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Functional versus Fractional Saturation
Measured versus Calculated Saturation
N-560 Operator's Manual
This N-560 measures functional saturation -- oxygenated hemoglobin
expressed as a percentage of the hemoglobin that can transport oxygen. It
does not detect significant amounts of dysfunctional hemoglobin, such as
carboxyhemoglobin or methemoglobin. In contrast, hemoximeters such
as the IL482 report fractional saturation -- oxygenated hemoglobin
expressed as a percentage of all measured hemoglobin, including
measured dysfunctional hemoglobins. To compare functional saturation
measurements to those from an instrument that measures fractional
saturation, fractional measurements must be converted as follows:
functional saturation =
When saturation is calculated from a blood gas partial pressure of oxygen
(PO
), the calculated value may differ from the SpO
2
pulse oximeter. This usually occurs because the calculated saturation was
not appropriately corrected for the effects of variables that shift the
relationship between PO
carbon dioxide (PCO
), 2,3-DPG, and fetal hemoglobin. See Figure 7.
2
Figure 7: Oxyhemoglobin Dissociation Curve
Principles of Operation
fractional saturation
100 - (% carboxyhemoglobin + % methemoglobin )
and pH, temperature, the partial pressure of
2
x 100
measurement of a
2
93
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