Continue OCs or hormone therapy
OCs and menopausal hormone replacement therapy produce measurable prothrombotic changes in the clotting system that appear to be directly related to the estrogen content. In theory, discontinuing the OC or hormone replacement therapy preoperatively would allow these changes to return to baseline and help prevent thromboembolic disease.
Although the risk of thromboembolic disease is 0.96% if patients are current users of OCs and 0.5% if they are not, studies have failed to confirm a clinical benefit of discontinuation.11 Further, the patient does not return to baseline for 4 to 6 weeks after ceasing therapy.
The potential risk of thromboembolic events also should be weighed against the risk of conception prior to surgery. We usually do not recommend discontinuation of OCs and hormone therapy before surgery, but give prophylaxis based on risk assessment.
Virchow’s triad: How intraoperative factors lead to clot formation
Venous stasis, vessel-wall trauma, and increased blood coagulability—the major contributors to perioperative DVT, known as Virchow’s triad—were identified more than 125 years ago.
Venous stasis. Intraoperatively, venous blood return from the lower extremities is reduced to less than half its normal rate,34 secondary to muscle relaxation during anesthesia, which causes venous dilation and reduced blood-flow velocity. Packing the abdominal contents may further impede blood return from the legs.
Resultant venous stasis causes platelet adhesion to the vein wall, followed by release of a thromboplastin-like substance that may trigger thrombus formation.
Blood flow increases in the immediate postoperative period with return of muscle tone, but remains significantly diminished for 21 days because of immobilization—specifically, lack of the usual pumping action of the leg muscles.
Vessel-wall trauma. Veins are highly likely to be damaged as vessels are skeletonized during major pelvic surgery, especially when malignancy is involved. Tissue injury activates the coagulation cascade by exposing blood to tissue thromboplastin (extrinsic path) and subendothelial collagen in the vessel wall, which activates factor XII (intrinsic path). Both pathways lead to conversion of factor X to its active form, factor Xa. Acting in concert with factor V, calcium, and phospholipids from platelet factor III, factor Xa catalyzes the conversion of prothrombin to thrombin. Thrombin regulates the conversion of fibrinogen to fibrin, the basic building block of a thrombus.
Increased blood coagulability. Clotting factors XI, IX, and VII increase following surgery, as do circulating platelets and platelet aggregation. This enhances coagulability, which persists from 72 to 96 hours after surgery but is usually balanced by the fibrinolytic system. Fibrinolysis is mediated primarily by plasmin, which digests fibrin and fibrinogen and activates factors V and VIII. If the fibrinolytic system is overwhelmed, the clotting system is unimpeded and thrombus formation may accelerate.
Pregnancy increases the risk of thrombosis, in part due to the progressive increase in resistance to activated protein C in the second and third trimesters. Risk is increased eightfold in women with inherited deficiency in any of the naturally occurring anticoagulants—antithrombin III, protein C, or protein S—compared with those with no deficiency.12
Preventive strategies
Two approaches to thromboembolic prophylaxis have been proposed, with prevention of fatal postoperative pulmonary emboli as a clear endpoint:
- Stratify a targeted group into levels of risk; then treat those at higher risk. Unfortunately, efforts to define risk have met with only partial success, due to limited availability of noninvasive screening, screening logistics, and expense. Further, specificity and positive predictive value of screening asymptomatic patients is low.
- Use prophylaxis in all patients in the targeted group, regardless of risk (TABLES 3 AND 4). This strategy seems effective. For example, in 2001, the Sixth American College of Chest Physicians Consensus Conference—the most recent of the consensus conferences—evaluated the risks of pulmonary embolus in 7,000 gynecologic surgery patients enrolled in prospective studies. Routine prophylaxis reduced fatal pulmonary emboli by 75%.13
The difficulty of defining patients at highest risk makes the concept of universal prophylaxis for a targeted group an attractive option unless a specific contraindication is identified.
TABLE 3
Risk stratification and prophylactic regimens
| LEVEL OF RISK | PATIENT CHARACTERISTICS | RECOMMENDED REGIMEN |
|---|---|---|
| Low | Less than 40 years of age | No specific recommendation for therapy |
| Undergoing uncomplicated minor surgery | Adequate hydration | |
| Requires less than 30 minutes of anesthesia | Aggressive early ambulation | |
| No additional risk factors | ||
| Moderate | Undergoing minor surgery with additional risk factors | Graduated compression stockings or SPCDs or |
| 40 to 60 years of age, undergoing minor surgery, and no additional risk factors | LDUH 5,000 U every 12 hours or | |
| Less than 40 years of age, undergoing major surgery, and no additional risk factors | LMWH (20 mg) or 2,500 U antifactor Xa once daily | |
| High | More than 60 years of age, undergoing minor surgery or with additional risk factors | LDUH 5,000 U every 8–12 hours or LMWH (40 mg) or 5,000 U antifactor Xa once daily |
| More than 40 years of age undergoing major surgery or with additional risk factors | SPCDs may be added | |
| Very high | More than 60 years of age, undergoing major surgery, with other risk factors such as cancer, prior venous thromboembolism, molecular hypercoagulable state, major trauma, spinal cord injury, or lower-extremity paralysis | Graduated compression stockings or SPCDs and LDUH 5,000 U every 8 hours or |
| Other risk factors such as cancer, prior venous thromboembolism, molecular hypercoagulable state, major trauma, spinal cord injury, or lower-extremity paralysis | LMWH (40 mg) or 5,000 U antifactor Xa once daily | |
| LDUH = low-dose unfractionated heparin; LMWH = low molecular weight heparin; SPCD = sequential pneumatic compression device | ||
| Note: Because perioperative prophylaxis is still evolving, these suggestions should not be considered inviolable. | ||
| Source: Modified from Geerts WH, et al.13 | ||