getemed VitaGuard VG310, Руководство по эксплуатации, Страница: 105 / 118

Algorithms and measuring principles 105
The SET® software goes through all the possible values for R (corre-
sponding to SpO
2
between 1 and 100%) and calculates the associated
interference components. An adaptive noise canceller, or ANC, then
takes this value N’(R) to calculate the amplitude of the noise energy,
or the so-called output power of the ANC. The result is a DST™ plot
(Fig. 78) that exhibits at least the arterial peak. This peak demon-
strates the particularly effective noise suppression for the affected
SpO
2
value when a precisely defined source of signal fluctuations, the
arterial pulse cycle, has been identified. Other, also higher peaks can
occur during venous fluctuations. Venous blood is saturated less with
oxygen, so the peak with the maximum SpO
2
value (in the right half
of the graph) always cor-
responds to the arterial
oxygen saturation. In
Fig. 78 the right peak cor-
responds to an SpO
2
of
97%. The DST calculation
is repeated every two
seconds on the latest raw
data over the preceding
four seconds.
Fig. 78 DST™ plot: relative noise cancellation as a function of SpO
2
The peak at 80% is caused by venous blood and would, with the con-
ventional method, have completely corrupted the measurement for a
false desaturation.
The
Perfusion
index (a percentage value) displayed on the monitor is
calculated from the electric signal of infrared light captured by the
sensor’s photodiode. This signal has a constant and a varying compo-
nent depending on whether the infrared light is absorbed by bone,
connective tissue, skin, or pulsatile blood.
The ratio of constant to variable signal component is used to calcu-
late the perfusion index, a value between 0 and 20%. When the
Perfusion
index is very small, SpO
2
and the pulse rate are no longer
monitored. When the
Sensitivity
in the SpO
2
menu is set to
Maxi-