Blood pressure (BP) is thepressure ofcirculating blood against the walls ofblood vessels. Most of this pressure results from theheart pumping blood through thecirculatory system. When used without qualification, the term "blood pressure" refers to the pressure in abrachial artery, where it is most commonly measured. Blood pressure is usually expressed in terms of thesystolic pressure (maximum pressure during oneheartbeat) overdiastolic pressure (minimum pressure between two heartbeats) in thecardiac cycle. It is measured inmillimetres of mercury (mmHg) above the surroundingatmospheric pressure, or inkilopascals (kPa). The difference between the systolic and diastolic pressures is known aspulse pressure,[1] while the average pressure during a cardiac cycle is known asmean arterial pressure.[2]
Blood pressure is one of thevital signs—together withrespiratory rate,heart rate,oxygen saturation, andbody temperature—that healthcare professionals use in evaluating a patient's health. Normal resting blood pressure in an adult is approximately 120 millimetres of mercury (16 kPa) systolic over 80 millimetres of mercury (11 kPa) diastolic, denoted as "120/80 mmHg". Globally, the average blood pressure, age standardized, has remained about the same since 1975 to the present,[when?] at approximately 127/79 mmHg in men and 122/77 mmHg in women, although these average data mask significantly diverging regional trends.[3]
Traditionally, a health-care worker measured blood pressure non-invasively byauscultation (listening) through astethoscope for sounds in one arm'sartery as the artery is squeezed, closer to the heart, by ananeroid gauge or amercury-tubesphygmomanometer.[4] Auscultation is still generally considered to be the gold standard of accuracy for non-invasive blood pressure readings in clinic.[5] However, semi-automated methods have become common, largely due to concerns about potential mercury toxicity,[6] although cost, ease of use and applicability toambulatory blood pressure or home blood pressure measurements have also influenced this trend.[7] Early automated alternatives to mercury-tube sphygmomanometers were often seriously inaccurate, but modern devices validated to international standards achieve an average difference between two standardized reading methods of 5 mm Hg or less, and astandard deviation of less than 8 mm Hg.[7] Most of these semi-automated methods measure blood pressure using oscillometry (measurement by a pressure transducer in the cuff of the device of small oscillations of intra-cuff pressure accompanying heartbeat-induced changes in the volume of each pulse).[8]
Blood pressure that is too low is calledhypotension, pressure that is consistently too high is calledhypertension, and normal pressure is called normotension.[9] Both hypertension and hypotension have many causes and may be of sudden onset or of long duration. Long-term hypertension is a risk factor for many diseases, includingstroke,heart disease, andkidney failure. Long-term hypertension is more common than long-term hypotension.
Blood pressure measurements can be influenced by circumstances of measurement.[10] Guidelines use different thresholds for office (also known as clinic), home (when the person measures their own blood pressure at home), andambulatory blood pressure (using an automated device over a 24-hour period).[10]
The risk of cardiovascular disease increases progressively above 90 mmHg, especially among women.[10]
Observational studies demonstrate that people who maintain arterial pressures at the low end of these pressure ranges have much better long-term cardiovascular health. There is an ongoing medical debate over what is the optimal level of blood pressure to target when using drugs to lower blood pressure with hypertension, particularly in older people.[13]
Blood pressure fluctuates from minute to minute and normally shows a circadian rhythm over a 24-hour period,[14] with highest readings in the early morning and evenings and lowest readings at night.[15][16] Loss of the normal fall in blood pressure at night is associated with a greater future risk of cardiovascular disease and there is evidence that night-time blood pressure is a stronger predictor of cardiovascular events than day-time blood pressure.[17] Blood pressure varies over longer time periods (months to years) and this variability predicts adverse outcomes.[18] Blood pressure also changes in response to temperature, noise, emotionalstress, consumption of food or liquid, dietary factors, physical activity, changes in posture (such asstanding-up),drugs, and disease.[19] The variability in blood pressure and the better predictive value of ambulatory blood pressure measurements has led some authorities, such as the National Institute for Health and Care Excellence (NICE) in the UK, to advocate for the use of ambulatory blood pressure as the preferred method for diagnosis of hypertension.[20]
