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The systemic arterial blood pressure rises to a maximum (the systolic pressure [PS]), during the ejection period, while it falls to a minimum (the diastolic pressure [PD]) during diastole and the iso(volu)metric period of systole (aortic valve closed) (^ A). Up to about 45 years of age the resting (sitting or recumbent) PD ranges from 60-90 mmHg (8-12kPa); PS ranges from 100-140 mmHg (13-19kPa) (^p. 208). The difference between PD and PS is the blood pressure amplitude or pulse pressure.

The mean blood pressure is decisive for peripheral arterial perfusion. It can be determined graphically (^A) from the invasively measured blood pressure curve (e.g., arterial catheter), or while recording such a curve by dampening down the oscillations until only the mean pressure is recorded.

In the vascular system the flow fluctuations in the great arteries are dampened through the "windkessel" (compression chamber) effect to an extent that precapillary blood no longer flows in spurts but continuously. Such a system consisting of highly compliant conduits and high-resistance terminals, is called a hydraulic filter. The arteries become more rigid with age, so that the PS rise per volume increase (AP/AV= elastance) becomes greater and compliance decreases. This mainly increases PS (^C), without necessarily increasing the mean pressure (the shape of the pressure curve is changed). Thoughtless pharmacological lowering of an elevated PS in the elderly can thus result in dangerous underperfu-sion (e.g., of the brain).

Measuring blood pressure. Blood pressure (at the level of the heart) is routinely measured according to the Riva-Rocci method, by sphygmomanometer (^ B). An inflatable cuff is fitted snugly around the upper arm (its width at least 40% of the arm's circumference) and under manometric control inflated to ca. 30 mmHg (4 kPa) above the value at which the palpated radial pulse disappears. A stethoscope having been placed over the brachial artery near the elbow, at the lower edge of the cuff, the cuff pressure is then slowly lowered (2-4 mmHg/s). The occurrence of the first pulse-synchronous sound (clear, tapping sound; phase 1 of Korotkoff) represents PS and is recorded. Normally this sound at first becomes softer (phase 2) before getting louder (phase 3), then becomes muffled in phase 4 and disappears completely (phase 5). The latter is nowadays taken to represent PD and is recorded as such.

Sources of error when measuring blood pressure. Complete disappearance of the sound sometimes occurs at a very low pressure. The difference between phases 4 and 5 (normally about 10 mmHg) is increased by conditions and diseases that favor flow turbulence (physical activity, fever, anemia, thyro-toxicosis, pregnancy, aortic regurgitation, AV fistula). If blood pressure is measured again, the cuff pressure must be left at zero for one to two minutes, because venous congestion may give a falsely high diastolic reading. The cuff should be 20% broader than the diameter of the upper arm. A cuff that is too small (e.g., in the obese, in athletes or if measurement has to be made at the thigh) also gives falsely high diastolic values, as does a too loosely applied cuff. A false reading can also be obtained when the auscultatory sounds are sometimes not audible in the range of higher amplitudes (auscultatory gap). In this case the true PS is obtained only if the cuff pressure is high enough to begin with (see above).

It is sufficient in follow-up monitoring of systemic hypertension (e.g., in labile hypertension from which fixed hypertension can often develop; ^D and p. 208) to measure blood pressure in one arm only (the same one every time, if possible). Nevertheless, in cases of stenosis in one of the great vessels there can be considerable, diagnostically important, differences in blood pressure between left and right arm (pressure on the right > left, except in dex-trocardia). This occurs in supravalvar aortic stenosis (mostly in children) and the subcla-vian steal syndrome, caused by narrowing in the proximal subclavian artery, usually of atherosclerotic etiology (ipsilateral blood pressure reduced). Blood pressure differences between arms and legs can occur in congenital or acquired (usually atherosclerotic) stenoses of the aorta distal to the origin of the arteries to the arms.

|— A. Aortic Pressure Curve (Invasive Measurement)

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