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Blood Pressure Measurement: An Overview

Blood pressure is traditionally defined as the intra-arterial pressure generated by the pulsatile flow of blood through an artery. With each beat of the heart a bolus of blood flows through the arteries and towards the peripheral circulation. During the cardiac cycle the heart experiences differing states of muscle contraction. There also exists a downstream resistance to flow, known as peripheral resistance. The dynamic pressure changes that are felt by the arterial wall are a direct result of the interaction between cardiac function and downstream resistance. Therefore, any form of examination of the dynamic pressure changes created by the heart would reflect, to some degree, cardiovascular function as well as the conditions that are existing in the peripheral circulation.

Since blood pressure is generated by cardiovascular activity, it is accepted worldwide as an index of cardiovascular performance. Therefore, impairment of cardiovascular function may often be detected through blood pressure changes. However, ease of determination is an additional factor which contributes to the acceptance of blood pressure as an index of cardiovascular performance. Due to current advances in technology, blood pressure may be measured using quick, convenient, noninvasive procedures that facilitate its integration into routine examinations.

Any type of measurement that is to be performed on human subjects is governed by its comfort, convenience and necessity. A procedure that is deemed impractical for routine use is of limited value to a physician. Invasive catheter procedures can obtain accurate blood pressure measurements in both central and peripheral locations of the body, providing the greatest flexibility with regard to measurement location. However, such measurements require surgical insertion procedures, and are seldom utilized for routine blood pressure measurements. Instead, noninvasive techniques are usually utilized for most cases. These non-invasive measurement techniques usually utilize a peripheral measurement site, and do not supply measurement information that directly pertains to the central circulation.

Many peripheral sites, such as the radial artery, have been used as measurement locations. However, the brachial artery is the site most often chosen for blood pressure measurement, and possesses attributes that fulfill the necessary requirements for most noninvasive techniques. For example, the brachial artery lies close to the surface of the skin, which allows a relatively clear measurement signal to be obtained. Also, due to the proximity of the brachial artery to the aorta, blood pressure measurements taken at the brachial artery correlate well with aortic blood pressures. Additionally, the fact that the brachial artery rests on the humerus facilitates uniform compression and occlusion of the artery. Lastly, the upper arm is a comfortable and convenient location for routine blood pressure measurement.

Blood pressure measurements attempt to determine pressures corresponding to various points of interest of the cardiac cycle. The cardiac cycle consists of the cyclic action of the heart spanning from systole to diastole. The events in the brachial artery that correspond to the points of interest of the cardiac cycle will be briefly described below (diagrams are exaggerated and are not to scale):

Systolic Blood Pressure (SBP)
during systole, the pressure generated by the heart reaches a maximum, resulting in maximum values for blood pressure and outward arterial wall distension.
Mean Arterial Pressure (MAP)
corresponds to a state of balance between the compressive and expansive forces acting on the arterial wall. There is no distension of the arterial wall, either outward or inward. This idealized case is very difficult to measure non-invasively.
Diastolic Blood Pressure (DBP)
during diastole, pressure reaches a minimum in the artery, with an inward distension of the arterial wall. However, the inward distension is much smaller in magnitude than the outward distension caused by systole. The lack of inward distension is due to the incompressible nature of the artery.