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Blood 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, the blood vessels that take blood away from the heart. Arterial pressure is most commonly measured via a sphygmomanometer, which historically used the height of a column of mercury to reflect the circulating pressure (see Non-invasive measurement). Today blood pressure values are still reported in millimetres of mercury (mmHg), though aneroid and electronic devices do not use mercury.
For each heartbeat, blood pressure varies between systolic and diastolic pressures. Systolic pressure is peak pressure in the arteries, which occurs near the beginning of the cardiac cycle when the ventricles are contracting. Diastolic pressure is minimum pressure in the arteries, which occurs near the end of the cardiac cycle when the ventricles are filled with blood. An example of normal measured values for a resting, healthy adult human is 115 mmHg systolic and 75 mmHg diastolic (written as 115/75 mmHg, and spoken as "one fifteen over seventy-five"). Pulse pressure is the difference between systolic and diastolic pressures.
Systolic and diastolic arterial blood pressures 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, disease, exercise, and momentarily from standing up. Sometimes the variations are large. Hypertension refers to arterial pressure being abnormally high, as opposed to hypotension, when it is abnormally low. Along with body temperature, blood pressure measurements are the most commonly measured physiological parameters.
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.
Units
The predominantly used unit for blood pressure measurement is mmHg (millimeter of mercury). Sometimes the unit kPa (kilopascal) is used, where 1 kPa = 7.50 mmHg.
Noninvasive measurement
The noninvasive 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.
Palpation methods
A minimum systolic value can be roughly estimated without any equipment by palpation, most often used in emergency situations. Palpation of a radial pulse indicates a minimum blood pressure of 80 mmHg, a femoral pulse indicates at least 70 mmHg, and a carotid pulse indicates a minimum of 60 mmHg. However, one study indicated that this method was not accurate enough and often overestimated patients' systolic blood pressure.[1] A more accurate value of systolic blood pressure can be obtained with a sphygmomanometer and palpating for when a radial pulse returns.[2] Because a diastolic pressure cannot be obtained by this method, blood pressures obtained by palpation are noted as "
Auscultatory methods
Auscultatory method aneroid sphygmomanometer with stethoscope
Mercury manometer
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, such as 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 (first Korotkoff sound). 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.
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 yield 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 results as well as possible.[4] 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 through an arterial line. 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
For some patients, blood pressure measurements taken in a doctor's office may not correctly characterize their typical blood pressure. In up to 25% of patients, the office visit blood pressure reading is higher than their typical blood pressure. This type of error is called white coat hypertension and can result from anxiety related to an examination by a health care professional.[5] The misdiagnosis of hypertension for these patients can result in needless and possibly harmful medication. On the other hand, in some cases a lower than typical blood pressure reading occurs at the doctor's office and these patients may fail to get needed treatment for hypertension.[6] Ambulatory blood pressure devices that take readings every half hour throughout the day and night have been used for identifying and mitigating these problems. Except for periods during sleep, home monitoring could be used for these purposes instead of ambulatory blood pressure monitoring.[7] Home monitoring may also be used to improve hypertension management and to monitor the effects of lifestyle changes and medication related to blood pressure.[8] Compared to ambulatory blood pressure measurements, home monitoring has been found to be an effective and lower cost alternative.
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.
When measuring blood pressure, an accurate reading requires that one not drink coffee, smoke cigarettes, or engage in strenuous exercise for 30 minutes before taking the reading. A full bladder may have a small effect on blood pressure readings, so if the urge to urinate exists, one should do so before the reading. For 5 minutes before the reading, one should sit upright in a chair with his or her feet flat on the floor and without any limbs crossed. The blood pressure cuff should always be against bare skin, as readings taken over a shirt sleeve are less accurate. During the reading, the arm that is used should be relaxed and kept at heart level, for example by resting it on a table.[11]
Since arterial pressure varies throughout the day, measurements intended to monitor changes over longer time frames should be taken at the same time of day to ensure that 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.
Automatic self-contained blood pressure monitors are available at reasonable prices, some of which are capable of Korotkoff's measurement in addition to oscillometric methods, enabling irregular heartbeat patients to accurately measure their blood pressure at home, which was not possible using the traditional devices.[citation needed]
Those using home arterial pressure monitoring devices are increasingly also making use of arterial pressure charting software.[12] These charting methods provide print outs for the patients physician and reminders on how often to check arterial pressure.
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