points. Since many oscillometric devices have not been validated, caution must be given as most are not suitable in clinical and acute care settings.
The term NIBP, for Non-Invasive Blood Pressure, is often used to describe oscillometric monitoring equipment.
Arterial blood pressure (BP) is most accurately measured invasively. Invasive arterial pressure measurement with intravascular cannulae involves direct measurement of arterial pressure by placing a cannula needle in an artery (usually radial, femoral, dorsalis pedis or brachial). This is usually done by an anesthesiologist or surgeon in a hospital.
The cannula must be connected to a sterile, fluid-filled system, which is connected to an electronic pressure transducer. The advantage of this system is that pressure is constantly monitored beat-by-beat, and a waveform (a graph of pressure against time) can be displayed. This invasive technique is regularly employed in human and veterinary intensive care medicine, anesthesiology, and for research purposes.
Cannulation for invasive vascular pressure monitoring is infrequently associated with complications such as thrombosis, infection, and bleeding. Patients with invasive arterial monitoring require very close supervision, as there is a danger of severe bleeding if the line becomes disconnected. It is generally reserved for patients where rapid variations in arterial pressure are anticipated.
Invasive vascular pressure monitors are pressure monitoring systems designed to acquire pressure information for display and processing. There are a variety of invasive vascular pressure monitors for trauma, critical care, and operating room applications. These include single pressure, dual pressure, and multi-parameter (i.e. pressure / temperature). The monitors can be used for measurement and follow-up of arterial, central venous, pulmonary arterial, left atrial, right atrial, femoral arterial, umbilical venous, umbilical arterial, and intracranial pressures.
Vascular pressure parameters are derived in the monitor's microcomputer system. Usually, systolic, diastolic and mean pressures are displayed simultaneously for pulsatile waveforms (i.e. arterial and pulmonary arterial). Some monitors also calculate and display CPP (cerebral perfusion pressure). Normally, a zero key on the front of the monitor makes pressure zeroing extremely fast and easy. Alarm limits may be set to assist the medical professional responsible for observing the patient. High and low alarms may be set on displayed temperature parameters.
Up to 25% of patients diagnosed with hypertension do not suffer from it, but rather from white coat hypertension (elevated arterial pressure specifically during medical exams, probably as a result of anxiety). Thus, well-performed, accurate home arterial pressure monitoring can prevent unnecessary anxiety, as well as costly and potentially dangerous therapy in many millions of people worldwide. Home arterial pressure monitoring provides a measurement of a person's arterial pressure at different times and in different environments, such as at home and at work, throughout the day. Home arterial pressure monitoring may assist in the diagnosis of high or low arterial pressure. It may also be used to monitor the effects of medication or lifestyle changes taken to lower or regulate arterial pressure levels.
The 2003 US Joint National Committee recommends the use of self monitoring of arterial pressure, before considering the more expensive ambulatory monitoring of arterial pressure, to improve hypertension management.[2] Both the Joint National Committee and the 2003 guidelines from the European Society of Hypertension and the European Society of Cardiology suggest that self monitoring might also be used as an alternative to ambulatory monitoring for the diagnosis of white coat hypertension.[3]
A study published in the May 2006 American Journal of Hypertension[4] compared home and ambulatory blood pressure monitoring methods in the adjustment of antihypertensive treatment. The study showed home arterial pressure monitoring is as accurate as a 24 hour ambulatory monitoring in determining arterial pressure levels. Researchers at the University of Turku, Finland studied 98 patients with untreated hypertension. They compared patients using a home arterial pressure device and those wearing a 24hr ambulatory monitor. Researcher Dr. Niiranen said that, "home blood pressure measurement can be used effectively for guiding anti-hypertensive treatment". Dr. Stergiou added that home tracking of arterial pressure, "is more convenient and also less costly than ambulatory blood pressure monitoring".
A clinical study published in the May 2007 edition of The American Journal of Hypertension[5] compared the accuracy of 3 different methods of taking arterial pressure in indicating cardiovascular health. The study aim was to assess the accuracy of home blood pressure monitoring (HBP), 24hr ambulatory blood pressure monitoring (ABP) and arterial pressure readings taken in a doctor’s office (OBP). The arterial pressure tests were compared to the left-ventricular mass index (LVMI). The LVMI was calculated from an echocardiogram of the heart and indicates cardiovascular organ damage, an indicator of arterial pressure. Researchers at The Columbia University Medical Center, New York found that home arterial pressure monitoring, over a 10 week period was a significant independent predictor of LVMI even after adjusting for age, sex and BMI (body mass index). They found that home monitoring over time is a better indicator of cardiovascular health than ambulatory readings or readings taken at the doctors’ office. The value of home monitoring increases over time with a number of measurements taken.
