Accutorr Plus Service Manual - Mindray

2.2.5 SpO2 Module: (Accutorr Plus Model with Datascope SpO2 only)
General Theory of SpO2 determination: A pulsatile arterial saturated oxygen monitor
detects the oxygenation level of the blood in the body’s arterial circulation. It is used to
continuously monitor the effectiveness of the pulmonary system (lungs). Specifically the
device, within limits, determines the fraction of hemoglobin molecules carrying oxygen
from the lungs to the body cells. Termed % SpO2, this fraction is normally about 97 percent.
The device measures the relative attenuation of two specific wavelengths of light (red and
infrared) by the arterial blood. A sensor from the instrument contains two sets of LED’s
to illuminate a portion of the body (e.g., a fingertip), and a single photo-detector to sense
the amount of light which exits. The two sets of LED’s are alternately pulsed so that the
circuitry can discriminate the infrared light. Each time the heart pulses arterial blood
into the finger, the photo-detector’s signals return to their original level. The electronic
instrumentation processes only this changing portion of the photo-detector’s outputs.
Thus, arterial blood (not skin, bone, venous blood, etc.).
The determination is based upon the assumption that hemoglobin and oxyhemoglobin are
the only two significant attenuators of light in the arterial blood. The device exploits the
difference in their optical attenuation characteristics. But since the detector’s signal is sensitive
to the combined attenuation of both molecules, the device must use two different wavelengths
to discriminate their individual contributions, and thus their relative concentration.
The SpO2 Module allows the Accutorr Plus Model with SpO2 to measure patient’s blood
stream saturated pulsatile oxygen level and pulse rate.
Datascope SpO2 Module Theory of Operation, Electrical Description
The SpO2 board consists of analog and digital sections. The analog section provides all
the patient finger probe excitation and analog signal processing. The digital section
controls the operation of the analog part of the board and provides communication to the
rest of the Accutorr Plus with SpO2 via the U14 DUART.
Analog Section
The operation of the analog section is controlled by the digital part of the board by:
Supplying CLKSEQ clock signal to the sequencer.
Latching signals D0..7, into control register with strobe signals
CONTROL-REG0*, CONTROL-REG1*, and CONTROL-REG2*.
Monitoring COMP* and SAT* signals.
The sequencer controls the front end of analog section. It provides RD-DRV and IR-DRV
signals for the LED drive circuit, which in turn alternately drive red and infrared LED
emitters in the probe. A single photo-detector on the opposite side of the finger
produces current pulses proportional to the amount of light received. The sequencer also
sets the gain of the current to voltage pre-amp and controls the demultiplexer. Gain
values for current to voltage pre-amp are derived from control register signals IR0, IR1,
RD0, and RD1. The fact that the sequencer has synchronous control of LED drive, gain
of current to voltage pre-amp, and demultiplexer makes it possible to set different
current to voltage gain values for infrared and red signals.
The control registers provide data for the DAC and set gain values of dc gain (DCG0,
DCG1), ac gain (ACG0, ACG1), and current to voltage pre-amp (IR0, IR1, RD0, RD1)
2-22 Revised 12/20/00
Accutorr Plus Service Manual
Chapter 2 - Theory of Operation