Page 216 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Arterial Blood Pressure and its Measurement
The systemic arterial blood pressure rises to a sound; phase 1 of Korotkoff) represents P S and
maximum (the systolic pressure [P S ]), during is recorded. Normally this sound at first be-
the ejection period, while it falls to a minimum comes softer (phase 2) before getting louder
(the diastolic pressure [P D ]) during diastole (phase 3), then becomes muffled in phase 4
and the iso(volu)metric period of systole (aor- and disappears completely (phase 5). The lat-
tic valve closed) (→ A). Up to about 45 years of ter is nowadays taken to represent P D and is re-
corded as such.
age the resting (sitting or recumbent) P D
Sources of error when measuring blood
ranges from 60–90 mmHg (8–12 kPa); P S
ranges from 100–140 mmHg (13–19 kPa) pressure. Complete disappearance of the
sound sometimes occurs at a very low pres-
(→ p. 208). The difference between P D and P S
is the blood pressure amplitude or pulse pres- sure. The difference between phases 4 and 5
(normally about 10 mmHg) is increased by
sure.
Heart and Circulation ripheral arterial perfusion. It can be deter- lence (physical activity, fever, anemia, thyro-
The mean blood pressure is decisive for pe-
conditions and diseases that favor flow turbu-
toxicosis, pregnancy, aortic regurgitation, AV
mined graphically (→ A) from the invasively
measured blood pressure curve (e.g., arterial
fistula). If blood pressure is measured again,
the cuff pressure must be left at zero for one
catheter), or while recording such a curve by
to two minutes, because venous congestion
dampening down the oscillations until only
In the vascular system the flow fluctuations
cuff should be 20% broader than the diameter
of the upper arm. A cuff that is too small (e.g.,
in the great arteries are dampened through the
7 the mean pressure is recorded. may give a falsely high diastolic reading. The
“windkessel” (compression chamber) effect to in the obese, in athletes or if measurement has
an extent that precapillary blood no longer to be made at the thigh) also gives falsely high
flows in spurts but continuously. Such a sys- diastolic values, as does a too loosely applied
tem consisting of highly compliant conduits cuff. A false reading can also be obtained
and high-resistance terminals, is called a hy- when the auscultatory sounds are sometimes
draulic filter. The arteries become more rigid not audible in the range of higher amplitudes
with age, so that the P S rise per volume in- (auscultatory gap). In this case the true P S is
crease (∆P/∆V = elastance) becomes greater obtained only if the cuff pressure is high
and compliance decreases. This mainly in- enough to begin with (see above).
creases P S (→ C), without necessarily increas- It is sufficient in follow-up monitoring of
ing the mean pressure (the shape of the pres- systemic hypertension (e.g., in labile hyperten-
sure curve is changed). Thoughtless pharma- sion from which fixed hypertension can often
cological lowering of an elevated P S in the el- develop; → D and p. 208) to measure blood
derly can thus result in dangerous underperfu- pressure in one arm only (the same one every
sion (e.g., of the brain). time, if possible). Nevertheless, in cases of ste-
Measuring blood pressure. Blood pressure nosis in one of the great vessels there can be
(at the level of the heart) is routinely measured considerable, diagnostically important, differ-
according to the Riva-Rocci method, by sphyg- ences in blood pressure between left and right
momanometer (→ B). An inflatable cuff is fit- arm (pressure on the right > left, except in dex-
ted snugly around the upper arm (its width at trocardia). This occurs in supravalvar aortic
least 40% of the arm’s circumference) and un- stenosis (mostly in children) and the subcla-
der manometric control inflated to ca. vian steal syndrome, caused by narrowing in
30 mmHg (4 kPa) above the value at which the proximal subclavian artery, usually of
the palpated radial pulse disappears. A stetho- atherosclerotic etiology (ipsilateral blood
scope having been placed over the brachial ar- pressure reduced). Blood pressure differences
tery near the elbow, at the lower edge of the between arms and legs can occur in congenital
cuff, the cuff pressure is then slowly lowered or acquired (usually atherosclerotic) stenoses
206 (2–4 mmHg/s). The occurrence of the first of the aorta distal to the origin of the arteries
pulse-synchronous sound (clear, tapping to the arms.
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
All rights reserved. Usage subject to terms and conditions of license.