Page 386 - Essential Haematology

372 / Chapter 27 Thrombosis and antithrombotic therapy

Table 27.5 Comparison of unfractionated heparin with low molecular weight heparin.

Unfractionated heparin Low molecular weight heparin
Mean molecular weight 15 4.5
in kilodaltons (range) (4 – 30) (2 – 10)
Anti - Xa : anti - IIa 1 : 1 2 : 1 to 4 : 1
Inhibits platelet function Yes No

Bioavailability 50% 100%
Half - life
intravenous 1 hour 2 hours
subcutaneous 2 hours 4 hours
Elimination Renal and hepatic Renal
Monitoring APTT Xa assay (usually not needed)

Frequency of heparin - induced High Low
thrombocytopenia
Osteoporosis Yes Less frequent

APTT, activated partial thromboplastin time.

enzymatic or chemical depolymerization of unfrac-
Heparin

activates tionated heparin. They have a greater ability to
inhibit factor Xa than to inhibit thrombin and
Antithrombin
interact less with platelets than standard heparin,
and so may have a lesser tendency to cause bleeding.
XI XIa

They also have greater bioavailability and a more
IX IXa
prolonged half - life in plasma, making once - daily
administration in prophylaxis or treatment feasible
VIII Inactivates
(Table 27.4 ).
PF3 X Xa
V
Indications
Prothrombin Thrombin
Heparin (usually LMWH) is routinely used in
Fibrinogen Fibrin DVT, PE and unstable angina pectoris. It is also
widely used in the prophylaxis of venous thrombo-
Figure 27.6 The action of heparin. This activates sis and is the drug of choice for women requiring

antithrombin which then forms complexes with anticoagulation in pregnancy because it does not
activated serine protease coagulation factors cross the placenta. It is also used during cardiopul-
(thrombin, Xa, IXa and XIa) and so inactivates them. monary bypass surgery, for maintaining the patency
of indwelling venous lines and in some case of DIC
irreversibly. In addition, heparin impairs platelet if the manifestations are predominantly vaso -

function. occlusive. There is some evidence that LMWH (or
Low molecular weight heparin (LMWH) prepa- warfarin) may improve the survival in cancer

rations (MW 2000 – 10 000) are produced by patients over and above the protection from venous