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Technical Information
rSO
2
Principles of Operation
Model X-100SP signal processor uses calculations based on the Beer-Lambert
law or Beer’s law, to determine cerebral and tissue oxygenation. The Beer-
Lambert law relates the absorption of light to the properties of the material
through which the light is traveling. The law states that there is a logarithmic
relationship between the concentration of compounds and the transmission of
light through it. By utilizing wavelengths of light that are absorbed by the
compounds to be measured, the concentration of the compounds can be
determined. For cerebral and tissue oximetry, the compounds of interest are
hemoglobin, deoxygenated hemoglobin, and tissue.
The oximetry sensors use a proprietary, patented arrangement of light emitters
(LEDs) and light detectors (photodiodes). This arrangement effectively provides
a “deep tissue” absorption measurement focused on the cerebrum. The
absorption measurement is largely unaffected by surface or near-surface
features, irregularities, or substances.
rSO
2
Accuracy Testing
At an independent research laboratory, rSO
2
accuracy testing was conducted
during induced hypoxia studies on healthy, non-smoking, light- to dark-skinned
subjects that were 18 years of age and older. The measured cerebral and tissue
hemoglobin saturation value (rSO
2
) of the sensors was compared to arterial/
venous hemoglobin oxygen (SavO
2
) value, determined from venous and arterial
blood samples. The model used for blood in the brain was 70% venous and 30%
arterial, which is applicable under normocapnic conditions. The venous blood
was drawn from the right jugular bulb. The accuracy of the sensors in
comparison to the blood gas analyzer samples measured over the rSO
2
range
of 45 – 100%. Accuracy data was calculated using the root-mean-squared
(A
rms
value) for all subjects, per ISO 80601-2-61, Medical Electrical
Equipment—Particular requirements for basic safety and essential performance
of pulse oximeter equipment.
