A. Anti-Factor Xa levels – Explanation
Heparin
There are two main types of heparin – unfractionated, ‘standard’ heparin or low molecular weight
heparin (LMWH). Heparins generally act by activating antithrombin III. Unfractionated heparin forms
a complex which inhibits thrombin, factors Xa, IXa, XIa and XIIa. LMWH however only increases the
action of antithrombin III on factor Xa
The table below shows the differences between standard heparin and LMWH:
| Standard heparin | Low molecular weight heparin
(LMWH) |
|
| Administration | Intravenous | Subcutaneous |
| Duration of action | Short | Long |
| Mechanism of action | Activates antithrombin III. Forms a complex that inhibits thrombin, factors Xa, IXa, Xia and XIIa |
Activates antithrombin III. Forms a complex that inhibits factor Xa |
| Side-effects | Bleeding
Heparin-induced thrombocytopaenia (HIT) |
Bleeding Lower risk of HIT and osteoporosis with LMWH |
| Monitoring | Activated partial thromboplastin time (APTT) | Anti-Factor Xa (although routine monitoring is not required) |
| Notes | Useful in situations where there is a high risk of bleeding as anticoagulation can be terminated rapidly |
Now standard in the management of venous thromboembolism treatment and prophylaxis and acute coronary syndromes |
Heparin-induced thrombocytopaenia (HIT)
- immune mediated – antibodies form which cause the activation of platelets
- usually does not develop until after 5-10 days of treatment
- despite being associated with low platelets HIT is actually a prothrombotic condition
- features include a greater than 50% reduction in platelets, thrombosis and skin allergy
- treatment options include alternative anticoagulants such as lepirudin and danaparoid
Both unfractionated and low-molecular weight heparin can cause hyperkalaemia. This is thought to
be caused by inhibition of aldosterone secretion.
Heparin overdose may be reversed by protamine sulphate, although this only partially reverses the
effect of LMWH.
