Evaluation of Anti-Xa Levels in Surgery Patients Receiving Fixed Dose Heparin

The purpose of this study is to determine if fixed dose heparin infusions at a rate of 500 units/hour are sufficient to maintain a target anti-Xa of 0.1-0.35 IU/mL for venous thromboembolism (VTE) prophylaxis in patients undergoing microvascular surgery. Additionally, a pilot protocol has been developed to titrate these heparin infusions to ensure patients have sufficient VTE prophylaxis. All patients will be enrolled in the observational arm of the study and receive anti-Xa level monitoring. Patients with out-of-range anti-Xa levels will cross over to the interventional arm of the study and receive real time heparin infusion dose adjustments per the pilot protocol. The primary outcome measured will be the percentage of patients with anti-Xa levels in the target range of 0.1-0.35 IU/mL while on a heparin infusion at 500 units/hour.