47
Human NIBP Nano
Owner’s Guide
of the periods of interruption are kept to a minimum. In the Human
NIBP soft ware, the periodic interruption of a finger blood pressure
measurement with constant cuff pressure levels to apply the PhysioCal
algorithm is simply referred to as an Auto calibration or ‘AutoCal’.
It is obvious from the previous sections that the unloaded diameter is
close to the average diameter at a pressure level where the amplitude of
the pulsations in the plethysmogram is largest. However, using only the
amplitude of the plethysmogram is relatively inaccurate. The PhysioCal
algorithm not only uses the amplitude, but also interprets the shape of
the plethysmogram during periods of constant cuff pressure. By analyzing
the plethysmogram at two or more pressure levels, it explores part of the
pressure-diameter relation and is able to track changes to the unloaded
diameter of a finger artery due to changes in smooth muscle tone.
The procedure at the start of a measurement is shown in Figure A–4. The
cuff pressure is increased in standard increments to a maximum level until
one heart beat is detected per level, and then decreased. The signal in the
plethysmogram increases to a maximum and then decreases. The cuff
pressure corresponding to the maximum signal in the plethysmogram,
combined with criteria to exclude venous pulsations, produces a good
indication that the finger artery is in an unloaded state. This cuff pressure is
assumed to correspond to the mean arterial pressure. Once the unloaded
state is established the PhysioCal procedure is carried out.
1
2
3
2 seconds
AutoCal
Start-up
AutoCal
C
uff P
ressur
e
(mmHg)
Pleth
y
smog
ram (DC
)
(V)
S
etpoint
(V)
200
100
0
1
0
0
1
The steps below correspond to the numbers in Figure A–4 above.
Figure A–4
PhysioCal Start-
up procedure and
ongoing adjustments.
The Plethysmogram
trace (middle) shows
more transmitted light
plotted upwards. The
Setpoint trace (bottom)
corresponds to the
total amount of light
transmitted through the
fi nger.