Various other factors, such as age andsex, also influence a person's blood pressure. Differences between left-arm and right-arm blood pressure measurements tend to be small. However, occasionally there is a consistent difference greater than 10 mmHg which may need further investigation, e.g. forperipheral arterial disease,obstructive arterial disease oraortic dissection.[21][22][23][24]
There is no accepted diagnostic standard for hypotension, although pressures less than 90/60 are commonly regarded as hypotensive.[25] In practice blood pressure is considered too low only ifsymptoms are present.[26]
Inpregnancy, it is the fetal heart and not the mother's heart that builds up the fetal blood pressure to drive blood through the fetal circulation. The blood pressure in the fetal aorta is approximately 30 mmHg at 20 weeks of gestation, and increases to approximately 45 mmHg at 40 weeks of gestation.[27]
The average blood pressure for full-term infants:[28]
In children the normal ranges for blood pressure are lower than for adults and depend on height.[30] Reference blood pressure values have been developed for children in different countries, based on the distribution of blood pressure in children of these countries.[31]
In adults in most societies, systolic blood pressure tends to rise from early adulthood onward, up to at least age 70;[32][33] diastolic pressure tends to begin to rise at the same time but start to fall earlier in mid-life, approximately age 55.[33] Mean blood pressure rises from early adulthood, plateauing in mid-life, while pulse pressure rises quite markedly after the age of 40. Consequently, in many older people, systolic blood pressure often exceeds the normal adult range,[33] if the diastolic pressure is in the normal range this is termedisolated systolic hypertension. The rise in pulse pressure with age is attributed to increasedstiffness of the arteries.[34] An age-related rise in blood pressure is not considered healthy and is not observed in some isolated unacculturated communities.[35]
Blood pressure generally refers to the arterial pressure in thesystemic circulation. However, measurement of pressures in the venous system and thepulmonary vessels plays an important role inintensive care medicine but requires invasive measurement of pressure using acatheter.
Venous pressure is the vascular pressure in avein or in theatria of the heart. It is much lower than arterial pressure, with common values of 5 mmHg in theright atrium and 8 mmHg in the left atrium.
Variants of venous pressure include:
Central venous pressure, which is a good approximation of right atrial pressure,[37] which is a major determinant of right ventricular end diastolic volume. (However, there can be exceptions in some cases.)[38]
Thejugular venous pressure (JVP) is the indirectly observed pressure over the venous system. It can be useful in the differentiation of different forms ofheart andlung disease.
Aortic pressure, also called central aortic blood pressure, or central blood pressure, is the blood pressure at the root of theaorta. Elevated aortic pressure has been found to be a more accurate predictor of both cardiovascular events and mortality, as well as structural changes in the heart, than has peripheral blood pressure (such as measured through thebrachial artery).[42][43] Traditionally it involved an invasive procedure to measure aortic pressure, but now there are non-invasive methods of measuring it indirectly without a significant margin of error.[44][45]
Certain researchers have argued for physicians to begin using aortic pressure, as opposed to peripheral blood pressure, as a guide for clinical decisions.[46][43] The way antihypertensive drugs impact peripheral blood pressure can often be very different from the way they impact central aortic pressure.[47]
If the heart is stopped, blood pressure falls, but it does not fall to zero. The remaining pressure measured after cessation of the heart beat and redistribution of blood throughout the circulation is termed the mean systemic pressure or mean circulatory filling pressure;[48] typically this is proximally ~7 mmHg.[48]
Overview of main complications of persistent high blood pressure.[49]
Arterial hypertension can be an indicator of other problems and may have long-term adverse effects. Sometimes it can be an acute problem, such as in ahypertensive emergency when blood pressure is more than 180/120 mmHg.[49]
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 theheart muscle tends to thicken, enlarge and become weaker over time.