The June 2007 AMNews; Newspaper for America's Physicians[6] released a study which showed arterial pressure readings taken in a doctors office are often unreliable. The American Medical Association newspaper quoted Prof Norman Kaplan from the University of Texas Southwestern Medical Center who said, "Of all the procedures done in a doctor's office, measurement of blood pressure is usually the least well performed but has the most important implications for the care of the patient." The paper explained that arterial pressure readings taken in a Doctors office can be falsely raised or lowered. This can be due to the presence of a Doctor or clinician which results in the patient experiencing white coat hypertension.
The American Heart Association website[7] states, "You may have what's called 'white coat hypertension'; that means your blood pressure goes up when you're at the doctor's office. Monitoring at home will help you measure your true blood pressure and can provide your doctor with a log of blood pressure measurements over time. This is helpful in diagnosing and preventing potential health problems."
Those using home arterial pressure monitoring devices are increasingly also making use of arterial pressure charting software. These charting methods provide print outs for the patients physician and reminders on how often to check arterial pressure.[8]
While statistically normal values for arterial pressure could be computed for a given population, it needs to be remembered that, not only does arterial pressure vary from person to person, it also varies in individuals from moment to moment. Additionally, since there's no guarantee the norm of the population in question should even be considered healthy, the relevance of such values would be questionable.
In children the observed normal ranges are lower; in the elderly, they are often higher, largely because of reduced flexibility of the arteries. Factors such as age, gender and race influence blood pressure values. Pressure also varies with exercise, emotional reactions, sleep, digestion and time of the day.
In the U.S., the optimal arterial pressure (sometimes referred to as the ‘gold standard’) targets are:[9][10][11]
Levels above 120 but below 140 mmHg in systolic pressure, or above 80 but below 95 mmHg in diastolic pressure, are referred to as "prehypertensive" and often progress to frankly hypertensive levels. However studies already extant reveal that there are fewer complications at, e.g., 115 mmHg systolic than 120, and in fact arterial pressure is a continuum with decreasing pathology associated with lower levels to well within the current "optimum" range.[12] "Some data indicates that 115/75 mm Hg should be the gold standard. Once arterial pressure rises above 115/75 mm Hg, the risk of cardiovascular disease begins to increase. Prehypertension is now considered to be a systolic pressure ranging from 120 to 139 or a diastolic pressure ranging from 80 to 89." (Excerpts from Mayo Clinic website). In the past, hypertension was only diagnosed if secondary signs of high arterial pressure were present, along with a prolonged high systolic pressure reading over several visits. In the U.S., this reactive stance has been soundly rejected in the light of recent evidence.
In the UK, mirroring abandoned earlier U.S. practice, nursing students continue to be taught that their patients’ readings should be considered ‘normal’ if in the range:
Clinical trials demonstrate that people who maintain arterial pressures at the low end of these pressure ranges have much better long term cardiovascular health. The principal medical debate is the aggressiveness and relative value of methods used to lower pressures into this range for those who don't maintain such pressure on their own. Elevations, more commonly seen in older people, though often considered normal, are associated with increased morbidity and mortality. The clear trend from double blind clinical trials (for the better strategies and agents) has increasingly been that lower arterial pressure is found to result in less disease.[citation needed]
The mean arterial pressure (MAP) is the average pressure measured over one complete cardiac cycle.
The up and down fluctuation of the arterial pressure results from the pulsatile nature of the cardiac output. The pulse pressure is determined by the interaction of the stroke volume versus the resistance to flow in the arterial tree.
The larger arteries, including all large enough to see without magnification, are low resistance (assuming no advanced atherosclerotic changes) conduits with high flow rates that generate only small drops in pressure. For instance, with a subject in the supine position, blood travelling from the heart to the toes typically only experiences a 5 mmHg drop in mean pressure.
Modern physiology developed the concept of the vascular pressure wave. This wave is created by the heart during the systole and originates in the ascending aorta. Much faster than the stream of blood itself, it is then transported through the vessel walls to the peripheral arteries. There the pressure wave can be palpated as the peripheral pulse. As the wave is reflected at the peripheral veins it runs back in a centripetal fashion. Where the crests of the reflected and the original wave meet, the pressure inside the vessel is higher than the true pressure in the aorta. This concept explains why the arterial pressure inside the peripheral arteries of the legs and arms is higher than the arterial pressure in the aorta.[13][14][15]
The endogenous regulation of arterial pressure is not completely understood. Currently, three mechanisms of regulating arterial pressure have been well-characterized:
These different mechanisms are not necessarily independent of each other, as indicated by the link between the RAS and aldosterone release. Currently, the RAS system is targeted pharmacologically by ACE inhibitors and angiotensin II receptor antagonists. The aldosterone system is directly targeted by spironolactone, an aldosterone antagonist. The fluid retention may be targeted by diuretics; however, the antihypertensive effect of diuretics is not due to its effect on blood volume. Generally, the baroreceptor reflex is not targeted in hypertension because if blocked, individuals may suffer from orthostatic hypotension and fainting.
