Classical thromboelastometry is performed by filling a cuvette with native whole blood and lowering a pin suspended by a torsion wire into the sample. The cup is rotated through 4° 45’ over 10 seconds with a 1 second rest period at each end. The torque of the cup is transmitted to the pin through the sample in the cup. The width of the tracing is proportional to the magnitude of the elastic shear modulus of the sample. Liquid blood has little or no torque so there will be no deflection even when the viscosity is high. As the blood clots and fibrin strands begin to form between the cup and the pin, the motion of the cup is transmitted to the pin.
The pin is guided by a ball bearing ensuring that all movement is limited to rotation. The movement of the pin is detected by an optical detection system and is transmitted to and processed by a computer with specific software. Results obtained by thromboelastometry are dependent on the activity of the plasma coagulation system, platelet function, fibrinolysis and many factors which influence these interactions, including several drugs.
Many limitations of classical thromboelastometry are overcome by the innovative rotation thrombelastometry (ROTEM®). Data obtained with ROTEM® correlate well with classical thromboelastometry (Calatzis et. al, 1996).
In ROTEM®, the pin (sensor) is fixed onto the tip of a rotating shaft, which is guided by a high precision ball bearing system. The shaft rotates back and forth (+/- 4.75 °; cycle time 10/min). It is connected with a spring for the measurement of elasticity. The exact position of the axis is detected by the reflection of light by a small mirror that is attached to the shaft.
The loss of the elasticity upon clotting of the sample leads to a change in the rotation of the shaft. This is detected by a CCD array and the data are analysed by a computer.
This opto-mechanical detection method provides a good protection against the impact of vibrations and mechanical shocks.