Both highsystolic pressure and highpulse pressure (the numerical difference between systolic and diastolic pressures) are risk factors.[49] Elevated pulse pressure has been found to be a stronger independent predictor of cardiovascular events, especially in older populations, than has systolic, diastolic, or mean arterial pressure.[52][53][54][55] In some cases, it appears that a decrease in excessive diastolic pressure can actually increase risk, probably due to the increased difference between systolic and diastolic pressures (ie. widened pulse pressure). If systolic blood pressure is elevated (>140 mmHg) with a normal diastolic blood pressure (<90 mmHg), it is calledisolated systolic hypertension and may present a health concern.[49][56] According to the 2017[57] American Heart Association blood pressure guidelines state that a systolic blood pressure of 130–139 mmHg with a diastolic pressure of 80–89 mmHg is "stage one hypertension".[49]
For those withheart valve regurgitation, a change in its severity may be associated with a change in diastolic pressure. In a study of people with heart valve regurgitation that compared measurements two weeks apart for each person, there was an increased severity ofaortic andmitral regurgitation when diastolic blood pressure increased, whereas when diastolic blood pressure decreased, there was a decreased severity.[58]
A large fall in blood pressure upon standing (typically a systolic/diastolic blood pressure decrease of >20/10 mmHg) is termedorthostatic hypotension (postural hypotension) and represents a failure of the body to compensate for the effect ofgravity on the circulation. Standing results in an increasedhydrostatic pressure in the blood vessels of the lower limbs. The consequent distension of the veins below thediaphragm (venous pooling) causes ~500 ml of blood to be relocated from the chest and upper body. This results in a rapid decrease in central blood volume and a reduction of ventricularpreload which in turn reduces stroke volume, and mean arterial pressure. Normally this is compensated for by multiple mechanisms, including activation of theautonomic nervous system which increasesheart rate,myocardial contractility and systemic arterialvasoconstriction to preserve blood pressure and elicitsvenous vasoconstriction to decrease venouscompliance. Decreased venous compliance also results from an intrinsicmyogenic increase in venoussmooth muscle tone in response to the elevated pressure in the veins of the lower body.
Other compensatory mechanisms include the veno-arteriolaraxon reflex, the 'skeletal muscle pump' and 'respiratory pump'. Together these mechanisms normally stabilize blood pressure within a minute or less.[61] If these compensatory mechanisms fail and arterial pressure and bloodflow decrease beyond a certain point, theperfusion of the brain becomes critically compromised (i.e., the blood supply is not sufficient), causinglightheadedness,dizziness, weakness orfainting.[62] Usually this failure of compensation is due to disease, or drugs that affect thesympathetic nervous system.[61] A similar effect is observed following the experience of excessive gravitational forces (G-loading), such as routinely experienced by aerobatic or combat pilots 'pulling Gs' where the extreme hydrostatic pressures exceed the ability of the body's compensatory mechanisms.
Some fluctuation or variation in blood pressure is normal. Variation in blood pressure that is significantly greater than the norm is known aslabile hypertension and is associated with increased risk of cardiovascular disease[63] brain small vessel disease,[64] and dementia[65] independent of the average blood pressure level. Recent evidence fromclinical trials has also linked variation in blood pressure to mortality,[66][67] stroke,[68] heart failure,[69] and cardiac changes that may give rise to heart failure.[70] These data have prompted discussion of whether excessive variation in blood pressure should be treated, even among normotensive older adults.[71]
Older individuals and those who had received blood pressure medications are more likely to exhibit larger fluctuations in pressure,[72] and there is some evidence that different antihypertensive agents have different effects on blood pressure variability;[65] whether these differences translate to benefits in outcome is uncertain.[65]
Cardiac systole and diastoleBlood flow velocity waveforms in the central retinal artery (red) and vein (blue), measured bylaser Doppler imaging in the eye fundus of a healthy volunteer.Schematic of pressures in the circulation
During each heartbeat, blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure.[73][unreliable medical source] The blood pressure in the circulation is principally due to the pumping action of the heart.[74] However, blood pressure is also regulated by neural regulation from the brain (seeHypertension and the brain), as well as osmotic regulation from the kidney. Differences in mean blood pressure drive the flow of blood around the circulation. The rate of mean blood flow depends on both blood pressure and the resistance to flow presented by the blood vessels. In the absence ofhydrostatic effects (e.g. standing), mean blood pressure decreases as thecirculating blood moves away from the heart through arteries andcapillaries due toviscous losses of energy. Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries andarterioles.[75] Pulsatility also diminishes in the smaller elements of the arterial circulation, although some transmitted pulsatility is observed in capillaries.[76] Gravity affects blood pressure via hydrostatic forces (e.g., during standing), and valves in veins,breathing, and pumping from contraction of skeletal muscles also influence blood pressure, particularly in veins.[74]
