System Processing Description
ARGUS VCM
SpO
2
Processing
Overview
Pulse oximetry works by applying a sensor to a pulsating arteriolar vascular bed. The
sensor contains a dual light source and photodetector. Bone, tissue, pigmentation, and
venous vessels normally absorb a constant amount of light over time. The arteriolar bed
normally pulsates and absorbs variable amounts of light during systole and diastole, as
blood volume increases and decreases. The ratio of light absorbed at systole and diastole
is translated into an oxygen saturation measurement (SpO
2)
. Because a measurement of
SpO
2
is dependent upon light from the sensor, excessive ambient light can interfere with
this measurement.
Pulse oximetry is based on two principles: that oxyhemoglobin and deoxyhemoglobin
differ in their absorption of red and infrared light (spectrophotometry), and that the
volume of arterial blood in tissue (and hence, light absorption by that blood) changes
during the pulse (plethysmography). The monitor determines SpO
2
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 (LED) in the
oximetry
OX
I
MAX
sensor serve as light sources; a photo diode serves as the photo
detector. Because oxyhemoglobin 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 arterial hemoglobin, the monitor uses
the pulsatile nature of arterial flow. During 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 SpO
2
measurements on the difference between maximum and minimum absorption
(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 Calibration
Because light absorption by hemoglobin is wavelength dependent and because the mean
wavelength of LEDs varies, an oximeter must know the mean wavelength of the
OXIMAX sensor’s red LED to accurately measure SpO
2
. During the monitoring, the
instrument’s software selects coefficients that are appropriate for the wavelength of that
individual sensor’s red LED; these coefficients are then used to determine SpO
2
.
Additionally, to compensate for differences in tissue thickness, the light intensity of the
sensor’s LEDs is adjusted automatically.
Measured versus Calculated Saturation
The measured SpO
2
value from an oximeter may differ from the saturation value that is
calculated from a blood gas partial pressure of oxygen (P
O2
). This usually occurs
because the calculated saturation was not appropriately corrected for the effects of
variables that shift the relationship between P
O2
and saturation: pH, temperature,
partial pressure of carbon dioxide (PC
O2
), 2, 3-DPG, and fetal hemoglobin.
68
ARGUS VCM
Service Manual
