Effects of acute normovolemic hemodilution on blood coagulation: comparison between tests of an in vivo and an in vitro model.

BACKGROUND AND OBJECTIVES Normovolemic hemodilution produces conflicting results on hemostasis, since works differ according to the fluids used, degree of hemodilution, method used to evaluate hemostasis, and the way used to produce hemodilution. The effects of hemodilution on hemostasis can depend on how it is done and whether an in vivo or an in vitro model is used. The objective of this study was to evaluate and compare hemostasis in two different degrees of hemodilution in both models. METHODS Thirteen patients underwent acute normovolemic hemodilution with a reduction in hematocrit to 30% and 20%. Volemia was maintained with Ringer's lactate. Blood samples for evaluation of hemostasis were collected at moments M1--before hemodilution, M2--20 minutes after obtaining a hematocrit of 30%, and M3--20 minutes after obtaining a hematocrit of 20%. Before hemodilution, blood samples were collected to perform hemodilution in a test tube. The degree of hemodilution in the test tube (in vitro) was the same as that produced in the patients (in vivo). Hemostasis was evaluated by the prothrombin time, partial activated thromboplastin time, and thrombin time, and by quantifying fibrinogen. RESULTS The behavior of the tests that evaluated hemostasis was identical in both models. An increase in PT, aPTT, and TT, as well as a reduction in the concentration of fibrinogen was observed. The higher degree of hemodilution was associated with greater compromise of coagulation. CONCLUSIONS The in vitro model can substitute the in vivo mode in the evaluation of hemostasis during acute normovolemic hemodilution.