Nonlinear viscoelasticity and the thrombelastograph: 1. Studies on bovine plasma clots.

To study the possible role of nonlinear viscoelastic effects in the thrombelastograph (TEG), clotting of bovine plasma was studied by both thrombelastography and with a controlled strain rheometer. Clot rheology is dominated by elastic effects at frequencies of interest. There is a well-defined regime of linear elasticity for strains less than around 2%, while at larger strains the clots show significant strain hardening. Oscillatory shear applied during clotting has little effect on the resulting clot provided that the strain is less than 2%, but leads to substantial weakening of clots formed at larger strains. The TEG operates within the regime of nonlinear elasticity, significantly obscuring the interpretation of TEG amplitude in terms of an elastic modulus. Comparing the results of standard TEG experiments with those conducted with a modified TEG, having no oscillation during clotting, shows that deformation during standard thrombelastography leads to weaker clots than are produced under quiescent conditions.