Nellcor N-395 - Principles of Operation; Oximetry Overview; Automatic Calibration

PRIN CIPLES OF OPERA TI0 N
Oximetry Overview
OXIMETRY OVERVIEW
Pulse oxiinetry is based on two principles: that oxyhemoglobii~
and deoxyheinoglobin differ in their absorption of red and
infrared light (i . e., spectropliotometry), and that the volume of
arterial blood in tissue (and hence, light absorption by that blood)
changes during the pulse
(i.e., pletliysmograpliy). A pulse
oximeter determines
Sp02 by passing red and infrared light into
an arteriolar bed and measuring changes in light absorption
during the pulsatile cycle. Red and infrared low-voltage
light-
emitting diodes (LEDs) in the oximetry sensor serve as light
sources; a photodiode serves as the photodetector.
Because
oxyhemoglobii~ 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
artevial hemoglobin, the monitor uses the
pulsatile nature of arterial flow.
Duriiig 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 monitor bases its
Sp02
ineasurements on the difference between maximum and minimum
absorption
(i.e., measurements at systole and diastole). By doing
so, it focuses on light absorption by pulsatile arterial blood,
eliminating the effects of nonpulsatile absorbers such as tissue,
bone, and venous blood.
Automatic Cali bration
Because light absorption by hemoglobin is wavelength dependent
and because the
mean wavelength of LEDs varies, an oximeter
must
hiow the mean wavelength of the sensor's red LED to
accurately
measure Sp02. During manufacturing, the mean
wavelength of the red LED is encoded in a resistor in the sensor.