Principles Of Pulse Oximetry; Introduction; Automatic Calibration; Functional Versus Fractional Saturation - Nellcor N-180 Service Manual

SECTION 2: PRINCIPLES OF PULSE OXIMETRY
Introduction
Automatic Calibration
Functional Versus Fractional Saturation
Measured Versus Calculated Saturation
INTRODUCTION
Pulse oximetry is based on two principles: that oxyhemoglobin and deoxyhe-
moglobin differ in their absorption of red and infrared light, and that the vol-
ume of arterial blood in tissue (and hence, light absorption by that blood)
changes during the pulsatile cycle. A pulse oximeter measures the oxygen
saturation of arterial hemoglobin (SpO2) by passing red and infrared light
into an arteriolar bed and determining light absorption. Two low-power light-
emitting diodes (LEDs) in the oximetry sensor serve as light sources (red,
nominally 660 nm; infrared, nominally 920 nm); a photodiode serves as the
photo detector.
Because oxyhemoglobin and deoxyhemoglobin differ in spectral absorption,
the relative absorption of red and infrared light by blood reflects hemoglobin
oxygen saturation. To identify arterial hemoglobin oxygen saturation, the
monitor relies on the pulsatile nature of arterial flow. At systole—as a new
pulse of arterial blood enters the vascular bed—blood volume, and hence light
absorption, increases. During diastole, blood volume and light absorption
'reach their lowest point. To measure SpO2, the monitor uses the difference
between minimum and maximum absorption of red and infrared light.
Accordingly, the effects of nonpulsatile absorbers (that is, tissue, bone, venous
blood) are eliminated. Red absorption and infrared absorption that are at-
. tributable to the pulsatile arterial blood are thus made distinct. .
AUTOMATIC CALIBRATION
Because light absorption by hemoglobin depends upon wavelength and the
mean wavelength of LEDs vary, an oximeter must store the mean wavelength
of the sensor red LED to accurately measure SpO2. (The mean wavelength of
the infrared LED is less crucial because the hemoglobin absorption curves are
relatively flat in the portion of the infrared spectrum used.) Patented mech-
anisms automatically calibrate the monitor for the sensor red wavelength
when the monitor is turned on. Calibration occurs periodically thereafter and
each time a new sensor is connected. During manufacturing, the mean
wavelength of the red LED is encoded in a resistor in the sensor. The monitor
software reads this resistor and selects appropriate calibration coefficients for
that sensor.
FUNCTIONAL VERSUS FRACTIONAL SATURATION
This monitor measures functional SpO2—oxygenated hemoglobin expressed
as a percentage of the hemoglobin that is capable of transporting oxygen.
Significant amounts of dysfunctional hemoglobin, such as carboxyhemoglobin
or methemoglobin are not detected. İn contrast, hemoximeters such as the II,
282 report fractional saturation—oxygenated hemoglobin expressed as a per-
centage of all measured hemoglobin, including measured dysfunctional
hemoglobins. To compare functional SpO2 measurements to those from an in
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