The PubMed and Web of Science databases were searched up to November 2020. The following search terms were used: intraoperative, thrombosis, microvascular surgery, salvage techniques, patency, patency test and free flap. The selected articles were reviewed on title and abstract to determine study eligibility. Searches were supplemented by hand searching the reference lists of studies and through personal discussion. After reviewing the search through the databases, 11 articles were withheld. Two supplementary articles and one book were also used and cited.

Original articles fulfilling the following eligibility criteria are included: 1) the study involved a microvascular anastomosis; 2) the study evaluated intraoperative thrombosis; 3) any type of patency test and/or technical salvage procedure was performed; 4) the article was published in English, French, German or Dutch. The exclusion criteria were: a) studies that evaluated thromboses in vessels without anastomosis; b) studies that evaluated macrovascular thrombosis.

Intraoperative assessment of the anastomosis starts with the inspection of the vessel. After removal of the temporary clips, two types of pulsations of the artery distal to the anastomosis can be observed: expansional and longitudinal (see Figure 1(a)) [9]. Expansional pulsations enlarge an artery circumferentially and indicate a patent anastomosis. Longitudinal pulsations lengthen an artery and

indicate a partially or completely occluded anastomosis. Besides absence of expansional pulsations, many early occluded anastomoses exhibit a white plug protrusion pushed through the suture-line. In other words, protrusion of a white plug is a strong indication of an occluded anastomosis [9].

If there is doubt about the patency of the microvascular anastomosis, the following test can be done. With the flicker test, closed forceps are placed underneath the anastomosis and with gentle upward pressure the artery becomes more narrow (see Figure 1(b)). During systole, blood flow through the narrowed patent anastomosis can be visualized through the thin and translucent arterial wall under the microscope [9]. In an occluded anastomosis, no flow of blood will be seen through the microscope. If the flicker test is inconclusive, the milking test can be performed. The milking test (also known as “Acland’s test”) involves milking the recipient artery with microforceps to empty and refill it with flow through the anastomosis (see Figure 2(a) & Figure 2(b)) [9]. Two microforceps are held closely together to squeeze the artery distal to the anastomosis. The artery is stroked by sliding the distal forceps further distally to produce a blood free segment of several millimeters long. The proximal forceps are then reopened and blood fills the distal vessel through a patent anastomosis.

Venous microvascular thrombosis tends to manifest itself after the first 24 hours and is not often seen intraoperatively [10]. Therefore the salvage procedures will mostly be performed during a revision-surgery. Thrombosis of venous microvascular anastomosis is often due to two factors: i.e. lower blood flow

and more fragile and thinner vein wall with an increased possibility of iatrogenic damage during microvascular surgery. The previously mentioned patency tests are also used for venous anastomosis. To avoid iatrogenic endothelial damage, cautious tissue handling is extremely important in venous patency testing. The flicker test is less traumatic for the vein and therefore primarily preferred above the milking test.

When the thrombus is not in the direct proximity of the anastomosis, balloon extraction with the fogarthy catheter may be indicated [11]. The technique consists of passing the catheter through the thrombus, gently inflate the balloon and withdraw the catheter including the thrombus. The catheter should be inserted cautiously to avoid intimal irritation. The catheter can be advanced both proximally and distally to the anastomosis. The balloon should not be overinflated and the correct size should be chosen according to the vessel diameter (see Figure 7).

A microvascular anastomosis can have inadequate blood flow caused by other means than a thrombus. Altered blood flow can be the result of torsion of the vessel due to excessive traction during the procedure [12]. Torsion can be managed by further dissecting the vessel into the tissue that provides less traction and untwisting will follow automatically (see Figure 8). If the vessel remains twisted despite dissection, re-anastomosis should be performed. Elongation also leads to a decrease in vessel diameter and as with mild torsion, this can be corrected by dissecting into the tissue. Failure of the anastomosis can also be due to an erroneously placed suture passing through the back wall of the vessel. Removal of this suture is necessary. Inadequate blood flow due to pressure of the surrounding tissues must also be ruled out and treated accordingly. Vasospasm can lead to a decrease in blood flow and pharmacological agents such as papaverine have an important role in the management.

