**15. The principles of the thromboelastography (TEG)**

Samples for analysis are stored in a citrated tube and must be analysed within 4 hours. Kaolin (a clotting factor) is added to two separate cuvettes both containing sufficient calcium chloride to reverse the effect of the citrate. One of the cuvettes also contains a surface coating of heparinase to deactivate any heparin in the sample. Both cuvettes are loaded onto the analyser for simultaneous analysis.

Each cuvette sits on a platform that oscillates through an angle of 4045´. The whole cycle lasts ten seconds including a 1 second rest period. A pin is freely suspended within the blood and is attached to a torsion wire which in turn is attached to a mechanical-electrical transducer. As the blood starts to clot within the cuvette, fibrin strands form between the blood and the pin and the torque is transmitted via the transducer to give a signal that is recorded by a computer (See figure 8).

The variables that are recorded are as follows: "r" (reaction time) is the period of time, in minutes, taken to form the initial fibrin strands after placement of blood in the TEG analyser.

Fig. 8. This image represents the TEG diagrammatically. (A) Shows a cross-sectional view of the apparatus. (B) Demonstrates the construction of the curve. (C) Shows the final printout of the TEG

Before this happens, the liquid blood in the cuvette exerts very little torque on the pin and the signal remains effectively zero (see figure 8). It is prolonged by congenital or acquired deficiencies in hepatic coagulation factors and anticoagulants such as unfractionated heparin, the low molecular weight heparins and warfarin. The "K" (K time) is the time, in minutes, after clot formation begins and the clot itself is of sufficient strength to produce a 20mm divergence of the line. This reflects the speed of clot strengthening. The "K" is reduced by an elevated fibrinogen and by effective platelet function and is prolonged by the same factors that prolong the "r". The "α" (alpha angle), measured in degrees, is the slope of the TEG tracing with respect to the r line (the line drawn between r1 and r2). It reflects the speed at which the clot is formed and behaves very much like the "K". If the "K" is undefined (i.e. the signal generated by the computer never reaches an amplitude of 20mm) such as in hypocoaguable states then the "α" becomes more important. The "MA" (maximum amplitude) is a measure of the maximum strength developed by the clot. It is dependent on fibrin formation and platelet function. Percentage clot lysis is estimated at 30 (LY30) minutes or 60 (LY60) minutes after the "MA" is reached. The percentages are calculated by using the following formula: LY30 = (MA - A30)/MA ×100 and LY60 = (MA – A60)/MA × 100, where A30 is the amplitude of the trace 30 minutes after "MA" is reached and A60 the corresponding value after 60 minutes. If the LY30 or LY60 are high then fibrinolysis is high. Finally, "C.I." (coagulation index) can also be derived from "r", "K", "α" and "MA"by substitution into the following equation: C.I. = 0.3258r – 0.1886K + 0.1224MA + 0.0759α – 7.7922. This equation has been derived after multiple regression of a large database of TEG tracings. The "C.I." reflects the patient's overall coagulation state and normal values range from – 3.0 to + 3.0 (43).

#### *Platelet mapping with TEG*

196 Perioperative Considerations in Cardiac Surgery

Thromboelastography was first described in 1948. Viscoelastic changes during coagulation are plotted against time(42). The abbreviation 'TEG' was used for this technique. TEG® is now a trademark for one manufacturer of thromboelastograph device, an alternative manufacturer uses the trademark ROTEM®. The principles are similar, described below.

Samples for analysis are stored in a citrated tube and must be analysed within 4 hours. Kaolin (a clotting factor) is added to two separate cuvettes both containing sufficient calcium chloride to reverse the effect of the citrate. One of the cuvettes also contains a surface coating of heparinase to deactivate any heparin in the sample. Both cuvettes are

Each cuvette sits on a platform that oscillates through an angle of 4045´. The whole cycle lasts ten seconds including a 1 second rest period. A pin is freely suspended within the blood and is attached to a torsion wire which in turn is attached to a mechanical-electrical transducer. As the blood starts to clot within the cuvette, fibrin strands form between the blood and the pin and the torque is transmitted via the transducer to give a signal that is

The variables that are recorded are as follows: "r" (reaction time) is the period of time, in minutes, taken to form the initial fibrin strands after placement of blood in the TEG analyser.

