Hemodynamic Measurements and Dynamics
of
Assisted Circulation
225
tonometry. This
is
Vascular unloading", as Marey termed it. The
tension within the wall of the artery under such circumstance is zero
when the transmural pressure is zero. The maximal oscillation was
found to correspond to mean arterial pressure. The oscillometric method
continued to gain popularity, although this technique is rather accurate
for systolic and mean blood pressure detection, its accuracy
is
much less
so
for diastolic pressure measurement.
Geddes
(1984)
has provided a detailed analysis and comparison of the
cuff technique and oscillometry.
Fig.
8.1.7
shows the corresponding
appearance and disappearance
of
the Korotkoff sounds
and
the
oscillations in cuff pressure.
Maximum oscillation in cuff pressure
corresponds to mean blood pressure as shown. Thus, in addition to
systolic and diastolic pressures, mean blood pressure can be obtained.
The radial pulse distal to brachial artery measurement site is also shown.
8.1.2.3
Noninvasive Blood Pressure Monitoring with Tonometer
There are several methods for noninvasive recording of blood pressure
waveforms, including the volume pulse method, the pressure pulse
method, the cantilever and optical deflection methods.
In the volume pulse method (Fig.
8.1.8),
the successful recording
hinges on the relationship between intravascular pressure distension and
radial displacement. The pressure pulse method (Fig.
8.1.8),
such as the
tonometer, is dependent on the interplays of contact stress, the
deformation stress and arterial pressure. Arterial pressure is actually a
fraction of the contact stress. Drzewiecki et al.
(1983)
have shown that
in arterial tonometery, with arterial flattening, shear stress becomes
negligible compared with the normal stress in the arterial wall and skin,
and uniform contact stress is developed over the transducer-skin
interface. This is the ideal state for pulse recording with tonometry.
Arterial tonometry for measuring blood pressure is based on the principle
that when a pressurized blood vessel is partially collapsed by an external
object, the circumferential stresses in the vessel wall are removed and the
internal and external pressures are equal. Since tonometers are basically
force transducers, they are useful only when applied to superficial
arteries with solid bone backing.
