•
Automatic Calibration
Because light absorption by hemoglobin is wavelength dependent and because the mean
wavelength of LEDs varies, a monitor must know the mean wavelength of the sensor’s red
LED to accurately measure SpO2. During manufacturing, the mean wavelength of the red
LED is encoded in a resistor in the sensor. During monitoring, the monitor reads this resistor
and selects coefficients that are appropriate for the wavelength of that sensor’s red LED;
these coefficients are then used to determine SpO2.
This resistor is read when the monitor is turned on, periodically thereafter, and each time a
new sensor is connected. Additionally, to compensate for differences in tissue thickness, the
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 carboxyhemoglobin or methemoglobin. In
contrast, some instruments 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:
Fractional saturation
Functional saturation = ×100
100 - (%carboxyhemoglobin + %methemoglobin)
•
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 monitor. 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 12-4): pH, temperature, the
partial pressure of carbon dioxide (PCO2), 2,3-DPG, and fetal hemoglobin.
12-9
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