Viscoelastic Studies Of Coagulation: Cell-Plasma Interactions

In order to examine the cell-plasma interactions during coagulation, a thrombelastograph was modified to measure two viscoelastic properties of blood: rigidity (G) and rate of stress relaxation (S). Citrated whole blood was collected from healthy donors and centrifuged to obtain platelet-rich and platelet-poor plasma and washed red cells. The blood was reconstituted at varying hematocrits (HCT) and platelet counts (PLT) and coagulation was initiated by adding CaCl₂. Factor XIII (FXIII) activity was inhibited with glycine ethyl ester (GEE). In normal blood, the rigidity increases rapidly after 6-8 min following addition of CaCl₂ with a rate constant (k_G) of 8.2 min and a saturated rigidity (G_sat ) of 22.3×l0³ dyn/cm² . S decreased in a biphasic minner: the first phase (ΔS1) lasting about 15 min and accounting for a 70% fall in S and with a rate constant (k_s1) of 6.7 min; the second phase (ΔS2) from 15 min on with a 15% drop in S and with a rate constant (k_s2) of 71.4 min. Increasing PLT resulted in a linearly increasing G up to 31×l0³ dyn/cm² for 3×10⁵ cells/mm³. Increasing HCT had no effect on G_sat , but increased k_G linearly for 10<HCT<50%. Inhibition of FXIII activity by GEE had no effect on G, but inhibited ΔS₂ while k_S2 was linearly dependent on GEE concentration. In cell-free plasma, ΔS1 was about 80% in 10 min with virtually no ΔS2. With the inclusion of even a small number of platelets, ΔS1 and ΔS2 returned to that found in normal blood.

It was concluded that platelets change the configuration of fibrin in the clot and alter the kinetics of orientation and crosslinking. Red cells have little mechanical influence but do seem to have some procoagulant activity.