Viscoelastic testing.pptx

Viscoelastic tests
Dr S Shrivastava

NECESSITY
• Hemorrhage is a major contributor to morbidity and mortality during
the perioperative period.
• Current methods of diagnosing coagulopathy have various limitations
including long laboratory runtimes, lack of information on specific
abnormalities of the coagulation cascade, lack of in vivo applicability,
and lack of ability to guide the transfusion of blood products.
• Viscoelastic testing offers a promising solution to many of these
problems. The two most-studied systems, thromboelastography (TEG)
and rotational thromboelastometry (ROTEM), offer similar graphical
and numerical representations of the initiation, formation, and lysis
of clot.

ADVANTAGES
• Transfusions of packed red blood cells (pRBC), plasma, and platelets
are all decreased in patients whose transfusions were guided by
viscoelastic tests rather than by clinical judgement or conventional
laboratory tests. Mortality appears to be lower in the viscoelastic
testing groups, despite no difference in surgical re-intervention rates
and massive transfusion rates
• Viscoelastic testing remains a relatively novel method to assess
coagulation status, and evidence for its use appears favorable in
reducing blood product transfusions, especially in cardiac surgery
patients.

REASON
• Perioperative coagulopathy is frequently multifactorial, including
dilution of plasma volume with intravenous fluid or packed red blood
cells (pRBC), consumption of coagulation factors from ongoing
bleeding, administration of antithrombotic medications such as
heparin or antiplatelet agents, or patient-specific conditions such as
end-stage liver disease, or inherited factor deficiencies.

REMEDY
• The correction of coagulopathy entails primarily the use of thawed
plasma, cryoprecipitate, or platelets. Plasma contains all plasma
coagulation factors. Cryoprecipitate is prepared from plasma, and
contains fibrinogen, factor VIII, von Willebrand factor, factor XIII, and
fibronectin, in a small infusion volume

ASSESSMENT
• Traditionally, to assess coagulopathy, the prothrombin time (PT) and
international normalized ratio (INR) were used to assess the extrinsic
and common pathways, while the activated partial thromboplastin
time (aPTT) was used to assess the intrinsic and common pathways.
• A laboratory fibrinogen level may supplement these basic
coagulation tests.
• More recently, viscoelastic point-of-care testing has shown promise in
diagnosing perioperative coagulopathy and targeting blood products
to specific clot defects.

Viscoelastic testing
• Viscoelastic testing in general refers to several commercially available
point-of-care tests that use a sample of patient blood to derive
various parameters pertaining to the quality of clot formed.
• The analyzer imitates sluggish venous blood flow and derives
measurements of the kinetics of each stage of clot initiation, strength,
and lysis.

MECHANICS
• A small sample of patient blood is placed into a cup, and a sensor rod
is inserted into the blood sample. Either the cup or the rod is then
gently rotated, with a subsequent clot forming between the cup and
rod. The change in speed and pattern of change are measured by a
computer and depicted as a graph. In thromboelastography (TEG), the
cup rotates, while in rotational thromboelastometry (ROTEM) the
sensor rod rotates

ADVANTAGES
• The point-of-care nature of the tests yields results quickly.
• The results are displayed in both graphical format as well as various
numerical measurements, with reference ranges, which can aid rapid
diagnosis of specific coagulopathies.
• They may be performed at various temperatures, ranging from 22 to
42 °C, to demonstrate the effects of acidosis, and hyperthermia or
hypothermia on coagulation.
• Probably most importantly, the tests are able to detect specific
defects in coagulation, such as hypofibrinogenemia, hyperfibrinolysis,
factor deficiency, and heparin effect

TEG
• The classical TEG uses a graphical display to show the initiation,
strengthening, and ultimately lysis of clot, and measures a number of
variables related to the graphic: reaction time (R), the time to clot
initiation; kinetics time (K), the time to reach a certain threshold of
clot strength; alpha (α) angle, slope between R and K; maximum
amplitude (MA), the maximum strength of the clot; A30, the strength
of the clot at 30 minutes; LY30, the degree of thrombolysis at 30
minutes (Figure 1). Abnormalities of any of these variables suggest
specific coagulopathies (Figure 2).

