Description
Functional oxygen saturation (SpO
2
)
175
The diagram above shows an example of the light
absorbed by the blood at 660 nm (red) and 920 nm
(infrared).
At 660 nm, the absorption and corresponding pulse
amplitude decrease with increasing O
2
saturation, but
rise at 920 nm. Since the absorption coefficients of
HbO
2
and Hb are known for both wavelengths, the
system can calculate how much of these two haemo-
globins is present. The quotient obtained by dividing the
oxygenated haemoglobin (HbO
2
) by the reduced and
oxygenated haemoglobin (Hb + HbO
2
) is known as the
functional saturation:
% SpO
2
(func) = 100 x
and refers to the haemoglobin capable of transporting
oxygen.
Dyshaemoglobins, HbCO and MetHb are normally
negligible, but may affect the accuracy of the measure-
ment.
Temperature measurement
Measuring principle
Temperature-dependent change in resistance of an
NTC resistor (NTC = negative temperature coefficient)
with linearization circuit.
HbO
2
HbO
2
+ Hb
Pressure measurement
Measuring principle
Piezoresistive change of resistance in a membrane.
Determination of PEEP and plateau pressure
PEEP (positive end-expiratory pressure) is the airway
pressure at the end of expiration.
The plateau pressure is the airway pressure measured
16 milliseconds before expiration begins.
O
2
measurement
Measuring principle of the galvanic cell
The O
2
sensor is based on the principle of a galvanic
cell.
Oxygen molecules from the gas mixture to be measured
diffuse through a plastic membrane into the electro-
chemical cell and are reduced on precious metal
electrodes.
A base electrode is oxidized at the same time. It is
depleted by the oxidation process and essentially
determines the service life of the sensor. The current
flowing through the cell is proportional to the partial
oxygen pressure in the gas mixture to be measured.
At constant pressure and constant temperature of the
gas mixture to be measured, the measured value is
directly proportional to the partial oxygen pressure.
Pulse amplitude
Time (s)
at 660 nm red
O
2
saturation
100% O
2
saturation
at 920 nm infrared
Pulse amplitude
Time (s)
O
2
ϑ
-
Plastic membrane
Plastic housing
Au cathode 1
Au cathode 2
Thermistors
Pb anode
Basic
electrolyte
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