Fig. 8. This image represents the TEG diagrammatically. (A) Shows a cross-sectional view of the apparatus. (B) Demonstrates the construction of the curve. (C) Shows the final printout

**15. The principles of the thromboelastography (TEG)** 

loaded onto the analyser for simultaneous analysis.

recorded by a computer (See figure 8).

of the TEG

**14. Thromboelastography** 

Platelet mapping studies measure the degree to which a patient's platelets can be activated by stimulation of either the thromboxane A2 (Txa2) receptor or the adenosine diphosphate (ADP) receptor. These receptors can be inhibited by aspirin and clopidogrel respectively, but the degree of inhibition varies between individuals. Thrombin activates platelets independent of the arachidonic acid and ADP receptors via the glycoprotein IIb/IIIa receptor.

In cardiac surgery, antiplatelet agents are often withheld to minimise bleeding complications, which can expose patients to increased risks of peri-operative myocardial ischaemia. Platelet mapping may have a role in determining the optimum time to operate, such that antiplatelet agents are withheld for long enough to decrease the bleeding risk, but no longer than necessary. In the medical management of coronary artery disease, this technique could potentially be used before antiplatelet therapy is commenced, to determine which antiplatelet agent is likely to be effective for an individual patient. It might also be used after antiplatelet therapy has been initiated to measure the response to therapy.

Platelet mapping can be assessed by the following method: 4 TEG channels (cups) are required. Cup 1 contains blood and kaolin (this activates the clotting cascade ex vivo) and the MA reflects thrombin activated platelets (MAthrombin). Thrombin in the blood sample is produced by trauma during the blood sampling procedure. Since the direct effect of thrombin on the IIb/IIIa receptor is not dependent on stimulation of either the Txa2 or ADP receptors, MAthrombin is an indirect measure of platelet function even in the presence of aspirin or clopidogrel. In all other 3 cups heparin is added to inhibit thrombin thus eliminating its direct action on the IIb/IIIa receptor. To the second cup 10μL reptilase and activated factor XIII (Activator F) is added which convert fibrinogen to fibrin by generating a cross-linked fibrin clot thus isolating the contribution of fibrin to the clot strength. Activator F has no effect on platelets so the "MA" reflects the action of fibrin only (MAfibrin) (44). To the third cup 10μL Activator F and 10μL arachidonic acid (AA) are added and the MAAA reflects platelet responsiveness to AA. To the fourth cup 10μL Activator F and 10μL ADP are added and the MAADP reflects platelet responsiveness to ADP. The relative response of platelets to either AA or ADP is calculated as follows with a normal reference value for each being above 80%:

Relative response of platelets to AA = (MAAA - MAfibrin) / (MAthrombin - MAfibrin) × 100.

Relative response of platelets to ADP = (MAADP - MAfibrin) / (MAthrombin - MAfibrin) × 100.

The platelet inhibition in response to an agonist is calculated by subtracting the percentage aggregation from 100 (45).

#### *Rotational Thromboelastometry (ROTEM®)*

ROTEM is based on the same principles as thromboelastography. Citrated whole blood is mixed with reagents in a stationary cuvette, then a pin is placed vertically into the blood and alternating clockwise and anticlockwise forces are applied to the pin. As the blood clots the pin's oscillation is impeded, and when fibrinolysis occurs the pin will move more freely.

The device records the changes in oscillation against time, creating the 'thromboelastogram' (TEM). The zero line indicates free movement of the pin. An amplitude of 100mm would indicate no movement of the pin, meaning the clot has reached the maximum possible firmness measurable by ROTEM. Combinations of reagents can be used to produce a series of TEMs that measure different aspects of the coagulation and fibrinolysis processes.

