Free flap surgeries have been performed in reconstructive surgery since late 1950s

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Free flap surgeries have been performed in reconstructive surgery since late 1950s [1]. Compared with localized flaps, free flaps have both flexibility and diverse donor site choices which eliminate the need for secondary surgery compared with skin grafting. Additionally, it has better cosmetic outcome. As a consequence of advancements in microsurgery, injuries that would have been deemed unsalvageable earlier and subsequently obligatory to be amputated are now being successfully repaired [2–5]. Compared with skin grafting, free flaps can not only repair deep extensive damages such as wounds having bones or tendons exposed, but also can cover functional wounds such as injuries of hands. Free flaps can be of many kinds. A colossal advantage of free flap is that free tissue transfer is suggested in the repair of wounds that are favorably complex, comprise a large surface area, and that exclude the use of local, pedicled flaps [6]. Free skin flap has become an essential surgical method in the field of repair and reconstruction as exposure of vital structures such as joints, bones, nerves, or blood vessels additionally demand free flap coverage [4].Since the free flap was invented, blood flow disorders have been always a major problem for micro and reconstruction surgeons. With the in-depth understanding of microsurgery and the improvement of microsurgical technology, the success rate of flap surgeries has reached a very high level, the success rate of free flap transplantation reported in many literatures has reached above 90% [7-10]. However, the risk of blood flow disorders remains unchanged. Therefore, continuous and accurate blood flow monitoring is required in order to detect blood flow disorders in time so that necessary measurements can be taken to avoid further complications which are crucial. An ideal blood flow monitoring should not cause any damage to patients and tissue flaps, it should have a good sensitivity, should be accurate and reliable, should be able to achieve continuous monitoring, and can be applied to all easily. Currently, the main means of blood flow monitoring is to go through some conventional clinical assessment, to judge the skin color, turgor, capillary refill, temperature, bleeding time and other indicators [11-13]. There is, however, no completely reliable technique that has been adopted universally [13,14]. The only universal form of monitoring is clinical observation [14,15]. The accuracy and sensitivity of clinical assessment are high. Daniel Chubb and others in their largest reported series did one review in 1140 cases of free flap transplantation, blood flow monitoring was performed using clinical observation alone, and the results of 94 cases showed blood flow disturbances and exploration. Of these, 4 cases had no blood flow disorders from the pedicle, and 52 flaps survived completely after treatment, 7 cases of partial survival, 31 cases of complete necrosis [16]However, there are some disadvantages as well. The disadvantage of clinical assessment is that it is highly subjective. In addition, the method of clinical observation requires high observation experience and difficult to continuously monitor. As a result, it affects the quality of flap condition assessment. This not only increases the complexity of post-operative consequence, but also greatly reduces the effectiveness of monitoring [17]. Among several indicators the most commonly used and most important indicator in the clinic is the color of the flap surface. It has several advantages. It has good sensitivity, accuracy, and most importantly non-invasive and non-contact methods, which can be observed by naked eye as well as are reliable and convenient [18]. Furthermore, this method might be less dependable in the observing the buried or muscle flaps or in patients with highly pigmented skin [32].Tension detection is often performed to evaluate the flap condition; capillary refill is more commonly used in partial or total amputated finger replantation. These are observed often and considered important indicators of the consequences of a free flap or replanted finger. In addition, the invasive detection method is puncture procedure done by sterile syringe to observe bleeding time and the color of the blood. If the bled blood is bright red in color, we can be determined about the patency of the feeding vessels, indicating good blood flow; however, when the bled blood is patent and quick but darker in color, turns red gradually indicates the venous reflux disorders. Less bleeding, slow bleeding or no bleeding indicates arterial blood supply disorders.Temperature detection is rarely performed, because in most of the wards, an external lamp is used to maintain the temperature after the replantation surgeries or flap transplantations, and by detecting its temperature, it is not possible to find blood circulation in real time. Because there are many deficits in clinical assessments, it is of great value to achieve accurate and sensitive blood flow monitoring through instruments [11]. At present, instruments both during intra- and post-operative monitoring flap perfusion used in repair and reconstruction surgery are: laser Doppler [19-21], ultrasound Doppler flowmeter [22], microdialysis [24,25], tissue pH monitoring [23], and transcutaneous oxygen partial pressure, heat transfer method, and so on.The handheld ultrasound Doppler flowmeter is presently the most widely used auxiliary instrument as a clinical observation method; however, it’s more important role is to detect the perforators of the skin we plan to design the flap before surgery, the same reasoning can be used to detect blood flow in blood vessels after surgery. With Doppler flow meters, pulse signals of blood flow from the blood vessels can be heard. Arterial pulsating creates strong echoes, and veins create a continuous but weak echoes. It may seem to be easier to use; however, the disadvantage of the ultrasound Doppler flow meter is that the surgery becomes more technical and requires an experienced doctor or nurse to perform it. For example, it is easy to slipup the blood vessel signal as the pedicle of the flap, it is not easy to get the exact location quickly, so there is a possibility to have time wasted. However, the operability and accuracy