A simple view of thehemodynamics of systemic arterial pressure is based aroundmean arterial pressure (MAP) and pulse pressure. Most influences on blood pressure can be understood in terms of their effect oncardiac output,[77]systemic vascular resistance, orarterial stiffness (the inverse of arterial compliance). Cardiac output is the product of stroke volume and heart rate. Stroke volume is influenced by 1) theend-diastolic volume or filling pressure of the ventricle acting via theFrank–Starling mechanism—this is influenced byblood volume; 2)cardiac contractility; and 3)afterload, the impedance to blood flow presented by the circulation.[78] In the short-term, the greater the blood volume, the higher the cardiac output. This has been proposed as an explanation of the relationship between high dietary salt intake and increased blood pressure; however, responses to increased dietary sodium intake vary between individuals and are highly dependent on autonomic nervous system responses and therenin–angiotensin system,[79][80][81] changes inplasma osmolarity may also be important.[82] In the longer-term the relationship between volume and blood pressure is more complex.[83] In simple terms, systemic vascular resistance is mainly determined by the caliber of small arteries and arterioles. The resistance attributable to a blood vessel depends on its radius as described by theHagen-Poiseuille's equation (resistance∝1/radius4). Hence, the smaller the radius, the higher the resistance. Other physical factors that affect resistance include: vessel length (the longer the vessel, the higher the resistance), blood viscosity (the higher the viscosity, the higher the resistance)[84] and the number of vessels, particularly the smaller numerous, arterioles and capillaries. The presence of a severe arterialstenosis increases resistance to flow, however this increase in resistance rarely increases systemic blood pressure because its contribution to total systemic resistance is small, although it may profoundly decrease downstream flow.[85] Substances calledvasoconstrictors reduce the caliber of blood vessels, thereby increasing blood pressure.Vasodilators (such asnitroglycerin) increase the caliber of blood vessels, thereby decreasing arterial pressure. In the longer term a process termed remodeling also contributes to changing the caliber of small blood vessels and influencing resistance and reactivity to vasoactive agents.[86][87] Reductions in capillary density, termed capillary rarefaction, may also contribute to increased resistance in some circumstances.[88]
In practice, each individual's autonomic nervous system and other systems regulating blood pressure, notably the kidney,[89] respond to and regulate all these factors so that, although the above issues are important, they rarely act in isolation and the actual arterial pressure response of a given individual can vary widely in the short and long term.
A schematic representation of the arterial pressure waveform over one cardiac cycle. The notch in the curve is associated with closing of the aortic valve.
The pulse pressure is the difference between the measured systolic and diastolic pressures,[90]
The pulse pressure is a consequence of the pulsatile nature of thecardiac output, i.e. the heartbeat. The magnitude of the pulse pressure is usually attributed to the interaction of thestroke volume of the heart, the compliance (ability to expand) of the arterial system—largely attributable to theaorta and large elastic arteries—and theresistance to flow in thearterial tree.[90]
A healthy pulse pressure is around 40 mmHg.[1] A pulse pressure that is consistently 60 mmHg or greater is likely to be associated with disease, and a pulse pressure of 50 mmHg or more increases the risk ofcardiovascular disease as well as other complications such as eye and kidney disease.[52] Pulse pressure is considered low if it is less than 25% of the systolic. (For example, if the systolic pressure is 120 mmHg, then the pulse pressure would be considered low if it is less than 30 mmHg, since 30 is 25% of 120.)[91] A very low pulse pressure can be a symptom of disorders such ascongestive heart failure.[52]
Elevated pulse pressure has been found to be a stronger independent predictor of cardiovascular events, especially in older populations, than has systolic, diastolic, or mean arterial pressure.[52][53] This increased risk exists for both men and women and even when no other cardiovascular risk factors are present. The increased risk also exists even in cases in which diastolic pressure decreases over time while systolic remains steady.[55][54]
