Arterial blood pressure and its measurement.

Pressure is defined as force per unit area. The actual value of the recorded pressure depends on dynamic influences, like acceleration and friction in a moving fluid, and on static influences like the effect of a gravitational field. In the arterial system, four mechanisms exert the main influence on the measured pressure: (1) cardiac output and peripheral resistance, on which the mean value of the pressure depends; (2) the flow amplitude and the characteristic impedance of the artery, on which the pressure amplitude depends; (3) the orthostatic pressure component; (4) the local velocity of flow, on which the recorded pressure depends, and is described by the Bernoulli equation. Under normal conditions, this influence is rather small. The measurement of arterial pressure can be done either invasively or with one of the following noninvasive methods: occlusive cuff method, vascular unloading, arterial tonometry, or measurement of the arterial pulse wave velocity. Each of these methods has certain advantages and disadvantages. It seems necessary to point out the fact that due to wave reflections in the arterial system, pressure values depend on local influences and differ from location to location. Most markedly, the influence of wave reflection can be demonstrated proximally, as well as distally, to the occlusive cuff in the Riva Rocci method. Once the blood pressure has been measured in a person, the problem of the assessment and interpretation of the measured values has to be considered. According to the decision of the WHO, certain limits are defined which permit a distinction between normotension and hypertension. In each case, of course, a physician has to consider the importance of individual conditions.