Welch Allyn PROPAQ CS - Manual (Page 127)

Service Manual Technical overview 123
Pulse oximetry option (SpO
2
) description
The monitor pulse oximetry option (SpO
2
) hardware includes two circuit boards, a side-
panel connector, and related hardware. The pulse oximetry option is installed in the pulse
oximetry option module attached to the rear of the monitor (also known as the Baqpaq) or
the expansion module when the printer or the CO
2
option are also present.
The two circuit boards include the SpO
2
Communications Processor (SCP Board) and the
SpO
2
Board. The SpO
2
Board provides all necessary circuitry to receive and process
oxygen saturation data from the oxygen sensor. The SCP Board prepares the data for
processing by the monitor's processor, and it handles all communications with the
monitor. The SCP Board also drives the SpO
2
speaker. The MSP Board replaces the SCP
Board in SpO
2
applications with CO
2
.
The SpO
2
option hardware consists of the following items:
• monitor/option interface circuit board (SCP Board)
• SpO
2
Board
• side-panel connector
• SpO
2
speaker
• the SpO
2
case attached to the rear of the monitor when no expansion module is
present
• when a printer is present, the expansion module case attached to the bottom of the
monitor
The SpO
2
signal provided by the oxygen transducer or sensor is first processed by the
SpO
2
Board where it is converted to digital data, then that data is passed to the SCP
Board where it is prepared for transmission to the monitor's main microprocessor.
An SpO
2
option isolated power supply, powered by monitor batteries, resides on the SCP
Board, supplying power to the SpO
2
circuits only.
A connector on the side panel allows connection of an oxygen transducer directly to the
monitor.
The oxygen transducer senses oxygen content of functional arteriolar hemoglobin through
the use of light passed through the sensor site. The reflective/absorptive characteristics
of hemoglobin at the wavelengths used allow the pulse oximetry circuits to obtain
changing saturation levels, which can be processed to obtain the oxygen saturation
percentage.
Wavelengths in the red and the infrared regions of light are produced by light emitting
diodes (LEDs) manufactured to emit light at the required wavelengths. The reflected light
is detected by a photo diode, which converts the light to an electrical signal. The LEDs are
switched on and off under control of the SpO
2
microprocessor. Both are not on at the
same time. As the system receives the signal from the photo diode, it knows whether the
signal was derived from the red or infrared LED. Because the photo diode is sensitive to
light emitted from sources other than the LEDs (ambient light), an ambient light
component is subtracted from the signal during the processing of the SpO
2
data.
The figure on page 125 shows a functional block diagram of a typical SpO
2
Board. The
SpO
2
Board consists of the following subsystems:
Note The oxygen transducers are not serviceable units.