Enoxaparin Dosing and the Prevention of Venous Thromboembolism in Plastic Surgery Patients

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Pannucci et al.1 believe that inadequate enoxaparin dosing causes “breakthrough” cases of venous thromboembolism. Unfortunately, their study design is flawed. One cannot compare 90-day venous thromboembolism risk by anti–factor Xa levels while simultaneously giving extra enoxaparin to patients with low levels. Higher doses for these individuals would theoretically remedy the low anti–factor Xa blood levels. Two studies would be needed, one to compare venous thromboembolism risk by anti–factor Xa level, and another to evaluate whether extra doses reduce risk. (Otherwise one could just as easily conclude that higher enoxaparin doses, not lower anti–factor Xa levels, increase the venous thromboembolism risk.) Regardless, the findings do not support the efficacy of additional dosing; all five venous thromboembolisms [five of 49 (10.2 percent)] occurred in the group that received higher doses of enoxaparin.1
Importantly, three of the five venous thromboembolisms were upper extremity thromboses in patients with central catheters.1 These secondary thromboses have a different cause (foreign body and intimal trauma2) related to the catheter,2 as opposed to venous stasis and valvular hypoxia.3 The venous thromboembolism literature typically evaluates primary venous thromboembolisms that originate in the lower extremities.4,5
Excluding the upper extremity thromboses, the 2.1 percent (two of 94) frequency of venous thromboembolism is similar to the 1.2 percent incidence (same for control and anticoagulated patients) among 3334 plastic surgery inpatients previously reported by Pannucci et al.6 The small sample size (n = 94) precludes any meaningful comparisons. Confounding variables include diagnosis (particularly cancer), procedure, anesthesia method, body mass index, central catheters, immobilization, length of hospitalization, and duration of enoxaparin administration (range, 1 to 40 days).
Testing for anti–factor Xa levels requires additional expense and inconvenience,1 but provides no diagnostic information regarding venous thromboembolisms. The authors1 dismiss ultrasound screening, referencing the 2012 American College of Chest Physicians guidelines.5 This low-grade (2C) recommendation was made for general and abdominal-pelvic surgery patients.5 The authors’ reluctance to take advantage of this highly accurate7 technology is puzzling. It is preferable to detect a small distal thrombosis of minimal or no clinical significance than to miss a deep venous thrombosis that may otherwise propagate undetected and cause a fatal pulmonary embolism. Clinical signs of venous thromboembolism are notoriously inaccurate (20 to 35 percent of clinical diagnoses are confirmed by ultrasound or venography7). Reliance on limb swelling is dangerous because, as the authors note,1 10 percent of symptomatic pulmonary embolisms present with sudden death.4
The authors1 claim that inadequate enoxaparin dosing “predicts” venous thromboembolism risk, despite the fact that 90 percent (44 of 49) of patients with low peak anti–factor Xa levels did not develop venous thromboembolisms, or 96 percent (47 of 49) if upper extremity thromboses are excluded. The authors1 cite professional guidelines to support risk stratification and chemoprophylaxis. Evidence-based medicine recognizes factual data, not institutional authority.8 Pannucci et al.9 recently found that the risk of venous thromboembolism is not significantly different (p = 0.08) when comparing patients by Caprini scores, and the bleeding risk is significantly increased by chemoprophylaxis (p = 0.02). Three study patients (3.2 percent) experienced bleeding, and one required a laparotomy.1 The sample size is too small to support the authors’ claim that extra doses of enoxaparin do not increase the risk of bleeding.
Enoxaparin is approved by the U.S. Food and Drug Administration for preventing venous thromboembolisms in joint replacement and general surgery patients,10 not plastic surgery patients. The recommended dose for venous thromboembolism prophylaxis in abdominal surgery is 40 mg subcutaneously once daily.11 Higher doses may represent an investigational use,12 exceeding the scope of off-label use. Numerous law firms solicit clients who have experienced bleeding complications, alleging that patients are not adequately informed of the risks.

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