Blood pressure For other uses, see Blood pressure (disambiguation). Blood pressure (BP) is the pressure exerted by circulat- ing blood upon the walls of blood vessels. When used without further specification, “blood pressure” usually refers to the arterial pressure in the systemic circulation. It is usually measured at a person’s upper arm. Blood pressure is usually expressed in terms of the systolic (maximum) pressure over diastolic (minimum) pressure and is measured in millimeters of mercury (mm Hg). It is one of the vital signs along with respiratory rate, heart rate, oxygen saturation, and body temperature. Normal resting blood pressure in an adult is approximately 120/80 mm Hg. Blood pressure varies depending on situation, activity, and disease states. It is regulated by the nervous and endocrine systems. Blood pressure that is low due to a disease state is called hypotension, and pressure that is consistently high is hypertension. Both have many causes which can range from mild to severe. Both may be of sudden onset or of long duration. Long term hypertension is a risk factor for many diseases, including kidney fail- ure, heart disease, and stroke. Long term hypertension is more common than long term hypotension in West- ern countries. Long term hypertension often goes unde- tected because of infrequent monitoring and the absence of symptoms. 1 Classification 1.1 Systemic arterial pressure The table presented here shows the classification of blood pressure adopted by the American Heart Association for adults who are 18 years and older. [1] It assumes the values are a result of averaging resting blood pressure readings measured at two or more visits to the doctor. [3][4] In the UK, clinic blood pressures are usually categorised into three groups; low (90/60 or lower), normal (between 90/60 and 139/89), and high (140/90 or higher). [5][6] Blood pressure fluctuates from minute to minute and nor- mally shows a circadian rhythm over a 24-hour period, with highest readings in the early morning and evenings and lowest readings at night. [7][8] Loss of the normal fall in blood pressure at night is associated with a greater fu- ture 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. [9] Various factors, such as age and sex, influence a person’s blood pressure and variations in it. In children, the normal ranges are lower than for adults and depend on height. [10] Reference blood pressure values have been developed for children in different countries, based on the distribu- tion of blood pressure in children of these countries. [11] As adults age, systolic pressure tends to rise and dias- tolic tends to fall. [12] In the elderly, systolic blood pres- sure tends to be above the normal adult range, [13] thought to be largely because of reduced flexibility of the arter- ies. Also, an individual’s blood pressure varies with ex- ercise, emotional reactions, sleep, digestion and time of day (circadian rhythm). Differences between left and right arm blood pressure measurements tend to be random and average to nearly zero if enough measurements are taken. However, in a small percentage of cases there is a consistent difference greater than 10 mm Hg which may need further investi- gation, e.g. for obstructive arterial disease. [14][15] The risk of cardiovascular disease increases progressively above 115/75 mm Hg. [16] In the past, hypertension was only diagnosed if secondary signs of high arterial pres- sure were present, along with a prolonged high systolic pressure reading over several visits. Regarding hypoten- sion, in practice blood pressure is considered too low only if noticeable symptoms are present. [2] Clinical trials demonstrate that people who maintain ar- terial pressures at the low end of these pressure ranges have much better long term cardiovascular health. The principal medical debate concerns the aggressiveness and relative value of methods used to lower pressures into this range for those who do not maintain such pressure on their own. Elevations, more commonly seen in older people, though often considered normal, are associated with in- creased morbidity and mortality. 1.2 Mean arterial pressure The mean arterial pressure (MAP) is the average over a cardiac cycle and is determined by the cardiac output (CO), systemic vascular resistance (SVR), and central ve- nous pressure (CVP), [18] MAP =(CO · SVR)+ CVP. 1