- See Hypertension for information about recognition and treatment of high blood
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.
Measurement
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.
Non-invasive measurement
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.
Auscultatory methods
Auscultatory method aneroid sphygmomanometer with stethoscope
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
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.
Invasive measurement
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.
Home monitoring
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]
Normal values
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]
Physiology
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]
Regulation
The endogenous regulation of arterial pressure is not completely understood. Currently,
three mechanisms of regulating arterial pressure have been well-characterized:
- Aldosterone release: This steroid hormone is
released from the adrenal cortex in response to angiotensin II or high serum
potassium levels. Aldosterone stimulates sodium retention and
potassium excretion by the kidneys. Since sodium is the main ion that determines the amount of fluid in the blood vessels by
osmosis, aldosterone will increase fluid retention, and indirectly, arterial pressure.
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.
Pathophysiology
High arterial pressure
-
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:
- immediately after awakening (before washing/dressing and taking breakfast/drink), while the body is still resting,
- immediately after finishing work.
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).
Low arterial 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]
Influential factors
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:
- Rate of pumping. In the circulatory system, this rate is called heart rate, the rate at
which blood (the fluid) is pumped by the heart. The higher the heart rate, the higher
(potentially, assuming no change in stroke volume) the arterial pressure.
- Volume of fluid or blood volume, the amount of blood that is present in the body. The
more blood present in the body, the higher the rate of blood return to the heart and the resulting cardiac output. There is some
relationship between dietary salt intake and increased blood volume, potentially resulting in higher arterial pressure, though
this varies with the individual and is highly dependent on autonomic nervous system response.
- Resistance. In the circulatory system, this is the resistance of the blood vessels. The higher the resistance, the higher the
arterial pressure. Resistance is related to size (the larger the blood vessel, the lower the resistance), as well as the
smoothness of the blood vessel walls. Smoothness is reduced by the buildup of fatty deposits on the arterial walls. Substances
called vasoconstrictors can reduce the size of blood vessels, thereby increasing blood
pressure. Vasodilators (such as nitroglycerin)
increase the size of blood vessels, thereby decreasing arterial pressure. Some types of omega-6 fatty acids, particularly from
olive oil, have been known to increase vascular smoothness.[citation needed]
- Viscosity, or thickness of the fluid. If the blood gets thicker, the result is an increase
in arterial pressure. Certain medical conditions can change the viscosity of the blood. For instance, low red blood cell
concentration, anemia, reduces viscosity, whereas increased red blood cell concentration increases viscosity. Viscosity also
increases with blood sugar concentration—visualize pumping syrup. It had been thought that aspirin and related
"blood thinner" drugs decreased the viscosity of blood, but studies found[17] that they act by reducing the tendency of the blood to clot
instead.
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.
Low arterial pressure
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
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.
See also
References
- ^ http://www.braun.com/medical/bloodpressure/downloads/measurement.DownloadPara.0001.File0.tmp.pdf
- ^ Seventh report of the Joint National Committee on prevention, detection,
evaluation, and treatment of high blood pressure. Hypertension 2003;42: 1206-52.
- ^ 2003 European Society of Hypertension-European Society of Cardiology
guidelines for the management of arterial hypertension. Guidelines committee. J Hypertens 2003;21: 1011-53.
- ^ American Journal of Hypertension May 2006.
- ^ The American Journal of Hypertension May 2007.
- ^ AMNews;Newspaper for America's Physicians June 2007.
- ^ American Heart Associationwebsite
- ^ Creating Your Own Blood Pressure Chart.
- ^ US National Library of Medicine
- ^ Hypertextbook
- ^ June 2006 American Medical Association Report
- ^ http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=16434724&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstractPlus
The risk of cardiovascular disease increases progressively throughout the range of arterial pressure, beginning at 115/75 mm
Hg.
- ^ Messerli FH, Williams B,Ritz E (2007).
"Essential hypertension". Lancet 370 (9587): 591-603.
- ^ O'Rourke M (1995). "Mechanical principles
in arterial disease". Hypertension 26 (1): 2-9. PMID 7607724.
- ^ Mitchell GF (2006). "Triangulating the
peaks of arterial pressure". Hypertension 48 (4): 543-5. PMID 16940226.
- ^ Textbook of Medical Physiology, 7th Ed., Guyton & Hall,
Elsevier-Saunders, ISBN 0-7216-0240-1, page 220.
- ^ Rosenson, R.S.; Wolff, D.; Green, D.;
Boss, A.H.; and Kensey, K.R. (February 2004). "Aspirin". Journal of Thrombosis and Haemostasis
2 (2): 340. ISSN 1538-7933. Retrieved on 2006-08-24.
External links
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