Intraoperative assessment of the anastomosis starts with the inspection of the vessel and the pulsation pattern. Flowchart 1 provides an overview of the steps undertaken in clinical patency testing. When removing the temporary clips, a suture-line bleeding can be observed due to needle holes and suture-line holes. This bleeding should stop quickly with the application of gentle pressure. When the blood leakage is managed, the anastomosis becomes clear to inspect the patency. Two types of pulsations can be noticed: longitudinal and expansional. Longitudinal pulsations, where the vessel is lengthened, in combination with a persistent suture-line bleeding are suggestive for a distal occlusion. Expansional pulsations, a circumferential enlargement of the anastomosis, is more indicative for a patent anastomosis. To determine the patency on pulsations pattern only is very hard and therefore, in practice, it should be verified with the flicker or milking test. In small fragile arteries and veins, flicker test is the preferred technique because the milking test is more likely to damage the endothelium and should be retained for the larger arteries in microvascular surgery. If the flicker test cannot clearly confirm a patent anastomosis, the milking test can be used to evaluate the patency.

Flowchart 1. Visual and tactile intraoparative patency testing.

There are two types of thromboses: a white plug and a red plug. Flowchart 2(a) gives a broad overview of which technique to use in what situation. The white plug consists of activated platelets and can easily be crumbled by gently squeezing the vessel wall with microforceps next to the suture line. White plugs can also be disassembled by in situ administration of abciximab, a glycoprotein IIb/IIIa receptor antagonist [13]. This is typically noticed as a white plug being protruded through the suture-line and forms itself mostly within the first 45 minutes after flow restoration [14]. Therefore the first 45 minutes are crucial and frequent intraoperative reassessment of the patency should be done. When the white plug formation remains unnoticed, it will continue to form a red plug. The red plug consists of the formation of a whole thrombus, including activated platelets, coagulation factors and blood cells. Whereas white plugs are platelet-rich and weakly adhesive, red plugs are fibrin-rich and strongly cohesive. The darker in color, the more difficult to disperse a plug will be, requiring some form of disassembly of the anastomosis and extraction of the thrombus [9].

Salvage technique of a red plug depends on the location of the thrombus. When the thrombus is located at the anastomosis, suture-line thrombectomy is the first option for both end-to-end/end-to-side anastomosis. If the thrombus is of significant size or excessive endothelium damage is suspected, anastomotic resection with complete re-anastomosis (either or not with vein graft interposition) is the preferred option to prevent recurrent thrombosis formation. When doing a resection of an end-to-end anastomosis, the thrombus and anastomosis are resected in total, the vessel ends are trimmed and hereafter reconnected. In an end-to-side fashion, the anastomosis is preferably redone using a different site on the recipient vessel because of injured endothelium [9]. Clip damage to the vessel wall is difficult to see, but even seemingly uninjured vessels likely have some degree of endothelial damage and therefore we suggest doing the re-anastomosis at a different site on the same or other recipient vessel. After intraoperative thrombus extraction, each vessel end should be irrigated with a heparin solution in order to prevent recurrent thrombosis. We suggest a heparin solution of 100 U/ml. Thrombectomy through a longitudinal arteriotomy is a valid option if the anastomosis is faultless with little endothelium damage and the small thrombus is located nearby the anastomosis. If the thrombus is located away from the anastomosis and beyond the surgeons reach, a balloon extraction with Fogarthy catheter is the choice of method. In the context of prevention, manipulation and damage of the endothelium increase the risk of developing an intraoperative thrombosis. Therefore, being very cautious with the endothelium throughout the whole microsurgical procedure is of utter importance.

A microvascular anastomosis can have inadequate blood flow caused by other means than a thrombus, see Flowchart 2(b). An erroneously placed stitch

(a)

(b)

Flowchart 2. (a) Thrombus occlusion salvage techniques; (b) No thrombus occlusion salvage techniques.

through the back wall of the vessel will cause a diminution in blood flow and is also thrombogenic. The misplaced stitch should be replaced accordingly. With the replacement of the stitch, inspection of the intima must be done to rule out intimal tear. Pressure on the pedicle can cause insufficient blood flow and the pressure should be relieved immediately. Torsion or elongation of the vessel due to excessive traction is treated either by dissection into the tissue or resection with complete re-anastomosis. Vasospasm can also be the cause of altered flow and is treated with pharmacological agents.