BT-550 Operation manual
43
P/N : 550-ENG-OPM-EUR-R01
Bistos Co., Ltd.
2018.03
- Operating Principles
The BT-550 MS board pulse oximeter is based on three principles:
1. Oxyhemoglobin and deoxyhemoglobin differ in their absorption of red and infrared light
(spectrophotometry).
2. The volume of arterial blood in tissue and the light absorbed by the blood changes during
the pulse (plethysmography).
3. Arterio-venous shunting is highly variable and that fluctuating absorbance by venous blood
is a major component of noise during the pulse.
The pulse oximeter of BT-550 as well as traditional pulse oximetry determines SpO
2
by passing red and
infrared light into a capillary bed and measuring changes in light absorption during the pulsatile cycle.
Red and infrared light-emitting diodes (LEDs) in oximetry sensors serve as the light sources, a
photodiode serves as the photodetector.
Traditional pulse oximetry assumes that all pulsations in the light absorbance signal are caused by
oscillations in the arterial blood volume. This assumes that the blood flow in the region of the sensor
passes entirely through the capillary bed rather than through any arterio-venous shunts. The
traditional pulse oximeter calculates the ratio of pulsatile absorbance (AC) to the mean absorbance(DC)
at each of two wavelengths, 660 nm and 905 nm:
S(660) = AC(660) / DC(660)
S(905) = AC(905) / DC(905)
The oximeter then calculates the ratio of these two arterial pulse-added absorbance signals:
R = S(660) / S(905)
This value of R is used to find the saturation SpO
2
in a look-up table built into the oximeter’s software.
The values in the look-up table are based upon human blood studies against a laboratory co-oximeter
on healthy adult volunteers in induced hypoxia studies.
The BT-550 MS board pulse oximeter assumes that arterio-venous shunting is highly variable and that
fluctuating absorbance by venous blood is the major components of noise during the pulse. BT-550 MS
board decomposes S(660) and S(905) into an arterial signal plus a noise component and calculates the
ratio of the arterial signals without the noise:
S(660) = S1 + N1
S(905) = S2 + N2
R = S1 / S2
Again, R is the ratio of two arterial pulse-added absorbance signals and its value is used to find the
saturation SpO
2
in an empirically derived equation into the oximeter’s software. The values in the
empirically derived equation are based upon human blood studies against a laboratory co-oximeter on
healthy adult volunteers in induced hypoxia studies.