The diagnosis of abnormalities in arterial pressure may require serial measurement. Since arterial pressure varies throughout the day, measurements should be taken at the same time of day to ensure the readings taken are comparable. Suitable times are:
It is sometimes difficult to meet these requirements at the doctor's office; also, some patients become nervous when their arterial pressure is taken at the office, causing readings to increase (this phenomenon is called white coat hypertension). Taking blood pressure levels at home or work with a home blood pressure monitoring device may help determine a person's true range of arterial pressure readings and avoid false readings from the white coat hypertension effect. Long term assessments may be made with an ambulatory blood pressure device that takes regular arterial pressure readings every half an hour throughout the course of a single day and night.
Aside from the white coat effect, arterial pressure readings outside of a clinical setting are usually slightly lower in the majority of people. The studies that looked into the risks from hypertension and the benefits of lowering the arterial pressure in affected patients were based on readings in a clinical environment.
Arterial pressure exceeding normal values is called arterial hypertension. In itself it is only an acute problem; see hypertensive crisis. But because of its long-term indirect effects (and also as an indicator of other problems) it is a serious worry to physicians diagnosing it.
All levels of arterial pressure put mechanical stress on the arterial walls. Higher pressures increase heart workload and progression of unhealthy tissue growth (atheroma) that develops within the walls of arteries. The higher the pressure, the more stress that is present and the more atheroma tend to progress and the heart muscle tends to thicken, enlarge and become weaker over time.
Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure, arterial aneurysms, and is the leading cause of chronic renal failure. Even moderate elevation of arterial pressure leads to shortened life expectancy. At severely high pressures, mean arterial pressures 50% or more above average, a person can expect to live no more than a few years unless appropriately treated.[16]
In the past, most attention was paid to diastolic pressure; but nowadays it is recognised that both high systolic pressure and high pulse pressure (the numerical difference between systolic and diastolic pressures) are also risk factors. In some cases, it appears that a decrease in excessive diastolic pressure can actually increase risk, due probably to the increased difference between systolic and diastolic pressures (see the article on pulse pressure).
Blood pressure that is too low is known as hypotension. The similarity in pronunciation with hypertension can cause confusion.
Low arterial pressure may be a sign of severe disease and requires urgent medical attention.
When arterial pressure and blood flow decrease beyond a certain point, the perfusion of the brain becomes critically decreased (i.e., the blood supply is not sufficient), causing lightheadedness, dizziness, weakness and fainting.
However, people who function well, while maintaining low arterial pressures have lower rates of cardiovascular disease events than people with normal arterial pressures.[citation needed]
The physics of the circulatory system, as of any fluid system, are very complex. That said, there are many physical factors that influence arterial pressure. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs or alcohol, obesity, excess weight and so-forth.
| In cardiac physiology, the rate and volume of flow are accounted for in a combined fashion by cardiac output which is the heart rate (the rate of contraction) multiplied by the stroke volume (the amount of blood pumped out from the heart with each contraction). It represents the efficiency with which the heart circulates blood throughout the body. |
Some physical factors are:
In practice, each individual's autonomic nervous system responds to and regulates all these interacting factors so that, although the above issues are important, the actual arterial pressure response of a given individual varies widely because of both split-second and slow-moving responses of the nervous system and end organs. These responses are very effective in changing the variables and resulting blood pressure from moment to moment.
Sometimes the arterial pressure drops significantly when a patient stands up from sitting. This is known as postural hypotension; gravity reduces the rate of blood return from the body veins below the heart back to the heart, thus reducing stroke volume and cardiac output.
When people are healthy, the veins below their heart quickly constrict and the heart rate increases to minimize and compensate for the gravity effect. This is carried out involuntarily by the autonomic nervous system. The system usually requires a few seconds to fully adjust and if the compensations are too slow or inadequate, the individual will suffer reduced blood flow to the brain, dizziness and potential blackout. Increases in G-loading, such as routinely experienced by acrobatic jet pilots "pulling Gs", greatly increases this effect. Repositioning the body perpendicular to gravity largely eliminates the problem.
Other causes of low arterial pressure include:
Shock is a complex condition which leads to critically decreased perfusion. The usual mechanisms are loss of blood volume, pooling of blood within the veins reducing adequate return to the heart and/or low effective heart pumping. Low arterial pressure, especially low pulse pressure, is a sign of shock and contributes to and reflects decreased perfusion.
If there is a significant difference in the pressure from one arm to the other, that may indicate a narrowing (for example, due to aortic coarctation, aortic dissection, thrombosis or embolism) of an artery.
Venous pressure is the vascular pressure in a vein or in the atria of the heart. It is much less than arterial pressure, with common values of 5 mmHg in the right atrium and 8 mmHg in the left atrium. Measurement of pressures in the venous system and the pulmonary vessels plays an important role in intensive care medicine but requires an invasive central venous catheter.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
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