Blood pressure

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

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 heartbeats) in the cardiac cycle. It is measured in millimetres 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, while the average pressure during a cardiac cycle is known as mean arterial pressure.

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 at approximately 127/79 mmHg in men and 122/77 mmHg in women, although these averages mask significant regional differences.

Traditionally, a healthcare worker measured blood pressure non-invasively by auscultation (listening) with a stethoscope for sounds in an arm artery as the artery is compressed by a sphygmomanometer. Auscultation is still generally considered the gold standard for non-invasive blood pressure readings in clinical settings. However, semi-automated methods have become common due to concerns about mercury toxicity, cost, ease of use, and suitability for ambulatory blood pressure or home blood pressure measurements.

Blood pressure is influenced by cardiac output, systemic vascular resistance, blood volume, and arterial stiffness. It varies depending on a person's situation, emotional state, activity level, and overall health. In the short term, blood pressure is regulated by baroreceptors, which act via the brain to influence the nervous system and endocrine system.

Blood pressure that is too low is called hypotension, pressure that is consistently too high is called hypertension, and normal pressure is called normotension. Both hypertension and hypotension have many causes and may occur suddenly or develop over time. Long-term hypertension is a risk factor for many diseases, including stroke, heart disease, and kidney failure.

Classification, normal and abnormal values

Systemic arterial pressure

Blood pressure measurements can be influenced by circumstances of measurement.[1] 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).[1]

The risk of cardiovascular disease increases progressively above a systolic blood pressure of 90 mmHg.[1]

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 adults.[4]

Blood pressure fluctuates from minute to minute and normally shows a circadian rhythm over a 24-hour period,[5] with highest readings in the early morning and evenings and lowest readings at night.[6][7] 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.[8] Blood pressure varies over longer time periods (months to years) and this variability predicts adverse outcomes.[9] 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.[10] 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.[11]

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.[12][13][14][15]

There is no accepted diagnostic standard for hypotension, although pressures less than 90/60 are commonly regarded as hypotensive.[16] In practice blood pressure is considered too low only if symptoms are present.[17]

Systemic arterial pressure and age

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.[18]

The average blood pressure for full-term infants:[19]

• Systolic 65–95 mmHg
• Diastolic 30–60 mmHg

In children the normal ranges for blood pressure are lower than for adults and depend on height.[21] Reference blood pressure values have been developed for children in different countries, based on the distribution of blood pressure in children of these countries.[22]

In adults in most societies, systolic blood pressure tends to rise from early adulthood onward, up to at least age 70;[23][24] diastolic pressure tends to begin to rise at the same time but start to fall earlier in mid-life, approximately age 55.[24] 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,[24] 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.[25] An age-related rise in blood pressure is not considered healthy and is not observed in some isolated unacculturated communities.[26]