In vivo effect of haemodilution with saline on coagulation.

EditorÐAs the in¯uence of i.v. ¯uids on the coagulation system is of importance and also sustains the discussion on optimal ¯uid therapy, we would like to comment on the results reported by Ng and colleagues 1 and the suggestions presented in the accompanying editorial. 2 The study design chosen by Ng and colleagues 1 to investigate coagulation and haemodilution might eliminate the in¯uence of tissue trauma provoked by surgery, but is not`without the effect of any confounding variables'. 2 First, the ratio of blood withdrawn:replacement with normal saline was 1:2, which suggests that the haemodiluted patients were hypovolaemic rather than haemodiluted. Second, all study subjects were patients with malignant disease, who are known to exhibit some state of hypercoagulability because of their underlying disease. Third, the shortening of the reaction time (r-time) by 30% and the coagulation time (k-time) by 36±45% was statistically different from the controls, but they also showed prolongation of the r-and k-times, although no intervention occurred. The question arises as to whether such changes are relevant in vivo, as they might be explained as an in vitro phenomenon attributable to sedimentation of red cells in the thrombelastograph (TEG) cup. Uraemic patients have been shown to have an increased bleeding time, but also exhibit thrombelasto-graphic signs of hypercoagulability measured by shorter r-and k-times, increased a-angle and maximum amplitude compared to normal controls. 3 Interestingly, the latter study showed a signi®cant negative correlation between haematocrit and signs of hypercoagulability. Moreover, isolated reduction of the haematocrit from 40 to 10% also resulted in thrombelastographic signs of hypercoagulability. 4 Reports of accelerated coagulation during haemodilution might therefore be in¯uenced by changes in red cell sedimentation occurring with changing haematocrit. It is likely that this effect also depends on the physicochemical properties of the dilution ¯uid used and the time needed to process the coagulation measurements. Our in vitro and in vivo data on the in¯uence of colloids and crystalloids on coagulation show an accelerated initiation of coagulation during moderate haemodilu-tion with Ringer's lactate and gelatin for the slow-reacting intrinsic TEG measurements only. 5 6 In contrast to the data of Ng 1 and Ruttmann, 2 our results for accelerated coagulation were always accompanied by a reduction in clot ®rmness. Furthermore, we were surprised that, in the study by Ng and colleagues, the a-angle increased by about 71% in the haemodiluted group at 30 min, although platelets and ®brinogen decreased markedly. …