are not that high in the application as the posture plays an important role to detect the perforators.Compared with the handheld ultrasound Doppler flowmeter, the implantable ultrasonic Doppler flowmeter is more accurate, sensitive, and achieves continuous monitoring [26, 27]. For implantable ultrasonic Doppler flowmeters, we need to fixate the probe near the pedicle of the blood vessel to monitor the arterial or venous blood flow. Generally, fixating the probe near the vein is more sensitive than monitoring the blood flow in a whole of the flap. A possible problem with using implanted Doppler ultrasound flowmeter to monitor the blood flow of the flap after surgery is that the Doppler ultrasound probe if not firmly fixated, it may move after surgery, resulting in false positive results. As well as in a short number of cases it was reported the association of pain with the implantable ultrasonic Doppler flowmeter [28].Near infrared spectroscopy (NIRS) is a method that can objectively and more accurately reflect the tissue oxygen saturation [29, 30]. Near-infrared light has a wavelength of 700 to 900 nm that can easily penetrate human tissues. Hemoglobin is basically the main absorption medium as well as the absorption spectra of oxygen, hemoglobin and deoxyhemoglobin are significantly different. Based on this principle, NIRS can detect the oxygen saturation level of different tissues. Moreover, NIRS can accurately and sensitively detect the flap’s blood flow detection method, and it can monitor embedded tissue flaps, which has greater advantages for bone flaps and muscle flaps, but relying on NIRS alone to monitor the tissue flaps reliability of blood flow needs to be performed in a large number of clinical trials [31].Color Doppler ultrasound is a non-implanted monitoring method that monitors the blood flow of the flap by detecting the blood flow rate and direction of the vascular pedicle [32]. Using this technique, a good judgment can be made about the anastomosis of the vascular pedicle of the flap. Color Doppler ultrasound is widely used in head and face reconstruction surgery, because this type of surgery is generally used for embedded tissue flaps, and it is safe and radiation free modality [33]. It can accurately and quantitatively monitor the blood flow of the vascular pedicle. Therefore, for cutaneous flaps or tissue flaps that may have blood flow disorders, a color Doppler ultrasound examination can avoid unnecessary exploration. However, it should be noted that when performing the ultrasound examination, the operator should accompany the patient for examination, because the ultrasound machine is very complex to use and the physician needs to know the position of the vascular pedicle and the anastomosis in order to make a judgment. As a result the whole monitoring process becomes very complicated and lengthy which is supported neither by the patients nor by the doctors.Microdialysis is an implantable, passive monitoring method. The microdialysis technique is to indirectly determine the blood flow status of the flap by sampling and analyzing the metabolites of the flap tissue [35]. The principle is to place a double-lumen microdialysis catheter in the flap tissue [35]. The catheter is connected to a semipermeable membrane. The isotonic fluid is pumped into the catheter, and the material is exchanged with the surrounding tissue through translucency. The concentration of the substance in the tissue can be obtained indirectly. Commonly monitored substances are glucose, lactic acid, pyruvate, and glycerol. If glucose is reduced, the ratio of lactic acid to pyruvate is reduced, which indicates anaerobic metabolism, indicating insufficient arterial blood supply. Increased glycerol levels suggest cell membrane damage, indicating arterial or venous blood flow disturbances. Microdialysis technology can detect vascular crisis before clinical manifestations [34]; however, the disadvantages are that the sampling time depends on the analysis time of the chromatographic method, and that samples cannot be divided and injected on to another chromatographic system in order to increase the number of analytes [35]. Additionally, microdialysis on conscious animals needs different instrumentation. At present, the application of microdialysis technology in the monitoring of blood supply of skin flaps is not being practiced enough, and the effectiveness of monitoring has been greatly different in the reported literature.Laser-Doppler flowmeter (LDF) is a frequently used technique for inspecting microvascular function [36]. This technique uses the optical Doppler effect to evaluate the changes in skin blood flow is frequently used as a test of skin microvascular vasodilator function [37]. LDF provides excellent temporal resolution. However there still remains some disadvantages as the spatial resolution provided by LDF lags behind.There are numerous methods reported for intra and post-operative flap monitoring which is discussed above and among them laser speckle contrast imaging (LSCI) allows a very fast assessment of an area of tissue, with a high spatial resolution [38]. When a wound is covered using microvascular free flaps, pedicled flap or other types of flaps, risk of tissue morbidity always remains, which depends on tissue perfusion supplied by the feeding vessels. Compromised circulation may end up causing post-operative loss of tissue viability or even revision procedures. It affects both the functional and aesthetic outcome of the surgery. LSCI have been currently a very popular method of monitoring the post-operative condition. It is reported to be accurate, sensitive and quick to investigate the tissue perfusion in real time. The major advantages of LSCI over most of the other methods are: 1) It does not need to be fixated to the skin and does not have any cable, letting it to be used in clinical conditions (e.g. wound healing) and simpler to use. 2) A larger area can be sampled and evaluated easily, thus lessening the known issues of vascular heterogeneity [39], and 3) After performing data collection, numerous sites can be chosen for analysis, allowing for better control of measurement options in post-processing and offline data analysis. In this study, Laser Speckle Contrast Imager (PeriCam PSI System, Perimed AB, Järfälla, Sweden) LSCI was used and we aimed to analyze if the variety of LSCI data can help in detecting flap necrosis.