The ROTEM machine has four channels, thus four TEMs can be generated simultaneously. By using combinations of coagulation triggers and coagulation inhibitors, a clearer understanding of the coagulation abnormalities can be obtained. The reference ranges for the parameters are not interchangeable between the different types of TEM, for example the MCF seen in the FIBTEM test will be significantly lower than that in INTEM or EXTEM, because clot firmness is due to the effect of fibrin only.


Parameters measured using the ROTEM are as follows:

Activator F has no effect on platelets so the "MA" reflects the action of fibrin only (MAfibrin) (44). To the third cup 10μL Activator F and 10μL arachidonic acid (AA) are added and the MAAA reflects platelet responsiveness to AA. To the fourth cup 10μL Activator F and 10μL ADP are added and the MAADP reflects platelet responsiveness to ADP. The relative response of platelets to either AA or ADP is calculated as follows with a normal reference

Relative response of platelets to AA = (MAAA - MAfibrin) / (MAthrombin - MAfibrin) × 100.

Relative response of platelets to ADP = (MAADP - MAfibrin) / (MAthrombin - MAfibrin) × 100. The platelet inhibition in response to an agonist is calculated by subtracting the percentage

ROTEM is based on the same principles as thromboelastography. Citrated whole blood is mixed with reagents in a stationary cuvette, then a pin is placed vertically into the blood and alternating clockwise and anticlockwise forces are applied to the pin. As the blood clots the pin's oscillation is impeded, and when fibrinolysis occurs the pin will move more freely. The device records the changes in oscillation against time, creating the 'thromboelastogram' (TEM). The zero line indicates free movement of the pin. An amplitude of 100mm would indicate no movement of the pin, meaning the clot has reached the maximum possible firmness measurable by ROTEM. Combinations of reagents can be used to produce a series

of TEMs that measure different aspects of the coagulation and fibrinolysis processes.

The ROTEM machine has four channels, thus four TEMs can be generated simultaneously. By using combinations of coagulation triggers and coagulation inhibitors, a clearer understanding of the coagulation abnormalities can be obtained. The reference ranges for the parameters are not interchangeable between the different types of TEM, for example the MCF seen in the FIBTEM test will be significantly lower than that in INTEM or EXTEM,

**Significance Clinical Application** 

Prolonged CT suggests anticoagulant drug activity or



coagulation factor abnormalities. Consider:

as protamine.

plasma)

value for each being above 80%:

aggregation from 100 (45).

**ROTEM Parameter [Measured Variable]**

[Delay from the start of the reaction to the start of clot

An amplitude of 2mm is taken to signify the start of

**Clotting time (CT)** 

formation.

clot formation.]

*Rotational Thromboelastometry (ROTEM®)* 

because clot firmness is due to the effect of fibrin only. Parameters measured using the ROTEM are as follows:

This represents:


the APTT for INTEM.

2mm amplitude.


Abnormalities that prolong the laboratory tests will prolong the CT in the comparable ROTEM test, but the endpoint for the laboratory test is fibrin formation, whereas in ROTEM a degree of fibrin stabilisation is required to attain



[Reproduced with permission of Tem International GmbH]

Fig. 9. ROTEM graph and basic parameters (a graph of an unstable clot has been chosen to enable illustration of the lysis parameters)

The available ROTEM tests are described below:


This value represents the inverse of LI (% remaining clot firmness + %

Fig. 9. ROTEM graph and basic parameters (a graph of an unstable clot has been chosen to

polymerisation]

**Relevant Parameters [Considerations if parameters are abnormal ]** 

**CT** [Factor deficiency (intrinsic pathway); Anticoagulant

**MCF, A(x), CFT, -angle** [Platelet contribution to clot

effects (heparin, thrombin inhibitors)]

**ML, LI(x)** [Hyperfibrinolysis]

firmness; Fibrinogen concentration; Fibrin

clot firmness lost = 100%) This value will increase if an extended measurement period is

used.

