Theory Of Operation; Oximetry - Respironics 950 Instruction And Service Manual

Theory of Operation

Oximetry

The pulse oximeter shines red and infrared light through the tissue and
detects the fluctuating signals caused by arterial blood pulses. The
ratio of the fluctuation of the red and infrared light signals received
determines the oxygen saturation content. Conditions such as steady
venous blood flow, skin thickness, fingernail thickness, etc., do not
affect the saturation reading because they are constant and do not
cause fluctuations.
The function, f, depends on the physical properties of the LEDs
producing the light. These physical properties are fixed by the
manufacturing material and processes.
Note that the pulse oximeter readings do not depend upon the absolute
light intensity, rather upon the fluctuations in light intensity.
Characteristics such as thick skin or skin pigmentation will affect the
min and max proportionally, thus the ratio min/max does not
change. However, if too little light passes through, the pulse oximeter
will not display values. Pulse oximeters use two different wavelengths
of light (red and infrared), providing the ability to determine one
component of blood. The pulse oximeter is calibrated to closely
approximate functional oxygen saturation values. These values will
closely approximate laboratory instrument fractional saturation values
if the dysfunctional hemoglobin saturation levels are negligible.
In summary, because the pulse oximeter performs all computations
from the internal software and there are no critical parts to drift, no re-
calibration is needed.
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