Chapter 8

### ARTERIAL PRESSURE

#### Definition

Lateral pressure exerted by the column of blood against the arterial wall is called the arterial pressure (blood pressure), and this pressure is referred to as the height of the column of blood supported by the force within a blood vessel. In a cardiac cycle, the highest pressure attained is the systolic pressure and the lowest pressure is the diastolic pressure. The equation "MAP – RAP = CO × TPR" is used to derive mean arterial pressure (MAP), where RAP is right arterial pressure, CO is cardiac output, and TPR is total peripheral resistance. Since RAP is very small, MAP (mm Hg) is the product of CO (liters per minute) and total peripheral resistance (mm Hg/liter/minute). The MAP is the geometric mean, and the calculation of MAP requires integration of pressure pulse.

An approximate estimate of MAP can be derived from the following equations:

#### Normal Arterial Pressure

Normal arterial pressure varies with age. It is approximately 70/50 mm Hg on the first day after birth, and gradually increases during the next several months to approximately 90/60 mm Hg. During subsequent years the rise is very slow and reaches 115/70 mm Hg at adolescence. There is a progressive increase in the pressure with age in the average population. The systolic pressure rises approximately 1 mm Hg/yr from 110 mm Hg at the age of 15 years. This probably reflects progressive reduction in arterial compliance. Diastolic pressure increases approximately 0.4 mm Hg/yr from 70 mm Hg at the age of 15 years. This rise probably reflects an increase in total peripheral resistance. The progressive increase in arterial pressure with age could also result from the effects of aging on the long-term blood pressure control mechanisms. Although average pressure in a population rises with age, the pressure never rises with age in certain people. MAP for a population is composed of individuals whose blood pressure does not change with advancing age and of individuals whose pressure increases with advancing age. There is no dividing line between normal and high blood pressure. An arbitrary level of normal blood pressure has been established to define those who have an increased risk of developing morbid cardiovascular events and/or clearly benefit from medical therapy. Arterial pressure is somewhat damaging. Life expectancy is inversely proportional to arterial pressures. The logic is to define hypertension at levels where treatment can provide benefits that outweigh risks. Males with normal diastolic pressure but elevated systolic pressure (> 158 mm Hg) have a 2.5-fold increase in cardiovascular mortality rates when compared to individuals with similar diastolic pressure but normal systolic pressure.

Arterial pressure is slightly higher in the right arm than in the left arm. Simultaneous measurement of blood pressure in both arms shows a difference of 10 mm Hg (both systolic and diastolic) in approximately 3% of normotensive and 6% of hypertensive ...

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