46
Human NIBP Nano
Owner’s Guide
generated by the air compressor, and thus causes changes in the finger
cuff pressure.
The original block diagram of the method as developed by Peñáz is shown
in Figure A–3. The arrows indicate a closed servo control loop. The signal
V (PG) of the photoplethysmograph, consisting of a light source (L), and
photocell (PC) built in a cuff (S) around the finger (F), is compared to a fixed
setpoint value (C1). A diff erence between signal and setpoint, is amplified
by DA, PID and PA, and drives the proportional valve (EPT). Cuff pressure
is monitored by the manometer (M). If the switch SW is closed, the servo
feedback loop is opened, and a steady cuff pressure (C2) can reach the
finger.
PhysioCal algorithm
The PhysioCal algorithm was developed by the Dutch physicist K.H.
Wesseling and his colleagues at TNO-Biomedical Instrumentation, to
enable the conversion of Peñáz’s volume-clamp method into ‘Finapres’
(standing for FINger Arterial Pressure) [Wesseling et al., 1995]. Their most
important development was an automated servo setpoint adjuster, the
PhysioCal procedure, which permitted the determination and periodic
updating of the unloaded arterial diameter.
Unfortunately, defining the correct unloaded diameter of a finger artery
is not straightforward. Changes in hematocrit, stress and smooth muscle
tone of the arterial wall aff ect the unloaded diameter. Therefore, the
unloaded diameter is usually not constant during a measurement and
has to be verified at intervals. Periods of constant cuff pressure are used
to adjust the correct unloaded diameter of the finger artery based on
the signal from the plethysmograph in the finger cuff . A disadvantage of
this method is that the measurement of blood pressure is temporarily
interrupted during such a period. Therefore, the frequency and duration
Figure A–3
Block diagram of the
volume clamp method of
Peñáz.
