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Thrombosis is a common source of failure in anastomoses, flaps, and vascular grafts. Ensuring vessel patency is critical to the success of microvascular procedures. Any tool that can accurately predict the patency of an anastomosis intraoperatively would enable the surgeon to detect and correct flow restrictions while the patient is still in the operating room.The authors used transit-time ultrasound technology in this pilot study to investigate whether a minimal cutoff value for quantitative postoperative blood flow (in milliliters per minute) could be established that would reliably predict sustained vessel patency at 24 hours postoperatively. Surgical end-to-end anastomoses were performed on 56 Sprague-Dawley rat femoral arteries with diameters ranging from 0.6 to 1.2 mm. Postoperative blood volume flow measurements were taken at 20-minute intervals up to 1 hour, and then again at 24 hours, to assess patency.Forty-seven anastomoses (83.9 percent ) were patent 24 hours after surgery. Nine anastomoses (16.1 percent ) thrombosed within 24 hours. Based on a receiver operating characteristic curve analysis, the optimal cutoff value for immediate postoperative flow for predicting thrombosis within 24 hours of microvascular anastomosis is 0.21 ml/minute.At 20 minutes postoperatively, blood flows greater than 0.30 ml/minute are highly suggestive of patency, and flows less than 0.21 ml/minute are highly suggestive of failure. The authors therefore recommend a minimal cutoff flow value of 0.30 ml/minute for vessels ranging from 0.6 to 1.2 mm in diameter to predict long-term postoperative vascular patency.