Background and objectiveDifferent types of hydroxyethyl starch (HES) affect blood coagulation differently. We studied the effects of HES 130/0.4 on coagulation in major orthopaedic surgery in relation to the pharmacological parameterin vivomolecular weight.Methods52 patients were randomly allocated to either HES 130/0.4 (6%, mean molecular weight 130 kDa, molar substitution 0.4) or HES 200/0.5 (6%, control) in a double-blind fashion. Colloidal volume requirements for intra- and postoperative haemodynamic stabilisation were compared. Safety analyses of this pharmacological study included a comparison of coagulation factor tests,in vivomolecular weight, and HES plasma concentrations.ResultsThe colloidal volumes given were similar at the end of surgery (1602 ± 569 for HES 130/0.4 vs 1635 ± 567mL for HES 200/0.5), 5h later (1958 ± 467 vs 1962 ± 398mL), and up to the first postoperative day (2035 ± 446 vs 2000 ± 424mL). HESin vivomolecular weight at the end of surgery was 88 707 ± 13 938 versus 158 374 ± 33 933Da (p < 0.001) and 5h later was 86 663 ± 16 126 versus 136 299 ± 26 208Da (p < 0.001). In parallel to the lowerin vivomolecular weight, factor VIII and von Willebrand factor returned to almost normal in the HES 130/0.4 group up to 5h postoperatively, but not in the control group (p < 0.05). Residual HES plasma concentrations after 24h were low in the HES 130/0.4 group (1.0 mg/mL), but higher in the control group (2.6 mg/mL).ConclusionHES 130/0.4 and HES 200/0.5 were found to be similar with regard to volume efficacy. Sensitive coagulation parameters returned more rapidly to normal in the HES 130/0.4 group. Lowerin vivomolecular weight and more rapid excretion of HES 130/0.4 are the likely explanations for the smaller influence on coagulation in this group.