ROTEM
• ROTEM uses an almost-identical graphical results display compared to
TEG, and also measures similar parameters on the graphic, but uses
different names . R time is clotting time (CT), K time is clot formation
time (CFT), MA is maximum clot firmness (MCF), and LY30 is clot lysis
(CL30). Of note, while the graphics and measurements of TEG and
ROTEM are akin conceptually, because of test reagent differences,
their values cannot be directly compared.

ROTEM
• The ROTEM system also markets multiple assays for analysis of
various aspects of the coagulation cascade. These include EXTEM, for
evaluating the extrinsic pathway, INTEM, for the intrinsic pathway,
FIBTEM, for evaluation of fibrinogen contribution to clot formation,
and HEPTEM and APTEM for evaluation of heparin effect or
thrombolysis reversal.
• FIBTEM A10 (clot strength at 10 minutes) has been found to correlate
to serum fibrinogen levels and also correlates to FIBTEM MCF
• Once FIBTEM MCF or A10 has been corrected, the EXTEM MCF or A10
can be analyzed, and if found to be low, suggests thrombocytopenia
as the cause of coagulopathy

ROTEM
• A comparison of EXTEM and APTEM is used to diagnose fibrinolysis—
if a fibrinolytic pattern is seen on EXTEM, the aprotinin in APTEM
should reverse the abnormality.
• HEPTEM and INTEM can be used together to demonstrate heparin-
induced coagulopathy, as a prolonged CT on the INTEM due to
heparin effect will normalize on the HEPTEM due to the addition of
heparinase in the assay.
• Thus, using multiple assays on ROTEM may yield more specific
diagnoses in coagulopathy compared to a classic TEG test.

BENEFITS
• The review of various studies concluded that TEG or ROTEM-guided
transfusion practice appeared to reduce overall mortality [7.4%
versus 3.9%]
• In TEG/ROTEM-guided transfusion management groups compared to
transfusion management guided by any other method, there was also
a statistically significant decrease in the proportion of patients
transfused with pRBC , overall plasma or platelet transfusion for
hemostasis.

ADVANTAGES
• Regarding the cost-effectiveness of viscoelastic testing in cardiac surgery
patients, the review concluded that it is cost-saving and more effective
than conventional coagulation tests, owing mostly to the decrease in blood
products transfused.
• Septic patients with disseminated intravascular coagulation (DIC) have
hypocoagulable profiles, whereas septic patients without DIC have
hypercoagulable profiles.
• In cirrhotic patients with severe coagulopathy, viscoelastic testing greatly
decreased use of blood products when using a TEG-guided transfusion
strategy prior to invasive procedures compared to the standard of care,
without an increase in bleeding events ,same as in cardiac surgery setting.

ADVANTAGES
• . Post-surgical patients at risk for thromboembolic events may benefit
from pre-operative viscoelastic testing for risk stratification.
• A prospective observational study by Collins et al. found FIBTEM to be
an independent predictor for progression to obstetric hemorrhage
greater than 2,500 cc.
• Various studies have linked dabigatran(DOACs) to abnormalities on
ROTEM, including prolonged EXTEM-CT, decreased A10, and
decreased FIBTEM .

ADVANTAGES
• Extracorporeal membrane oxygenation (ECMO) is a method of
replacing pulmonary gas exchange and/or supporting cardiac function
in patients with refractory pulmonary or cardiac failure. Patients on
ECMO require persistent systemic anticoagulation to avoid circuit
clotting and machine failure, typically accomplished with heparin
infusion. Viscoelastic tests have been investigated as a method of
monitoring the anticoagulation status of ECMO patients
• Viscoelastic tests may also play a role in monitoring anticoagulation
for patients on Ventricular Assist Device.

SONOCLOT ANALYSIS
• At present the treatment of postoperative bleeding remains empirical
because of the perceived need for immediate correction of the
haemostatic defect and the lack of readily available measures of all
phases of clot formation and breakdown, including the strength of
the clot.
• Recently, interest has been shown in the perioperative use of
viscoelastic methods of assessing the clotting mechanism. Two such
methods exist: these are Sonoclot analysis (SCT) and
thrombelastography (TEG). Both methods evaluate all phases of clot
formation and retraction from a single sample of blood, and allow
assessment of coagulation factor, fibrinogen and platelet activity, in
addition to measures of clot maturation and lysis.

