222
Dynamics
of
the Vascular System
catheterization in the diagnostic setting has been discussed by Li and
Kostis (1984).
8.1.2
Noninvasive Blood Pressure Measurements
8.1.2.1
Auscultatory Measurement of Blood Pressure
The auscultatory Korotkoff sound method remains the most popular form
of noninvasive blood pressure measurement in the clinical setting. This
method however, lacks accuracy when compared to invasive catheter
technique.
Errors of
5-10
mmHg error is common. This technique,
however,
is
simple to employ and has surprisingly high repeatability. It
allows both systolic and diastolic pressures to be determined.
Figure 8.1.5 illustrates the modern auscultatory method. The cuff is
inflated to a pressure exceeding the expected systolic arterial pressure
(Ps).
During the inflation of the cuff with cuff pressure exceeding that of
the systolic pressure, the segment of the artery under the cuff is forced to
collapse, either partially or completely. The cuff is then allowed to
deflate slowly
at
a
few mmHg per second. This is accomplished through
a needle valve which allows air to escape, hence dropping the cuff
pressure. During deflation, the initial arterial lumen opening that is is
detected is the systolic pressure. The first vascular sounds that emerge is
generally referred to as phase
1,
define
P,.
When either the vascular
sounds become muffled (phase
IV)
or disappear completely (phase
V),
the diastolic pressure
(Pd)
is obtained. This technique has an estimated
accuracy of 5-
10
mmHg. There remains debate as to whether phase
IV
or
V
is
a
better indicator of diastolic pressure. Figure 8.1.6 illustrates the
Korotkoff vascular sounds recorded in a brachial artery.
When vascular
reactivity is altered with hand-grip, the spectral content is shifted, such
that the Korotkoff sound intensity
is
increased, together with a higher
observed blood pressure (Fig.
8.1.6;
Matonick and Li,
1999).
The width and length of the cuff are important considerations in the
application
of the
auscultatory method,
typically
a
width to
circumference ratio of 0.4 is used. There
is
an optimum width of the cuff,
narrower than optimum cuffs tend to impose an arterial stress below that
of cuff pressure and can result in an overestimation of blood pressure.
A
