Paper I
2024 December (Supplementary) (2019 Scheme) · 100 marks · 180 min

Question

Mention two low molecular weight heparins. Explain their mechanism of action. What are the differences between unfractionated heparin and low molecular weight heparins. (1+2+3)

Q76 marksShort Essays

Answer

Two low molecular weight heparins: Enoxaparin, Dalteparin.

Mechanism of action LMWHs bind and potentiate antithrombin III, but their shorter chain length means they preferentially inactivate factor Xa, with much less effect on thrombin (IIa) compared to unfractionated heparin (which inactivates both roughly equally) — the smaller molecule cannot bridge antithrombin to thrombin simultaneously the way the larger unfractionated heparin chain can.

Differences between unfractionated heparin and LMWH

FeatureUnfractionated heparinLMWH
ActionInactivates both Xa and IIaPredominantly inactivates Xa
BioavailabilityVariable, unpredictableHigh, predictable
MonitoringRequires aPTT monitoringGenerally not required
DosingIV infusion, frequent adjustmentFixed-dose subcutaneous, once/twice daily
Half-lifeShorterLonger
Reversal by protamineCompleteOnly partial
Heparin-induced thrombocytopeniaHigher incidenceLower incidence

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