[Comparative study of low molecular dextran or hydroxyethyl starch as a volume substitute in hemodilution therapy]

Hydroxyethyl starch in balanced electrolyte solution (Hextend)--pharmacokinetic and pharmacodynamic profiles in healthy volunteers

Hextend is a new plasma volume expander containing 6% hydroxyethyl starch (HES) in a physiologically balanced medium of electrolytes, glucose, and lactate (weight average, molecular weight 670 kDa, molar substitution 0.75). This open-label study was designed to investigate the pharmacokinetic and pharmacodynamic profiles of Hextend in 21 healthy volunteers. We infused Hextend 10 ml/kg IV over 20 min and determined serum concentrations of HES at selected intervals over a 7-day period. Serum concentration-time curves indicated mixed pharmacokinetic behavior reflecting a two-compartment model in most subjects. The median serum half-life over 7 days was 38.2 h. The balanced formulation of the suspension medium did not seem to affect distribution, metabolism, or excretion of Hextend when compared with similar HES. Pharmacodynamic analysis demonstrated decreases in some plasma components compatible with the infusion of that volume of fluid and the duration of plasma volume expansion. Other plasma components remained unchanged, reflecting the benefit of a balanced electrolyte solution. Hemodilution was observed for 24--48 h after short-term infusion of Hextend. Some hemostatic indices showed moderate changes, and serum amylase demonstrated a temporary increase. Our study suggested that Hextend has pharmacokinetic and pharmacodynamic profiles that are similar to those of other HES.

IMPLICATIONS

Hextend is a new plasma volume expander containing 6% hydroxyethyl starch in a physiologically balanced medium. This open-label volunteer study demonstrated that it has pharmacokinetic and pharmacodynamic profiles similar to those of established HES.

[Hydroxethyl starch: effects on hemostasis]

HES are high-polymeric glucose compounds obtained via hydrolysis and subsequent hydroxyethylation from the highly-branched amylopectin contained in maize. Initially, the HES were only characterized by their in vitro molecular weight (Mw), without consideration of the in vivo hydrolysis by alpha-amylase. The degree of substitution and the molar substitution ratio quantify the hydroxyethylation. The glucose units can be substituted at carbon 2, 3 and 6 leading to various substitution patterns. This pattern is described with the C2/C6 hydroxyethylation ratio. The higher the degree of substitution and the C2/C6 ratio, the less the starch is metabolized. The in vitro Mw, the degree of substitution and the C2/C6 ratio are the main determinants of the in vivo Mw which is clinically relevant. Haemorrhagic complications that occur after infusing larger volumes of HES can be avoided with a starch of low in vivo Mw. This is not only due to a lesser effect on the coagulation system which prevents an acquired type I von Willebrand syndrome, but also to a smaller decrease in platelet volume, since platelet volume and platelet function are positively correlated. In addition, HES with low in vivo Mw has significantly better rheological effects than HES with a high in vivo Mw, as high Mw macromolecules affect plasma viscosity negatively. Furthermore high Mw HES macromolecules lead to a distinctive decrease in fibronectin concentration that reflects saturation of the reticuloendothelial system. Another advantage of low in vivo Mw HES is its rather short half-life. Patients with an increased bleeding risk, microcirculatory disturbance or affected RES should receive HES with low in vivo Mw. In the future, HES should be mainly characterized by the in vivo and not the in vitro Mw.

Haemodilution with medium molecular weight hydroxyethyl starch in patients with peripheral arterial occlusive disease stage IIb

The basic therapy of peripheral arterial occlusive disease stage IIb, according to Fontaine, is exercise. It should be determined whether a mild hypervolaemic haemodilution with hydroxyethyl starch or Ringer lactate can produce a further increase in walking distance. For this purpose, three groups of 25 patients each were formed. One group exercised three times weekly and the second group, in addition to exercise, underwent a mild hypervolaemic to isovolaemic dilution therapy with HES for a period of 6 weeks. In the final group the haematocrit was reduced to the same extent by venesections and volume substitution using Ringer lactate. The walking distance in the HES group increased from 216 m to 311 m (44%), in the Ringer lactate group from 214 m to 258 m (20%), and in the exercise group from 213 m to 242 m (14%). On comparison of the groups, the increase in pain-free walking distance in the HES group differs significantly (P less than 0.05) from that achieved in the other groups. It was demonstrated that haemodilution with HES in combination with exercise brings about a clinical effect three times that achieved by exercise alone. Venesection with subsequent administration of Ringer lactate and exercise is superior to exercise alone but markedly inferior to the combination therapy with HES.

Haemodilution with dextran 40 and hydroxyethyl starch and its effect on cerebral microcirculation

The effect of haemodilution with Ringer's solution, hydroxyethyl starch (HAES) 200/0.5 10% and dextran 40 on cerebral blood flow (CBF) was measured in ten cats by means of the hydrogen clearance technique. As expected from theoretical considerations the haemodilution effect was short and not significant with Ringer's solution, but was 25% with HAES and 35% with dextrane 40. The corresponding rise in CBF was significant in both the latter but not with Ringer's solution. CBF was similarly highly correlated with diminution of haematocrit (Hct). The different effects of the substances on CBF could all be explained by their different effects on the Hct. Data analysis, together with recent literature, suggests that the dominating factor determining CBF was the O2-transport capacity, which in these experiments was in close relation to Hct. The results support the assumption that the increase of CBF by haemodilution is caused by a regulatory mechanism and not by a change of rheological parameters.

[Hemodilution in patients with stage IIb peripheral arterial occlusive disease: prospective randomized double-blind comparison of middle-molecular weight hydroxyethyl starch and low-molecular weight dextran solution]

Isovolemic hemodilution is a simple method of treating patients with peripheral arterial occlusion disease and hematocrit values of 43% or more. Dextran, a plasma substitute, has been used for that purpose since the early 1950s. However, the use of dextran in some diseases, e.g., hemorrhagic diathesis, kidney insufficiency, and microangiopathy is not without risk. Anaphylactic reaction, too, has occurred in some cases. Since the end of the 1970s hydroxyethyl starch, a plasma substitute, is available for the therapy of microcirculatory disorders and hemorrhagic diathesis. The side effects of hydroxyethyl starch are less serious and anaphylactic reactions are less frequent and moderate in extent. The clinical efficacy of both substances was compared in this survey. It could be stated that middle molecular hydroxyethyl starch is even clinically superior to low molecular dextran because the distance that the patients could walk increased significantly.