44
Human NIBP Nano
Owner’s Guide
•
When cuff pressure is below mean blood pressure, the arterial
diameter is relatively large and therefore the plethysmographic
signal is low. Since compliance is low in this state the amplitude
of the pulsation is small and the plethysmogram only varies
significantly during diastole.
•
When cuff pressure is above mean blood pressure, the artery
is collapsed during a substantial part of every heart beat. The
plethysmographic signal is high (no absorption of light by arterial
blood), and only varies significantly during systole.
•
When the cuff pressure level equals the average intra-arterial
pressure, the plethysmogram resembles an inverted pressure
waveform. [
Note:
The end-diastolic period of the plethysmographic
waveform is analyzed during an AutoCal.]
•
When the top of the plethysmographic signal at end-diastole is
too sharp, mean cuff pressure is below mean arterial pressure.
When it is too flat, cuff pressure is too high. The shape of the
plethysmogram of course also depends on the state of the artery,
which depends on the smooth muscle tone (see also Figure A–2).
[During an AutoCal the interpretation ‘too sharp’ or ‘too flat’ always
has to be verified by analyzing the plethysmogram at one or more
other pressure levels.]
The pressure-diameter relationship
The relationship between changes in arterial diameter and changes in
intra-arterial pressure depends on the mechanical properties of the
artery. When the artery is very compliant (low rigidity), diameter changes
are relatively large, whereas diameter changes in stiff arteries are small. It
is a well known fact that arterial compliance depends on the transmural
pressure, i.e., the pressure diff erence between the pressure inside the
artery and the pressure of the surrounding tissue [Langewouters 1984].
At high transmural pressures, when cuff pressure is low, the arterial
diameter is relatively large. Therefore, the artery is distended and
becomes stiff , causing small diameter changes. When transmural pressure
is low, i.e., when cuff pressure is high with respect to mean blood pressure,
the artery is almost collapsed, and diameter changes will be also small.
At near zero transmural pressure, i.e., when the pressure inside the artery
equals the pressure in the finger cuff , diameter changes are largest. This
observation was first published by Marey in 1876 [Marey 1876]. Figure A–2
shows a stylized pressure-diameter (p-d) plot of a finger artery measured
in vitro.
Near zero transmural pressure, the p-d curve is steepest. Therefore, for a
given pressure change, the diameter change will be largest.