[Reproduced with permission of Tem International GmbH]

enable illustration of the lysis parameters)

**Name of Test [Test description]** 

[Mild intrinsic coagulation pathway activation]

**INTEM** 

The available ROTEM tests are described below:

**Significance Clinical Application** 

considered.

An abnormally high ML usually indicates hyperfibrinolysis, so antifibrinolytic drugs should be

**ROTEM Parameter [Measured Variable]** 

**Maximum Lysis (ML)**

amplitude (relative to the peak at MCF) that has occurred by the end of the measurement period given as a percentage of MCF. This is the % clot firmness

[The decrease in

lost.]


A suitable combination of TEMs for a patient undergoing cardiac surgery would be INTEM, FIBTEM and HEPTEM. The INTEM is performed to look at CT (prolonged in consumptive coagulopathy and the heparin effect on the intrinsic pathway) and also the α-angle and MCF (to assess whether fibrinogen or platelet supplementation is needed). The HEPTEM is compared with INTEM to determine the degree of Heparin effect. FIBTEM is performed to define the fibrinogen contribution to the haemostatic plug.

As recommended at the European ROTEM meeting 2007, a minimum of 3 sets of ROTEM should be performed; a baseline, a set whilst on CPB, and one after CPB. Further sets are to be performed if the patient is still bleeding. These tests can be performed quickly and interpreted safely between 10-15 minutes from the start of the tests.

The FIBTEM test is used clinically to guide administration of fibrinogen (46), but evidence for the treatment trigger or therapeutic target has not been conclusively established. It has been suggested that a FIBTEM MCF of 7mm is adequate for haemostasis in orthopaedic surgery, but a higher target of 22mm has been discussed for cardiac surgery. Caveats for ROTEM use:

The INTEM and EXTEM are not very sensitive to mild coagulation factor deficiencies, and are insensitive to defects of primary haemostasis (platelet aggregation). The EXTEM test may still be normal when INR is as high as 4, and may show pathological values with very high heparin levels.

#### *Thromboelastography (TEG) versus Rotational Thromboelastometry (ROTEM)*

As discussed above, the techniques have a common origin. Both systems generate graphs of clot firmness between a cuvette and pin versus time. In the TEG system the cuvette oscillates around a pin throughout the clotting process, whilst in the ROTEM system the cuvette is fixed and the pin rotates.

By convention, the parameters obtained from the graphs have been given different names in the two tests. A comparison of the nomenclature is given in the table below from an article by Luddington (42).


Copyright of Clinical & Laboratory Haematology is the property of Blackwell Publishing Limited and its contents may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.

#### Fig. 10. Nomenclature of TEG and ROTEM

#### *Clinical application of near patient test results and evidence base for decision-making*

There are several studies in the literature that highlight the usefulness of the TEG in cardiac surgery. Shore-Lesserson et al demonstrated in a prospective randomized controlled trial of high risk cardiac surgical patients (n=105) that fewer patients randomized to the TEG arm were transfused fresh frozen plasma (p < 0.002) or platelets (p < 0.05) compared to those patients managed by standard coagulation tests in the postoperative period (47). Total blood product transfusion was less in the TEG arm and this reached statistical significance. TEG was used to guide protamine, platelet, FFP and antifibrinolytic therapy. The authors concluded that TEG –based transfusion algorithm reduced transfusion requirements.

Spiess et al assessed the usefulness of TEG in predicting postoperative haemorrhage and need for reoperation. They found that TEG was a better predictor (87% accuracy) than coagulation profile (51%) or activated clotting time (30%) (48). Another study by the same author concluded that costs and risks associated with allogenic blood transfusions could be reduced with the introduction of a TEG based haemostasis protocol in cardiac surgery (49). Tuman found that TEG predicted postoperative haemorrhage more accurately than routine coagulation tests (88% versus 33% respectively) (50).

