Theory Of Operations; Pulse Oximetry - GE Marquette 418793-001 Service Manual

Theory of Operations

Pulse Oximetry

Revision D
OXIMETER ASSEMBLY: Theory of Operations
APEX pulse oximeters shine light (red and infrared) through perfused
tissue and detect the fluctuating signals caused by arterial blood
pressure pulses. Well-oxygenated blood is bright red, while poorly
oxygenated blood is dark red. The pulse oximeter determines functional
oxygen saturation of arterial hemoglobin from this color difference by
measuring the ratio of absorbed red and infrared light as the blood
volume fluctuates with each heart beat. Since steady conditions (steady
venous blood flow, skin thickness, bone, finger nails, etc.) do not cause
fluctuations, they do not affect the saturation reading.
Mathematically:
Anything that affects the intensity of the light such as thick or colored
skin affects the maximum and minimum proportionally and thus the
ratio minimum/maximum does not change. However, if too little light
gets through, the pulse oximeter does not function.
APEX pulse oximeters use two different wavelengths of light (colors) and
thus have the ability to determine one component of blood. APEX pulse
oximeters are calibrated to closely approximate functional oxygen
saturation values. APEX pulse oximeter oxygen saturation values will
closely approximate laboratory instrument fractional saturation values if
the dysfunctional hemoglobin saturation levels are negligible. If the
dysfunctional hemoglobin is carboxyhemoglobin or methemoglobin, then
the difference between the oxygen saturation value displayed by the
APEX pulse oximeter and the oxygen saturation values determined by
the laboratory instrument are greater as the dysfunctional hemoglobin
levels rise approximately in accordance with the following formulas:
1. SpO2 = O2Hb + COHb + MetHb
2. SaO2 = 100 x O2Hb /(100 - COHb - MetHb)
Where:
a. SpO2 = APEX pulse oximeter determined and displayed oxygen
saturation in percent
b. O2Hb = Fractional oxyhemoglobin saturation in percent
c. COHb = Carboxyhemoglobin saturation in percent
d. MetHb = Methemoglobin saturation in percent
e. SaO2 = Functional oxygen saturation in percent
APEX Telemetry Transmitter
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