SC 7000 and SC 9000XL Patient Monitors Service Manual
ASK-T898-03-7600 Siemens Medical Systems, EM-PCS Danvers 29
7k9kXLTM.c2.CD_ROM.fm/04-99/kaupp
NOT A CONTROLLED DOCUMENT
10.3Cardiac Output
The two thermistor signals are connected to a precision resistor network to
linearize voltage vs temp curve of the thermistor. The thermistor signals are
filtered and clamped before amplification. Two calibration voltages are also
sampled by the A/D converter to correct amplifier offset and gain errors.
The catheter also has a reference resistor which is read for calibration.
The front bezel switches on the HemoMed are converted to unique voltages
when pressed, allowing a voltage to be sent to the A/D converter, which can
be decoded by the front end processor into the corresponding switch
closure.
Figure 2-15 etCO
2
Sensing Process Functional Block Diagram
11etCO
2
Module
The etCO
2
module non-invasively monitors end-tidal CO
2
using a technique
that relies on the selective absorption properties of the CO
2
to specific
frequencies of infra-red radiation. See Figure 2-15.
In the sensor a thick film infra-red source is pulsed at a rate of
approximately 87 Hz, generating a broad- band spectrum of IR. Selective
filtering separates this into two narrow regions, one inside and one outside
the band of CO
2
absorption. The detector associated with the filter outside
the band of CO
2
absorption records the maximum level of the source
energy since the signal it receives is not affected by CO
2
. It provides a
baseline which serves as a Reference for the level of CO
2
in the airway.
The other detector senses a filtered energy level modified by the presence
of CO
2
. As the level of CO
2
increases, the CO
2
gas molecules in the airway
absorb more of the light energy and less signal reaches the detector. This
signal, converted by the detector, is referred to as the Data signal. Current
through the thick-film source is bidirectional to offset the tendency of
particles within the source to migrate when exposed to a strong
unidirectional electric field caused by current flow only in one direction. This
keeps the structure of the source uniform and enhances system integrity
and life of the product.
To acquire a precise level of CO
2
, both channels are simultaneously
sampled and the level of CO
2
is determined from the ratio of the Data and
the Reference channels. The ratio is compared to a look-up table in
memory to establish the correct value in units of mmHg.
The module then sends the results to the host system for further processing
and display.
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