11-18
îš„ 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 SpO
2
. 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 SpO
2
.
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.
îš„ 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 - (%carboxyhemoglobin + %methemoglobin)
×100
îš„ Measured versus Calculated Saturation
When saturation is calculated from a blood gas partial pressure of oxygen (PO
2
), the calculated
value may differ from the SpO
2
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 PO
2
and saturation (Figure 11-8): pH, temperature, the partial pressure of
carbon dioxide (PCO
2
), 2,3-DPG, and fetal hemoglobin.
Figure 11-8 Oxyhemoglobin Dissociation Curve
Saturation (%)
SpO
2
%
Temperature
PCO2
2,3-DPG
PH
Temperature
PCO2
2,3-DPG
Fetal Hb
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