Functional Versus Fractional Saturation; Measured Versus Calculated Saturation - Nellcor OxiMax N-600x Service Manual

Technical Discussion

Functional versus Fractional Saturation

Measured versus Calculated Saturation

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The pulse oximeter 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 =
100 - (% carboxyhemoglobin + % methemoglobin )
When saturation is calculated from a blood gas partial pressure of oxygen (PO
the calculated value may differ from the %SpO
This usually occurs when the calculated saturation was not appropriately
corrected for the effects of variables that shift the relationship between PO
pH, temperature, the partial pressure of carbon dioxide (PCO
fetal hemoglobin. See Figure 21.
pH
100
Temperature
PCO
2
2,3-DPG
Fetal Hb
50
0
Figure 21: Oxyhemoglobin Dissociation Curve
fractional saturation
measurement of a pulse oximeter.
2
pH
Temperature
PCO
2
2,3-DPG
50
PO (mmHg)
2
x 100
and
2
), 2, 3-DPG, and
2
100
),
2