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Introduction: Unfractionated heparin (UFH) has been used for decades to prevent thrombotic events during vascular surgery. Although it is known that UFH has a complex and nonlinear pharmacokinetics, with great individual variability, anticoagulation monitorization in vascular surgery is not routine and a standard empirical dose is often used. Activated clotting time (ACT) has been shown to be a simple, reliable and inexpensive way to monitor UFH anticoagulant effect, being routinely used during cardiac surgery. However, heparinisation remains a dilemma in vascular surgery and few studies emphasized the role of anticoagulation monitoring in this setting. Objectives: To investigate whether a fixed heparin dose of 5000 IU in arterial vascular surgery results in adequate and homogeneous heparinisation in all patients. Secondary endpoints: to identify preoperative factors for heparin response, intraoperative events and outcomes. Methods: This observational prospective pilot study included 30 consecutive patients undergoing arterial vascular surgery. ACT monitoring was performed before clamping and at 3, 30 and 60 minutes after 5000 IU UFH bolus. Preoperative and intraoperative data were also accessed. A target ACT of ≥ 200 s was set, taking in account of the lowest ACT value admitted by vascular surgery recommendations. Results: The average ACT value increased to 210.20 ± 28.82 s (1.61 ± 0.25 times vs baseline) 3 minutes after bolus, then declined to 191.60 ± 21.86 s and 173.4 ± 21.37 s after 30 and 60 minutes, respectively. Three minutes after UFH bolus, 53% patients had ACT ≥ 200 s, decreasing to one third and 7% at 30 and 60 minutes, respectively. Even when weight-based, a correlation between heparin dose per kilogram and ACT change was not found (r = 0.187; p = 0.322). There was also no correlation between ACT values and preoperative hemoglobin, platelet count, creatinine clearance or INR. There was a positive correlation between preoperative aPTT and intraoperative ACT measurements (r = 0.432; p = 0.017). There was no difference between ACT values and previous antithrombotic/anticoagulant therapy and between intraoperative ACT and intraoperative blood loss. Conclusions: This study confirms that administrating a fixed or even a weight-based heparinisation is insufficient to provide consistent anticoagulation levels in all patients. Perioperative anticoagulation should be monitored and ACT-based. Larger clinical RCT's are warranted.