Biomedical Engineering Reference
In-Depth Information
by initiating the CPB circuit is known to prolong ACT measurements with
different coagulation activators. However, the use of ACT measurements
has been questioned because of the inherent test variability and the inter-
ference with many peri-operative factors such as aprotinin infusion, hemo-
dilution, patient hypothermia and quantitative or qualitative platelet
abnormalities.
32
The term thrombelastography (TEG) is used to describe the trace pro-
duced from the measurement of the viscoelastic changes associated with
fi brin polymerization. TEG gives a graphic representation of clot formation
and subsequent lysis. Blood is incubated at 37 °C in a heated cup. Within
the cup is suspended a pin connected to a detector system. This is a torsion
wire or an optical detector. The cup and pin are oscillated relative to each
other. The movement is initiated from either the cup or the pin. As fi brin
forms between the cup and pin the transmitted rotation from the cup to pin
or impedance of the rotation of the pin is detected at the pin and a trace
generated.
TEG has been used for recent years as a guide to blood product and
drug administration during cardiac and hepatic surgery.
33
It is capable of
providing a robust, inexpensive, snapshot of hemostasis at the bedside.
For laboratory use, the need for an anticoagulated sample has led to
stricter controls over sample handling. There is more work to be carried
out particularly with regard to standardization and reagent optimization
before this potential can be fully evaluated. Cell saver devices and
platelet gel procedures are becoming more and more popular in the
era of blood management during CPB which both are operated by a
perfusionist.
6.6.2 Inline blood gas analysis
Arterial blood gas analyses are essential to monitor gas exchange in criti-
cally ill patients and during anesthesia for major surgery. Usually, arterial
blood samples are drawn intermittently and analyzed in a central laboratory
or by a point-of-care blood gas analyzer. During the past decade, marked
advances in continuous intravascular blood gas monitoring (CIBM) have
been achieved by miniaturization of the sensors measuring
P
O
2
,
P
CO
2
and
pH. CIBM appears to be desirable at least in selected patient groups, pro-
vided the technique proves to be reliable and cost-effective.
P
O
2
,
P
CO
2
and
pH can be measured by electrochemical and photochemical/optical
sensors.
34
Electrochemical sensors have been used for intravascular
P
O
2
measure-
ment. The principle is that used in a modifi ed Clark electrode. A small
polarizing potential is maintained between the platinum cathode and
the silver anode. The electrodes are immersed in an electrolyte solution
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