Functional Versus Fractional Saturation; Measured Versus Calculated Saturation - Nellcor N-395 Operator's Manual

During monitoring, the instrument9 s software reads this resistor
value and selects coefficients that are appropriate for the
wavelength of that individual sensor's red LED; these
coefficients are then used to determine SpOz. This resistor value
is read when the inonitor is turned on, periodically thereafter, and
each time a new sensor is connected.
Additionally, to compensate for differences in tissue thickness,
the light intensity of the sensor's LEDs is adjusted automatically.
Functional versus Fractional Saturation
This monitor 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 carboxyheinoglobin or
methemoglobin. In contrast, hernoximeters such as the IL482
report fractional saturation
-
oxygenated hemoglobin expressed
as a percentage of all measured hemoglobin, including measured
dysfunctioilal hemoglobins. To compare functioilal saturation
measureineizts to those from an instrument that measures
fractional saturation, fractional measurements must be converted
as follows:
fractional saturation
functional saturation
=
x
I00
100
-
(% carboxyhemoglobin + % methemoglobin)
Measured versus Calculated Saturation
When saturation is calculated froin a blood gas partial pressure of
oxygen (P02), the calculated value may differ from the SpOz
ineasurement 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 2 1): pH, temperature, the partial pressure of
carbon dioxide (PC02), 2,3-DPG, and fetal hemoglobin.