• Sonoclot analysis In 1889 Hayem suggested that changes in the viscosity of
blood might form the basis for a test of coagulation function.
• in 1975 Von Kaulla, Ostendorf and van Kaulla described the Sonoclot
analyser, a device which measures the changing impedance to movement
imposed by the developing clot on a small probe vibrating at an ultrasonic
frequency in a coagulating blood sample
• The Sonoclot analyser (Sienco Inc., Morrison) has a hollow, open-ended
disposable plastic probe, mounted on an ultrasonic transducer. The probe
vibrates vertically a distance of 1 m at a frequency of 200 Hz, and is
immersed to a fixed depth in a cuvette containing a 0.4-ml sample of whole
blood or plasma

• The increasing impedance to vibration of the probe as the sample
clots is detected by the electronic circuits driving the probe and
converted to an output signal, on a paper chart recorder, which
reflects the viscoelastic properties of the developing clot [40]. The
continuous output curve, or “signature” (see fig. 2), describes the
whole coagulation process in vitro, from the start of fibrin formation,
through polymerization of the fibrin monomer, platelet interaction
and eventually to clot retraction and lysis.

• A plain cuvette, containing no activator, is usually used to derive the
SCT signature. Two other types of cuvette are also available which are
better suited to intraoperative coagulation monitoring. Both contain a
celite activator: the cuvettes with a low concentration of activator
(“red cap” tubes) speed up the analysis but still allow assessment of
platelet function, while the cuvettes with a high concentration of
activator (“white cap” tubes) merely measure ACT which is
comparable with the ACT obtained with the Hemochron method.

SIGNATURE ANALYSIS
• . The onset time, also known as the Sonoclot ACT or SonACT, is the
time in seconds until the beginning of fibrin formation. This is defined
as an upward deflection of 1 mm on the 100-mm wide recorder chart
and is calculated automatically by the machine. This measurement
corresponds to the conventional ACT measurement using the
Hemochron method, provided cuvettes containing a high
concentration of activator are used to derive the SCT.
• The rate of fibrin formation from fibrinogen is indicated by the
gradient of the primary slope (R1). The rate is expressed as a
percentage of the peak amplitude per unit time; R1 values between
15 and 45 % are considered normal.

• An inflection point, or shoulder, on the upstroke is often seen
between the R1 and R2 slopes, although this is a variable feature of
the trace. The shoulder represents the start of contraction of the
fibrin strands by the action of the platelets

• The secondary slope (R2) reflects further fibrinogenesis, fibrin
polymerization and platelet–fibrin interaction.. The peak impedance (PEAK)
reflects completion of fibrin formation and has two variables. The time to
peak amplitude is an index of the rate of conversion of fibrinogen to fibrin,
although the R2 slope is modified by the dynamic combination of fibrin
formation and early clot retraction. The amplitude of the peak is an index
of fibrinogen concentration.
• The downward slope (R3) after the peak is produced as platelets induce
further contraction of the completed clot and the clot mass decreases as
serum is squeezed out of the clot matrix. The number of available platelets
and the level of platelet function are therefore key determinants of the R3
value. When platelet function is poor, or the number of platelets is small,
there is minimal, or protracted, inflection in the R1/R2 upstroke and a
shallow R3 gradient with little decrease in the signal after the peak .

• R3 represents number and function of platelets
• In patients with accelerated fibrinolysis, there is decrease in signal
after the R3 slope.
• SCT distinctly differentiates between platelet-rich and platelet-poor
plasma and has been used primarily for assessing platelet function

• . When transfer of the blood sample to the point of analysis takes
longer than 3 min, a platelet protective anticoagulant is
recommended, such as the Diatube H (Diagnostica Stago, France).
This tube contains a 3.2 % citrate anticoagulant, combined with
theophylline, adenosine and dipyridamole.
• Most of the conventional tests end when fibrin strands are formed
but viscoelastic techniques such as Sonoclot begin as the first fibrin
strands become evident and continue through to clot lysis and
retraction.

• Sonoclot is not a better test of fibrinolytic activity than routine tests
such as measurement of fibrin degradation products, but it may
produce results more quickly and prove to be a useful guide to
intraoperative fibrinolytic activity. However, TEG is more useful as a
monitor of this aspect of coagulation function.
• Both SCT and TEG measure the viscoelastic properties of a
developing clot, and so monitor haemostasis as a dynamic process
rather than at isolated end-points as in routine coagulation tests. In
both techniques a single blood sample is all that is required to
evaluate all phases of coagulation

Viscoelastic testing.pptx

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