ADInstruments Human NIBP Nano Owner's Manual page 56

In the case of high frequency oscillations the servo system loop gain is
automatically reduced until oscillations stop, and an extra servo controller
filter may be switched in. An AutoCal is scheduled aft er two beats to
confirm the setpoint and servo-system loop gain. Aft er oscillations stop
the loop gain is brought back up if necessary to guarantee suff icient gain
for a proper waveform measurement. This 'gain-up' process may take
several heart beats to complete.
The number of beats between AutoCals gets
lower if, during an AutoCal, the Human NIBP
Nano didn't get a good signal
The AutoCal frequency follows a fixed pattern unless there are problems
getting a good signal. The standard number of beats between AutoCals
generally follows the pattern 10, 10, 10, 20, 30, 40, 50, 60, 70 and then stays
at 70 beats thereaft er. Occasionally, if there is a temporary problem in
the signal, the counter may go down by 2 or 3 beats during the interval
before the next AutoCal. Aft er an AutoCal countdown, the next AutoCal is
performed and the result of this is used as adjustment for the following
countdown.
Hence, if stability of the setpoint cannot be confirmed – for example, if
there are movement artifacts or if the change in setpoint is considered too
large – the frequency of AutoCal will increase automatically.
A note on blue fi ngertips
During a normal finger blood pressure measurement some coloring of
the finger tip distal from the finger cuff is observed. The arteries below
the finger cuff are unloaded, that is, the arterial walls are held at zero
transmural pressure and they are therefore at their unstressed diameter
(see also Figure A–4). Unstressed arteries still have about 60% of their
original distended diameter or approximately 1/3 or 1/2 their original
cross-sectional area and volume. The other blood vessels below the cuff
are collapsed. Therefore, all the vessels behind the cuff , in the finger tip,
are at instantaneous arterial pressure including the micro-circulation
and the veins. Thus blood entering the finger tip during systole can only
leave during diastole, when the pressure in the vessels of the finger tip is
higher than the pressure more proximal from the cuff . This causes some
congestion.
During a blood pressure measurement, the blood in the finger tip is less
eff iciently refreshed. The reduced washout of metabolites and increased
de-oxygenation of the venous blood causes blue finger tips. Note,
however, that the PhysioCal algorithm always checks that the artery is
suff iciently open during a measurement. Inflow and outflow from the
51
Human NIBP Nano Owner's Guide