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.
Intended Use, Principles of Operation and Limitations
When comparing PCO
2
/PO
2
values displayed by the SDM with
PaCO
2
/PaO
2
values obtained from arterial blood gas (ABG)
analysis, pay attention to the following points:
• Carefully draw and handle blood samples.
• Blood sampling should be performed in steady state
conditions.
• The PaCO
2
/PaO
2
value obtained from ABG analysis should be
compared to the SDM’s PCO
2
/PO
2
reading at the time of blood
sampling.
• In patients with functional shunts, the sensor application site
and the arterial sampling site should be on the same side of
the shunt.
• If the menu-parameter ‘Severinghaus Correction Mode’
is set to ‘Auto’, the PCO
2
values displayed by the SDM are
automatically corrected to 37 °C (regardless of the patient’s
core temperature). When performing the ABG analysis, be
sure to properly enter the patient’s core temperature into the
blood gas analyzer. Use the blood gas analyzer’s ‘37 °C-PaCO
2
’
value to compare with the SDM’s PCO
2
value.
• Verify proper operation of the blood gas analyzer. Periodically
compare the blood gas analyzer’s barometric pressure against
a known calibrated reference barometer.
Pulse Oximetry
Principles of Operations of Pulse Oximetry
The SDMS uses pulse oximetry to measure functional oxygen
saturation (SpO
2
) and pulse rate (PR). Pulse oximetry
is based on two principles: firstly, oxyhemoglobin and
deoxyhemoglobin differ in their absorption of red and infrared
light (spectrophotometry) and secondly, the volume of arterial
blood in tissue (and hence, light absorption by that blood)
changes during the pulse (plethysmography).
Pulse oximeter sensors pass red and infrared light into a
pulsating arteriolar vascular bed and measure changes in light
absorption during the pulsatile cycle. Red and infrared low-
voltage light-emitting diodes (LED) serve as light sources and
a photodiode serves as photodetector. The software of a pulse
oximeter uses the ratio of absorbed red to infrared light to
calculate SpO
2
.
Pulse oximeters use the pulsatile nature of arterial blood flow
to differentiate the oxygen saturation of hemoglobin in arterial
blood from the one in venous blood or tissue. During systole,
a new pulse of arterial blood enters the vascular bed: blood
volume and light absorption increase. During diastole, blood
volume and light absorption decrease. By focusing on the
pulsatile light signals, effects of nonpulsatile absorbers such as
tissue, bone and venous blood are eliminated.
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