Mindray PM-9000, Operation Manual

Page: 208 / 378

SpO
2
Monitoring
12-24
absorption (i.e., measurements at systole and diastole). By doing so, it focuses on
light absorption by pulsatile arterial blood, eliminating the effects of nonpulsatile
absorbers such as tissue, bone, and venous blood.
„ 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.
„ 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 - (%carboxyhemo %methemoglobin)
×100
„ 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
12-8): pH, temperature, the partial pressure of carbon dioxide (PCO
2
), 2,3-DPG, and
fetal hemoglobin.