Nellcor N-200 - Measured Versus Calculated Saturation; Oxyhemoglobin Dissociation Curve

Principles of Operation
13
Measured versus Calculated Saturation
When saturation is calculated from a blood gas partial pressure
of oxygen (PO2), the calculated value may differ from the SpO2
measurement of a pulse oximeter. This usually occurs because
the calculated saturation was not appropriately corrected for
the effects of variables that shift the relationship between PO2
and saturation (Figure 1): pH, temperature, the partial
pressure of carbon dioxide (PCO2), 2,3-DPG, and fetal
hemoglobin.
0
10050
Saturation (%)
pH
Temperature
PCO
2
2,3-DPG
PO
2
(mmHg)
100
50
pH
Temperature
PCO
2
2,3-DPG
Fetal Hb
Figure 1: Oxyhemoglobin Dissociation Curve