Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" refers to the pressure in a brachial artery, where it is most commonly measured. Blood pressure is usually expressed in terms of the systolic pressure (maximum pressure during one heartbeat) over diastolic pressure (minimum pressure between two heartbeats) in the cardiac cycle. It is measured in millimeters of mercury (mmHg) above the surrounding atmospheric pressure, or in kilopascals (kPa). The difference between the systolic and diastolic pressures is known as pulse pressure,[1] while the average pressure during a cardiac cycle is known as mean arterial pressure.[2]
Blood pressure is one of the vital signs—together with respiratory rate, heart rate, oxygen saturation, and body 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, 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 by auscultation (listening) through a stethoscope for sounds in one arm's artery as the artery is squeezed, closer to the heart, by an aneroid gauge or a mercury-tube sphygmomanometer.[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 to ambulatory 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 a standard 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 is influenced by cardiac output, systemic vascular resistance, blood volume and arterial stiffness, and varies depending on person's situation, emotional state, activity and relative health or disease state. In the short term, blood pressure is regulated by baroreceptors, which act via the brain to influence the nervous and the endocrine systems.
Blood pressure that is too low is called hypotension, pressure that is consistently too high is called hypertension, 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, including stroke, heart disease, and kidney 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), and ambulatory 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, emotional stress, consumption of food or liquid, dietary factors, physical activity, changes in posture (such as standing-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 and sex, 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. for peripheral arterial disease, obstructive arterial disease or aortic 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 if symptoms are present.[26]
In pregnancy, 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 termed isolated systolic hypertension. The rise in pulse pressure with age is attributed to increased stiffness 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 the systemic circulation. However, measurement of pressures in the venous system and the pulmonary vessels plays an important role in intensive care medicine but requires invasive measurement of pressure using a catheter.
Venous pressure is the vascular pressure in a vein or in the atria of the heart. It is much lower than arterial pressure, with common values of 5 mmHg in the right atrium and 8 mmHg in the left atrium.
Variants of venous pressure include:
Normally, the pressure in the pulmonary artery is about 15 mmHg at rest.[40]
Increased blood pressure in the capillaries of the lung causes pulmonary hypertension, leading to interstitial edema if the pressure increases to above 20 mmHg, and to pulmonary edema at pressures above 25 mmHg.[41]
Aortic pressure, also called central aortic blood pressure, or central blood pressure, is the blood pressure at the root of the aorta. 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 the brachial 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]
Disorders of blood pressure control include high blood pressure, low blood pressure, and blood pressure that shows excessive or maladaptive fluctuation.
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 a hypertensive 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 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, and arterial aneurysms, and is the leading cause of chronic kidney failure.[49] Even moderate elevation of arterial pressure leads to shortened life expectancy.[49] 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.[50] For people with high blood pressure, higher heart rate variability (HRV) is a risk factor for atrial fibrillation.[51]
Both high systolic pressure and high pulse 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 called isolated 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 with heart valve regurgitation, a change in its severity may be associated with a change in diastolic pressure. In a study of