voltage light-emitting diodes (LED) serve as light sources; a photonic diode serves as the
photo detector.
Because oxyhemoglobin and deoxyhemoglobin differ in light absorption, the amount of red
and infrared light absorbed by blood is related to hemoglobin oxygen saturation. To
identify the oxygen saturation of arterial hemoglobin, the oximeter uses the pulsatile nature
of arterial flow.
During systole, a new pulse of arterial blood enters the vascular bed, and blood volume
and light absorption increase. During diastole, blood volume and light absorption reach
their lowest point.
The oximeter bases its SpO
2
measurements on the difference between maximum and
minimum absorption (measurements at systole and diastole). By doing so, it focuses on
light absorption by pulsatile arterial blood, eliminating the effects of non-pulsatile absorbs
such as tissue, bone and venous blood.
Wavelength
The sensor contains LEDs that emit red light at a wavelength of approximately 660 nm and
infrared light at a wavelength of approximately 900 nm.
7.2 Functional Versus Fractional Saturation
This oximeter 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, hemoximeter such as the IL482 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- ( %Carboxyhem %Methemoglobin)
7.3 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 pulse oximeter. This usually
occurs because the calculated saturation was not appropriately corrected for the effects of
variables that shift the relationship between PO
2
and pH, the partial pressure of carbon
dioxide (PCO
2
), 2,3-DPG, and fetal hemoglobin.
Summary of Contents for VITAL TEST
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