7.1 Efficacy in prophylaxis of VTE in surgical patients

A meta-analysis of RCTs of spinal/epidural block versus general anaesthesia reported reduction in postoperative morbidity and mortality (including VTE).⁶⁵ Evidence level 1+

Spinal or epidural anaesthesia may be preferred to general anaesthesia where appropriate and feasible.

7.2 Risk of vertebral canal haematoma when combined with pharmacological prophylaxis of VTE

The increasing use of pharmacological prophylaxis for VTE has raised concerns amongst anaesthetists that spinal or epidural block may be followed by an increased risk of vertebral canal haematoma. Most cases of vertebral canal haematoma occur spontaneously, at an estimated incidence of only one per million population per year,¹⁴⁶ often associated with disordered coagulation,¹⁴⁷ which is also one of the major causal factors in the cases reported after spinal/epidural block.¹⁴⁸ Although this is a rare complication, its serious nature requires that some precaution is taken to minimise its incidence.¹⁴⁹ Advice on the use of spinal/epidural block in patients receiving pharmacological prophylaxis for thromboembolism has been published.¹⁵⁰ Technical difficulty during instrumentation of the vertebral canal is an associated factor, with epidural block (especially with catheter insertion) carrying a greater risk than spinal anaesthesia. Catheter removal is also a time of risk.¹⁴⁸ The skill and experience of the anaesthetist may therefore be important.

In 1997-98 American reports documented over 40 cases of vertebral canal haematoma occurring within a five year period in patients receiving the LMW heparin, enoxaparin.^40,41 Most followed spinal or epidural block, although a few followed diagnostic lumbar puncture. The report also confirmed the risk factors identified in the earlier review:¹⁴⁸ epidural catheters carried the highest risk at both removal and insertion. However, this is in direct contrast to the European experience, where, although enoxaparin has been available for much longer, only two cases of vertebral canal haematoma have been reported. In a further study, the incidence of vertebral canal haematoma after spinal or epidural block in patients receiving enoxaparin was estimated at 1 in 2,250,000 in Europe, but at 1 in 14,000 in the United States.¹⁵¹ The main factor contributing to this variation was the difference in the recommended dose of enoxaparin used: 40mg once daily, starting 12 hours before surgery for high risk European patients, compared with 30mg twice daily, starting one hour after surgery for American patients. In the European patients, the peak effect of the drug (occurring at four - six hours post-administration) would be past at the time of block administration, minimising risk. In the American patients, although there would be no effect at the institution of a block, the long half life of enoxaparin (10-12 hours) could lead to accumulation, producing an overt effect on coagulation and leading to bleeding at the time of catheter removal.¹⁵²

7.2.1 ORAL ANTICOAGULANTS (see section 3.8)

Full anticoagulation is an absolute contraindication to spinal or epidural block. Most authorities recommend that the INR be 1.5 or lower for institution of a block or removal of a catheter.^153,154,155 In most cases, the patient with a higher INR who is to undergo major elective surgery will have it delayed so that coagulation can revert to normal. Management is as outlined elsewhere^6,156 (see also SIGN guideline on perioperative transfusion).¹⁵⁷

7.2.2 UNFRACTIONATED HEPARIN (see section 3.4)

Thromboprophylaxis with low-dose subcutaneous heparin (5,000 IU 8-12 hourly or 7,500 IU 12-hourly) does not usually prolong the APTT and large numbers of patients have received spinal or epidural block without sequelae.¹⁵⁸ However, a transient elevation of the APTT may occur¹⁵⁹ and some anaesthetists prefer not to institute spinal or epidural block within 4-6 hours of a dose, and to administer heparin after institution of the block.¹⁵⁰ Similar considerations apply to catheter removal. The platelet count should be checked before the block is instituted or the catheter removed if the patient has been receiving any heparin preparation for more than a few days, to exclude heparin-associated thrombocytopenia (see section 3.4.5).

7.2.3 LOW MOLECULAR WEIGHT HEPARINS

Evidence suggests that standard European dosing regimens (e.g. enoxaparin 20-40mg once daily) are not associated with any increased risk as long as the block is instituted, or the catheter removed, 10-12 hours after drug administration. A first dose can be given immediately after block administration or catheter removal.¹⁶⁰

7.2.4 ANTIPLATELET AGENTS (ASPIRIN)

There is little or no evidence that aspirin increases risk,¹⁶⁰ although interactions with other agents such as heparins or warfarins may occur.¹⁴⁹

7.2.5 RECOMMENDATIONS

When instituting spinal/epidural block prior to elective surgery, epidural catheter removal or diagnostic lumbar puncture, the following precautions should be taken: Aspirin: proceed normally, but remember interactions UFH: proceed normally but exercise caution or administer 4-6 hours before block or delay first dose until after block performed or until after surgery LMWH: administer 10-12 hours before block Warfarin: if INR <1.5 proceed normally if INR >=1.5 delay surgery or consider alternative anaesthetic or anaesthetic technique if surgery is urgent.

When there is difficulty or bleeding during the block procedure it is essential that this is recorded and greater vigilance ensured during the postoperative period. It may also be advisable to omit or delay the next dose of thromboprophylactic agent.

If perioperative pharmacological prophylaxis is to be omitted altogether in a patient who would normally receive it, a mechanical method should be used instead.

In patients requiring emergency surgery who have already received a thromboprophylactic drug, the agent used, the dose, and the time interval since the last administration should be noted and related to recommendations above.

Any decision should be based on the balance of risks and benefits, will often require discussion with the patient, and should be documented fully.

Management and prophylaxis of the patient in labour are considered in section 9.