As discussed above, the techniques have a common origin. Both systems generate graphs of clot firmness between a cuvette and pin versus time. In the TEG system the cuvette oscillates around a pin throughout the clotting process, whilst in the ROTEM system the cuvette is

By convention, the parameters obtained from the graphs have been given different names in the two tests. A comparison of the nomenclature is given in the table below from an article

Copyright of Clinical & Laboratory Haematology is the property of Blackwell Publishing Limited and its contents may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for

There are several studies in the literature that highlight the usefulness of the TEG in cardiac surgery. Shore-Lesserson et al demonstrated in a prospective randomized controlled trial of high risk cardiac surgical patients (n=105) that fewer patients randomized to the TEG arm were transfused fresh frozen plasma (p < 0.002) or platelets (p < 0.05) compared to those patients managed by standard coagulation tests in the postoperative period (47). Total blood product transfusion was less in the TEG arm and this reached statistical significance. TEG was used to guide protamine, platelet, FFP and antifibrinolytic therapy. The authors concluded that TEG –based transfusion algorithm

Spiess et al assessed the usefulness of TEG in predicting postoperative haemorrhage and need for reoperation. They found that TEG was a better predictor (87% accuracy) than coagulation profile (51%) or activated clotting time (30%) (48). Another study by the same author concluded that costs and risks associated with allogenic blood transfusions could be reduced with the introduction of a TEG based haemostasis protocol in cardiac surgery (49). Tuman found that TEG predicted postoperative haemorrhage more accurately than routine

*Clinical application of near patient test results and evidence base for decision-making* 

*Thromboelastography (TEG) versus Rotational Thromboelastometry (ROTEM)* 

fixed and the pin rotates.

by Luddington (42).

individual use.

Fig. 10. Nomenclature of TEG and ROTEM

reduced transfusion requirements.

coagulation tests (88% versus 33% respectively) (50).

Avidan et al (51) showed in a prospective randomised comparison trial that following algorithms based on point of care tests or on structured clinical practice with standard laboratory tests did not decrease blood loss, but did reduce the transfusion of all blood components (p < 0.05) after routine cardiac surgery, when compared with clinician discretion.

Finally, Westbrook et al showed in a prospective randomised controlled study a nonstatistically significant trend toward less blood product usage in a strict protocol utilizing TEG compared to physician's choice based on clinical experience and standard laboratory coagulation tests. The study also demonstrated a cost saving benefit (52).

Use of an automated protamine titration system, such as the commercially available pointof-care Hepcon®, has been shown to be associated with higher heparin and lower protamine doses. This can potentially decrease activation of the coagulation and inflammatory cascades, with possible advantages of decreased postoperative bleeding and blood product requirement.(23)

#### *Blood product administration in cardiac surgery and the cardiothoracic ICU*

If a patient bleeds in the postoperative period, it is important to distinguish between 'surgical' and 'medical' bleeding. A 'medical bleed' from the microvasculature occurs due to a coagulation abnormality, and this abnormality must be detected and corrected with blood products or pharmacological agents to halt the bleeding. In contrast a 'surgical bleed' is from a cardiac or large vessel injury and will often require resternotomy to re-establish haemostasis surgically. Misdiagnosis of 'surgical bleeding' can lead to unnecessary resternotomy, whilst failure to diagnose a 'surgical bleed' may lead to delay and futile use of blood products. A progressive, untreated 'surgical bleed' will lead to a combined 'medical' AND 'surgical' bleed. Likewise, a 'medical bleed' that is not correctly treated may result in a pericardial tamponade that needs surgical intervention.

Factors to be considered in post-operative haemorrhage include:


Where possible, administration of blood products should be minimised(53). With donor screening and testing of donated blood, the risk of blood-borne disease transmission has decreased. Uncertainties regarding prion-related disease transmission remain, and there is also evidence that blood transfusion during cardiac procedures is associated with worse short- and long-term outcomes (41, 54). The measures below are adapted from the 2011 Blood Conservation Clinical Guidelines published by the Society of Thoracic Surgeons and the Society of Cardiovascular Anaesthesiologists.(55)
