Why is heparin (unfractionated heparin (UFH) or low molecular weight heparin (LMWH)) administered before high-risk surgery?

Why Heparin is Administered Before High-Risk Surgery

Heparin (both unfractionated heparin [UFH] and low molecular weight heparin [LMWH]) is administered before high-risk surgery primarily to prevent venous thromboembolism (VTE), which significantly reduces mortality and morbidity in surgical patients. 1

Mechanism and Rationale

Heparin works by activating antithrombin (AT), which inhibits thrombin and factor Xa in the coagulation cascade. This prevents the formation of blood clots during and after surgery when patients are at highest risk due to:

• Immobility during and after surgery
• Hypercoagulable state induced by surgical trauma
• Venous stasis during prolonged procedures
• Cancer-associated hypercoagulability (in oncologic surgeries)

Timing of Administration

The timing of heparin administration depends on the type:

• UFH: Usually given 2 hours before surgery at a dose of 5,000 IU subcutaneously and continued every 8-12 hours postoperatively 2
• LMWH: Can be administered:
  • 12 hours before surgery
  • 12-24 hours after surgery
  • 4-6 hours after surgery (starting with half-dose, followed by usual high-risk dose the next day) 1

Effectiveness in Preventing VTE

Heparin prophylaxis significantly reduces VTE risk:

• Meta-analyses show heparin prophylaxis reduces VTE incidence from 30.6% to 13.6% in surgical patients 1
• LMWH provides approximately 70% relative risk reduction for all thrombi and proximal vein thrombi compared to placebo 1
• Prophylaxis with heparin is particularly important in high-risk surgeries such as orthopedic, abdominal, pelvic, and cancer surgeries 1

Choice Between UFH and LMWH

LMWH has several advantages over UFH:

• Once-daily administration versus 2-3 times daily for UFH
• Better pharmacokinetic profile
• Lower risk of heparin-induced thrombocytopenia
• More predictable dose-response relationship
• Less binding to plasma proteins, macrophages, and endothelial cells 1

A meta-analysis of randomized studies in elective hip surgery showed LMWH was superior to UFH with:

• 60% reduction in proximal venous thrombosis (OR 0.40)
• 30% reduction in total venous thrombosis (OR 0.70)
• 70% reduction in pulmonary embolism (OR 0.30) 1

More recent evidence from trauma patients shows LMWH is superior to UFH in reducing:

• Mortality (OR 0.64)
• Overall VTE events (OR 0.67)
• Pulmonary embolism (OR 0.53)
• Deep vein thrombosis (OR 0.73) 3

Duration of Prophylaxis

• For general surgery: At least 10 days postoperatively 1
• For major abdominal or pelvic cancer surgery: Extended prophylaxis up to 30 days postoperatively reduces VTE risk by 60% without increasing bleeding risk 1
• For orthopedic surgery: Prophylaxis should continue until the patient is fully mobile 2

Special Considerations

Bridging Therapy

For patients already on oral anticoagulants requiring surgery:

• LMWH bridging is started 36-48 hours after the last dose of warfarin
• Last pre-operative LMWH dose should be given approximately 24 hours before surgery
• Half the total daily dose is recommended the day prior to surgery
• Resumption should occur at least 24 hours after surgery for low-to-moderate bleeding risk procedures, and 48-72 hours for high bleeding risk procedures 1

Neuraxial Anesthesia

When spinal/epidural anesthesia is planned:

• The first dose of LMWH should be delayed until after epidural catheter removal
• If not feasible, the catheter should be removed at least 8 hours after the last LMWH dose
• Avoid other drugs that impair hemostasis (e.g., NSAIDs) 1

Common Pitfalls to Avoid

1. Inappropriate timing: Administering heparin too close to surgery can increase bleeding risk; too late may not provide adequate prophylaxis
2. Overlooking renal function: LMWH is cleared renally and should be dose-adjusted or avoided in patients with severe renal impairment
3. Failure to recognize high-risk patients: Not providing prophylaxis to patients with multiple risk factors (age >60, cancer, previous VTE)
4. Inadequate duration: Stopping prophylaxis too early, especially in high-risk patients
5. Overlooking contraindications: Active bleeding, severe thrombocytopenia, or recent neurosurgery

Heparin prophylaxis is now standard of care for most patients over 40 years of age undergoing major surgery and for younger patients with a history of venous thromboembolism 4.