Background: No clear recommendations exist regarding the optimal dosing of unfractionated heparin (UFH) during vascular surgery. Moreover, little is known about the effect of the UFH bolus downstream of the arterial clamp, where stasis and inflammation can possibly alter the anticoagulation obtained. Methods: The aim of our prospective observational study was to assess anticoagulation below the arterial clamp and its clinical impact on the quality of revascularization. Thirty-six patients American Society of Anaesthesiologists physical status (ASA) grade I-III undergoing open revascularization surgeries were included. A baseline activated coagulation time (ACT) was obtained. Thirty minutes after a single bolus of 5,000 units of UFH, we measured an upstream ACT via a radial arterial catheter and an ACT below the arterial clamp via surgeon sampling. The quality of revascularization was assessed with preoperative and postoperative ankle-brachial and toe-brachial indexes (TBIs). Results: The upstream postheparin ACT was significantly higher than the downstream postheparin ACT, with a mean difference of 24.3 sec (P < 0.0001). In 7 patients, the downstream ACT was lower than the baseline ACT. The upstream and downstream heparin concentrations were similar. There was no relationship between the downstream ACT and either ankle-brachial index improvement (28 patients, P = 0.51) or TBI improvement (27 patients, P = 0.21). Conclusions: Our study demonstrates a significant difference between the ACT above and below the arterial clamp without any clinical impact of this possibly insufficient anticoagulation. Further investigations are warranted to determine the optimal dose of UFH in vascular surgery. Trial registration: Clinicaltrials.gov, NCT02477072.