[The new heparins].

The new, so-called molecular weight heparins (mean MW : about 5,000 daltons) are less heterogeneous than the old, fractionated heparins (mean MW: about 15,000 daltons). Both types of preparations contain molecules with affinity for antithrombin III (AT III) and anticoagulant activity, as well as other molecules without affinity for AT III but which are thought to contribute to the antithrombotic activity of heparin. The anticoagulant effect of low molecular weight heparins (defined as their ability to prolong the activated cephalin time) in 3 to 4 times lower than that of standard heparin, but their activity against the activated factor X (Xa) is approximately the same. The half-life of the anti-Xa activity of low molecular weight heparins is twice that of standard heparin. The bioavailability of low molecular weight heparins administered subcutaneously is greater than 90%, as against less than 30% with standard heparin. Clinical trails have shown that one single subcutaneous injection of low molecular weight heparin 2 hours before an operation, then every 24 hours reduces the incidence of isotopic venous thrombosis in general surgery at least as much as 2 daily injections of calcium heparinate 5000 I.U. Low molecular weight heparins are virtually devoid of the pro-platelet aggregation activity exhibited by standard heparin.