Note: The SDMS measures and displays functional oxygen
saturation: the amount of oxygenated hemoglobin expressed
as a percentage of the hemoglobin that can transport oxygen.
The SDMS does not measure fractional saturation: oxygenated
hemoglobin expressed as a percentage of all hemoglobin, in-
cluding dysfunctional hemoglobin such as carboxyhemoglobin
or methemoglobin.
Good to know!
Oxygen saturation measurement techniques – including pulse
oximetry – are not able to detect hyperoxemia.
Due to the S-shape of the oxyhemoglobin dissociation curve
(ODC) SpO
2
alone cannot reliably detect hypoventilation in
patients being administered with supplemental oxygen.
Limitations of Pulse Oximetry
The following clinical situations or factors may limit the
correlation between functional oxygen saturation (SpO
2
) and
arterial oxygen saturation (SaO
2
) and may cause the loss of
the pulse signal:
• dysfunctional hemoglobins (COHb, MetHb)
• anemia
• intravascular dyes, such as indocyanine green or methylene
blue
• low perfusion at the measurement site (e.g. caused by inflated
blood pressure cuff, severe hypotension, vasoconstriction in
response to hypothermia, medication, or a spell of Rynaud’s
syndrome)
• venous pulsations (e.g. due to use of the forehead, cheek
or earlobe as a measurement site on a patient in steep
Trendelenburg position)
• certain cardiovascular pathologies
• skin pigmentation
• externally applied coloring agents
(e.g. nail polish, dye, pigmented cream)
• prolonged and/or excessive patient movement
• exposure of the sensor to high ambient light levels
• defibrillation
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