Cardiac surgery patients are at increased risk for venous thromboembolism (VTE). Prevention is the most critical strategy to reduce VTE-associated morbidity and death. However, there is a lack of data on the optimal approach to VTE prophylaxis in this population of high-risk patients. This study aimed to assess whether the standard dose of enoxaparin, the subcutaneous injection of 40 mg of enoxaparin daily, achieves adequate anti–factor Xa (aFXa) levels for VTE prophylaxis in patients after open heart surgery.

All patients with open heart surgery with cardiopulmonary bypass from August to December 2022 who received at least 3 consecutive doses of subcutaneously administered enoxaparin were included in the study. Patients receiving therapeutic anticoagulation, patients who underwent cardiac transplantation or placement of ventricular assist device, and patients with renal insufficiency were excluded. Serum aFXa was measured 0.5 to 1 hour before the fourth dose to attain the steady-state trough levels.

Data were completed for 44 patients. The target aFXa level was between 0.10 and 0.20 IU/mL for the avoidance of both underanticoagulation (≤0.10 IU/mL) and overanticoagulation (>0.20 IU/mL). The mean was 0.049 IU/mL with SD of 0.026 IU/mL, which was statistically significantly lower than the lower end of the target aFXa values (0.10 IU/mL; t₄₃ = −13; P < .001; d = −1.9; 99% CI, −0.059 to −0.043).

The daily subcutaneous administration of 40 mg of enoxaparin leads to subprophylactic aFXa levels for most patients who undergo cardiac surgery. Further studies on the clinical relevance are warranted.