GE HEALTHCARE E-PRESTN, Technical Manual

Page: 135 / 622

Cardiac Output Modules, E-COPSv, E-COP
4 - 7
2062973-004
Invasive blood pressure measurement section
An is5 V supply is connected to the input of the pressure transducer bridge circuit. A
differential voltage, which depends on blood pressure and input supply voltage, is calculated
from the bridge circuit output using the following formula:
Uout = Uin x Pressure x 5 V, where Uin = 5 V

Uout = 25 V x Pressure [mmHg]
Pressure amplification is performed by the instrumentation amplifier. The gain of the amplifier
is set so that the level of the signal transferred to the A/D converter stays within the
measurement range even when there are circumstantial offsets or offsets caused by the
pressure transducer. The input filter before the amplifier attenuates high frequency
disturbances.
A FET switch cuts the measurement current and detects the existence of the pressure
transducer. The existence of the pressure transducer is also checked digitally by a jumper next
to the connector.
Figure 5 Pressure transducer’s principle of operation
SvO
2
measurement section
The SvO
2
algorithm is part of the measurement board software. The algorithm consists of five
different parts: initialization, calibration, signal processing and SvO
2
calculation, automatic
gain control, and signal quality analysis.
Initialization
When the optical module is connected to the COPSv module, a number of start-up procedures
are performed prior to normal operation. These procedures include transfer of calibration
factors from the optical module to the COPSv module and initialization of LED currents.
Calibration
The system is calibrated according to either in-vitro or in-vivo calibration. In-vitro calibration is
performed before the oximetry catheter is removed from the package with the catheter tip still
inside the calibration cup. The resulting calibration factor is calculated on the basis of the
measured ratio of red and infrared signals and the ideal ratio for the calibration cup. In-vivo
calibration is performed when the catheter is inserted into the patient’s pulmonary artery. The
resulting calibration factor is based on the measured ratio of red and infrared signal and the
Hgb and SvO
2
values measured in a laboratory. If the calibration is skipped, the result of an old
calibration is used instead and the ‘Not calibrated’ message is displayed in the SvO
2
parameter
window.
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