ROTEM data had come up recently in Question 26 from the second paper of 2014. The best explanation of what the curves actually mean can be found at the Practical Haemostasis website. The LITFL TEG/ROTEM page also offers a good succinct explanation of what one can expect from normal and abnormal TEG tracings. ROTEM, however, is somewhat different to TEG; the machine offers four tracings (INTEM, EXTEM, FIBTEM and APTEM) which need to be interpreted together.
TEG tracings and the meaning of the various measured variables are discussed in the chapter dedicated to the Viscoelastic tests of clotting function. For the trainee with many months to spare, there are also detailed notes on this topic at the Haemoview website:
INTEM is similar to the APTT. This test uses phospholipid and ellagic acid as activators and provides information similar to that of the APTT. Intrinsic pathway is being tested.
EXTEM is similar to the PT. The test uses Tissue Factor as an activator and provides information similar to the PT. Extrinsic pathway being tested. The addition of tissue factor greatly speeds up reaction time (R or CT) and ensures that MA will be established within 10 minutes, but at the cost of all the useful information which can be derived from the R/CT variables.
HEPTEM excludes the effects of heparin. INTEM and HEPTEM are virtually identical in their reference interval, being essentially the same test run with and without heparinase. HEPTEM uses lyophilised heparinase for neutralising heparin, and reports a result which uncovers any coagulopathy which might coexist alongside heparinisation. The use of HEPTEM is ideal for situations where the patient is heavily heparinised (eg. on bypass) and the anaesthetist is interested in the degree of residual coagulopathy one might expect after the heparin is reversed with protamine. When comparing INTEM and HEPTEM, a significantly shortened HEPTEM CT would suggest a heparin effect (i.e. the CT is shortened by adding heparinase).
APTEM excludes fibrinolysis. This test uses aprotinin to inhibiting fibrinolytic proteins. It is otherwise identical to EXTEM. A shortened CT and a higher MCF in an APTEM test (compared to EXTEM) suggests that hyperfibrinolysis is taking place (i.e. the clot forms faster and stronger in the presence of fibrinolysis inhibitor such as aprotinin). In such a situation, one might reach for the tranexamic acid.
FIBTEM isolates fibrinogen function. By using a platelet inhibitor (cytochalasin D) this test blocks the platelet contribution to clot formation, leaving only the clotting proteins. Thus, one can observe the contribution of functional fibrinogen to clot formation. Obviously, without platelets the clot is totally useless from a haemostatic standpoint, and this is well demonstrated by the FIBTEM trace which rarely even reaches an amplitude of 20mm at its maximum. A FIBTEM MCF of less than 9mm suggests that there is insufficient fibrinogen level, and a MCF in excess of 25mm suggests that there is an excess of fibrinogen and perhaps some sort of procoagulable state.
ECATEM tests for direct thrombin inhibitors. This test uses Ecarin and so is similar to the Ecarin Clotting Time (ECT). Ecarin is a prothrombin activator, derived from the venom of saw-scaled viper Echis carinatus. In the presence of direct thrombin inhibitors, ECT will be prolonged; however with heparin or warfarin ECT will be normal.
Typically, INTEM EXTEM FIBTEM and APTEM will be performed, with HEPTEM and ECATEM being optional extras. Each technique has its own reference ranges, which no sane person is ever expected to memorise. The CICM examiners are unlikely to severely penalise the candidate who fails to accurately recall the normal HEPTEM alpha-angle value. In order to satisfy the author's autistic attachment to completeness, the reference values are offered in the table below
The CT and CFT are in seconds, alpha-angle is in degrees, MCF and A10 are in mm, ML and LI30 are a percentage of MCF.
These graphics were prepared and laid out in a matter which resembles Question 26 from the second paper of 2014. In fact, the ROTEM values are the same as those used by the college in their paper.
Continuing with the trend of ripping off the manufacturer's ROTEM education literature, here are some ROTEM images which demonstrate the effect of low fibrinogen:
The interpretation of this graph is not especially laborious. The cardinal abnormality is the massively prolonged CT and CF of the INTEM graph, which suggests that something has killed the intrinsic pathway of the clotting cascade. The CT returns to normal in the HEPTEM graph, which is essentially just an INTEM test with adde heparinase. The presence of heparinase seems to have reversed all of the coagulopathy - the CFT, alpha-angle and MCF have all returned to normal. Therefore, this patient has no coagulation problems other than the heparin.
One of the major arguments for the use of point-of-care instruments like TEG and ROTEM is their ability to detect fibrinolysis as the cause of bleeding, whcih no other convenient test can actually do. This is what fibrinolysis looks like in a ROTEM:
In this instance, the standout feature is the rapid decrease in amplitude in all tests except APTEM - to zero. In the APTEM test, aprotinin is added; fibrinolytic enzymes are inhibited, and clot stability is restored. Note that all the other variables are intact - clotting is initiated normally and clot propagation is generally unaffected.