Ameta-analysis in 2000 showed that a 10 mmHg increase in pulse pressure was associated with a 20% increased risk of cardiovascular mortality, and a 13% increase in risk for all coronary end points. The study authors also noted that, while risks of cardiovascular end points do increase with higher systolic pressures, at any given systolic blood pressure the risk of major cardiovascular end points increases, rather than decreases, with lower diastolic levels. This suggests that interventions that lower diastolic pressure without also lowering systolic pressure (and thus lowering pulse pressure) could actually be counterproductive.[92] There are no drugs currently approved to lower pulse pressure, although some antihypertensive drugs may modestly lower pulse pressure, while in some cases a drug that lowers overall blood pressure may actually have the counterproductive side effect of raising pulse pressure.[93]
Pulse pressure can both widen or narrow in people withsepsis depending on the degree ofhemodynamic compromise. A pulse pressure of over 70 mmHg in sepsis is correlated with an increased chance of survival and a more positive response toIV fluids.[94][95]
Theendogenous,homeostatic regulation of arterial pressure is not completely understood, but the following mechanisms of regulating arterial pressure have been well-characterized:
Aldosterone release: Thissteroid hormone is released from theadrenal cortex in response to activation of the renin-angiotensin system, high serumpotassium levels, or elevatedadrenocorticotropic hormone (ACTH). Renin converts angiotensinogen to angiotensin I, which is converted byangiotensin converting enzyme to angiotensin II. Angiotensin II then signals to the adrenal cortex to release aldosterone.[101] Aldosterone stimulatessodium retention and potassium excretion by the kidneys and the consequent salt and water retention increases plasma volume, and indirectly, arterial pressure. Aldosterone may also exert direct pressor effects on vascular smooth muscle and central effects on sympathetic nervous system activity.[102]
These different mechanisms are not necessarily independent of each other, as indicated by the link between the RAS and aldosterone release. When blood pressure falls many physiological cascades commence in order to return the blood pressure to a more appropriate level.
The blood pressure fall is detected by a decrease in blood flow and thus a decrease inglomerular filtration rate (GFR).
Decrease in GFR is sensed as a decrease in Na+ levels by themacula densa.
The macula densa causes an increase in Na+ reabsorption, which causes water to follow in viaosmosis and leads to an ultimate increase inplasma volume. Further, the macula densa releases adenosine which causes constriction of the afferent arterioles.
Taking blood pressure with a sphygmomanometerMeasuring systolic and diastolic blood pressure using a mercury sphygmomanometer
Arterial pressure is most commonly measured via asphygmomanometer, which uses the height of a column of mercury, or ananeroid gauge, to reflect the blood pressure by auscultation.[4] The most common automated blood pressure measurement technique is based on theoscillometric method.[103] Fully automated oscillometric measurement has been available since 1981.[104] This principle has recently been used to measure blood pressure with a smartphone.[105] Measuring pressureinvasively, by penetrating the arterial wall to take the measurement, is much less common and usually restricted to a hospital setting. Novel methods to measure blood pressure without penetrating the arterial wall, and without applying any pressure on patient's body are being explored,[106] for example, cuffless measurements that uses only optical sensors.[107]
In office blood pressure measurement,terminal digit preference is common. According to one study, approximately 40% of recorded measurements ended with the digit zero, whereas "without bias, 10%–20% of measurements are expected to end in zero"[108]
Blood pressure levels in non-human mammals may vary depending on the species. Heart rate differs markedly, largely depending on the size of the animal (larger animals have slower heart rates).[109] The giraffe has a distinctly high arterial pressure of about 190 mm Hg, enabling blood perfusion through the 2 metres (6 ft 7 in)-long neck to the head.[110] In other species subjected to orthostatic blood pressure, such asarboreal snakes, blood pressure is higher than in non-arboreal snakes.[111] A heart near to the head (short heart-to-head distance) and a long tail with tightintegument favor blood perfusion to the head.[112][113]
As in humans, blood pressure in animals differs by age, sex, time of day, and environmental circumstances:[114][115] measurements made in laboratories or under anesthesia may not be representative of values under free-living conditions. Rats, mice, dogs and rabbits have been used extensively to study the regulation of blood pressure.[116]
Blood pressure and heart rate of various mammals[114]
Hypertension in cats and dogs is generally diagnosed if the blood pressure is greater than 150 mm Hg (systolic),[117] althoughsight hounds have higher blood pressures than most other dog breeds; a systolic pressure greater than 180 mmHg is considered abnormal in these dogs.[118]
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