5
IV. THEORY
OF
OPERATION
The principle of differential light absorption is used by a pulse oximeter to determine the oxygen saturation of
arterial blood (SpO
2
). Red light and infrared light are differentially absorbed by oxygenated and deoxygenated
hemoglobin. The pulse oximeter has a sensor with light emitting diodes (LEDs) that provides these wavelengths of
light for transmittance through a measurement site, usually a finger. Based on the relative absorption of these two
wavelengths of light at the measurement site, the pulse oximeter determines the relative amount of oxygenated and
deoxygenated hemoglobin, which is calculated as SpO
2
.
In order to make this calculation independent of skin color, finger size, etc., the pulse oximeter uses only the time
varying light absorption component generated by the patient’s pulse. In addition, the pulse oximeter uses the period
of pulsation to measure the pulse rate.
The FingerSim™ absorbs light very much like a human finger. The overall red and infrared light absorption of the
FingerSim™ approximates the overall light absorption of a typical finger. In addition, the red and infrared photo
spectrometric light absorption of the inner solution approximates arterial blood as seen by the oximeter at 80%, 90%
and 97% oxygen saturation levels (see Figures 3, 4, and 5).
Minor SpO
2
variations will be seen between oximeter manufacturers because standards correlating red and infrared
light absorption to oxygen saturation in pulse oximetry are not available. Each manufacturer has developed its own
correlation and inevitably some differences have developed (see “Health Devices” June 1989). In addition, minor
SpO
2
variations between sensors will be observed due to the fact that red and infrared emitting light sources vary
slightly between sensors.
The FingerSim™ enables the healthcare professional to repeatedly test and evaluate the pulse oximeter system
(oximeter and sensor) under controlled light absorption conditions.
The time varying light absorption component required by a pulse oximeter is created in the FingerSim™ by
rhythmically pressing the color coded end. This creates a volume change in the distal (sensor) end of the
FingerSim™, analogous to the heart creating blood pressure waves that force blood into the finger. The amplitude
and rate of the pulse wave can be varied by changing the applied pressure and interval.
