Blood Pressure

Description

Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure is understood to mean arterial blood pressure, i.e., the pressure in the large arteries, such as the brachial artery (in the arm). The pressure of the blood in other vessels is lower than the arterial pressure.

The peak pressure in the arteries during the cardiac cycle is the systolic pressure, and the lowest pressure (at the resting phase of the cardiac cycle) is the diastolic pressure. Typical values for the arterial blood pressure of a resting, healthy adult are approximately 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg), with large individual variations.

Blood pressure is not static, but undergoes natural variations from one heartbeat to another or throughout the day (in a circadian rhythm); it also changes in response to stress, nutritional factors, drugs, or disease.

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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 sounds) 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 (mm Hg). Normal ranges for blood pressure in adult humans are:

Systolic between 90 and 135 mmHg (or 90 and 135 torr, 12 to 18 kPa)
Diastolic between 50 and 90 mmHg (or 50 and 90 torr, 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 flowed through the vessels.

How Blood Pressure Works

The mean blood pressure in the arteries supplying the body is a result of the heart pumping blood from the veins back into the arteries.

The mean blood pressure value is determined by the volume of blood the heart is pumping per minute, termed cardiac output, versus the resistance of the 20,000 to 30,000 arterioles, termed total peripheral resistance, through which the blood must flow to reach the capillaries and then veins.

The up and down fluctuation of the arterial blood pressure results from the pulsatile nature of the cardiac output. The pulse pressure is determined by the interaction of the stroke volume versus the volume and elasticity of the major arteries.

The larger arteries, including all large enough to see without magnification, are low resistance (assuming no advanced atherosclerotic changes) and conduit vessels with high flow rates but producing very little pressure drop. For instance, about 5 mmHg mean pressure decrease in the blood flow traveling from the heart all the way to the toes is typical, assuming the individual is supine (horizontal with respect to gravity).

Blood Pressure Regulation

The endogenous regulation of blood pressure is not completely understood. Currently, three mechanisms of regulating blood pressure have been well-characterized: