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Blood pressure Measurement

Blood pressure (BP) is most accurately measured invasively by placing a cannula into a blood vessel and connecting it to an electronic pressure transducer. This invasive technique is regularly employed in intensive care medicine, anesthesiology, and for research purposes, but it is associated with complications such as thrombosis, infection, and bleeding. Therefore, the less accurate techniques of manual or oscillometric measurement predominate in routine examinations.

Mechanical sphygmomanometer with aneroid manometer and stethoscopeMost often, arterial blood pressure is measured manually using a sphygmomanometer. This is an inflatable (Riva Rocci) cuff placed around the upper arm, at roughly the same vertical height as the heart in a sitting person, attached to a manometer. The cuff is inflated until the artery is completely occluded. Listening with a stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When blood flow barely begins again in the artery, a "whooshing" or pounding sound (first Korotkoff sound) is heard. The pressure is noted at which this sound began. This is the systolic blood pressure. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound). This is the diastolic blood pressure.

Oscillometric methods are used in long-term measurement as well as in clinical practice. Oscillometric measurement (also termed NIBP = Non-Invasive Blood Pressure) is incorporated in many bedside patient monitors. It relies on a cuff similar to that of a sphygmomanometer, which is connected to an electric pump and a pressure transducer. The cuff is placed on the upper arm and is automatically inflated. When pressure is gradually released, the small oscillations in cuff pressure that are caused by the cyclic expansion of the brachial artery are recorded and used to calculate systolic and diastolic pressures.

Values are usually given in millimetres of mercury (mmHg). Normal ranges for blood pressure in adult humans are:

Systolic between 90 and 135 mmHg (12 to 18 kPa)
Diastolic between 50 and 90 mmHg (7 to 12 kPa)
In children the observed normal ranges are lower; in the elderly, they are more often higher. Clinical trials demonstrate that people who maintain blood pressures in low end of these pressure ranges have much better long term cardiovascular health and are considered optimal. The principal medical debate is the aggressiveness and relative value of methods used to lower pressures into this range for those who don't maintain such pressure on their own. Elevations, more commonly seen in older people, though often considered normal, are associated with increased morbidity and mortality. The clear trend from double blind clinical trials (for the better strategies and agents) has increasingly been that lower ends up being demonstrated to result in less disease/better outcomes long term.

Incidentally, the absolute BP is obtained by adding the atmospheric pressure (760 mmHg at sea level) to the values obtained by the sphygmomanometer. If the BP had not been greater than the atmospheric pressure, the blood vessels would collapse and the blood would have never flown through the vessels!


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