Clinical application of perforator flaps has gradually become popular in reconstruction surgeries since proposed in 1989(1). The perforator flap refers to a kind of flap that is vascularized by small-diameter vessels which passes either through or in between the deep tissues (mostly muscle)(7). It belongs to the category of axial vascular flaps. The application conforms to the principle of tissue transplantation-"well repair of recipient site, less damage to donor site". This kind of flap has the advantage of flexible design and fast recovery, which makes it suitable for the reconstruction of tissue defects. It is widely used in various repair and reconstruction operations to achieve a better aesthetic appearance and physiological function. As the number of flap operations keeps increasing, flap extraction and transfer techniques are improving continuously, though blood flow dysfunction still remains the most common postoperative complication-up to 7% according to Salvatore, et al(8). Improper treatment may eventually lead to flap ischemic necrosis. The causes of postoperative blood flow dysfunction include arterial, venous, and microcirculation. Arterial compromise is a situation that progresses rapidly. It is more harmful than venous congestion and must be dealt with urgently. In contrast, venous crisis is relatively easier to identify and progresses more slowly, also the affected tissue may have a greater survival rate. The main reasons for venous failure are mechanical obstruction, anatomical variation, or technical errors due to inadequate length pedicle, inadequate venous drainage, compression and kinking of the vein(9). The occurrence of venous congestion is due to insufficient venous return, resulting in stasis in the affected flap, which leads to platelet and fibrinogen accumulation and microcirculation intravascular thrombosis(10). All of these changes in microcirculation may persist even after the mechanical obstruction is relieved. If not treated in time, it will occlude the capillaries and inflow arteries. Ischemic necrosis will be inevitable. In these cases, reconstruction of physiological venous outflow is essential to avoid continuous pathophysiologic changes in microcirculation. Once the mechanical obstruction is lifted, different non-surgical methods could be used as salvage treatments for congested flaps, such as anticoagulant therapy (heparin and prostaglandin E1), manual massage to encourage drainage of the congested blood, placement of external catheters for venous drainage, subcutaneous injection of low-molecular-weight heparin or recombinant tissue plasminogen activator, blood-letting with intermittent needle puncture, and the prescription of medical leeches(11). However, the salvage rate of non-surgical methods above is less than 80% according to Yu, et al(11).
In recent years, NPWT has been increasingly used for wounds since it was first come up with in 1997 by Morykwas and Argenta. They presented the conclusion that 125 mmHg subatmospheric pressure could increase the blood flow fourfold and decrease tissue bacterial count with an animal based experiment, both continuously and intermittently(3). Up to date studies have pointed out that NPWT promotes wound healing mainly from two aspects, fluid-based and mechanical way(12). It increases tissue perfusion and improve microcirculation by increasing capillary caliber and density(4, 13). The existence of mechanical forces, as well as change in endothelial morphology and increase in blood flow stimulates endothelial proliferation, capillary budding and angiogenesis(14), promoting granulation tissue formation(13). Previous studies also have shown that NPWT may promote the formation of small blood vessels by changing the microstructure of capillaries and endothelial cells, promoting the vitality of flaps, and thereby increase blood flow reversely(15). Moreover, scholars have found that negative pressure creates a hypoxia-sensitive environment, which promotes the early nonspecific immunity of neutrophils(16) in the inflammatory phase and increases tendency of collagen formation(17) in the proliferation phase. Besides, it also promotes collagen fiber production and increase the tensile strength and stability of wounds, substantially promoting the whole process of healing. On the other side, continuous drainage by NPWT can reduce the occurrence of hematoma and effusion in order to eliminate dead space and reduce infection by reducing excess interstitial fluid(18). This physically results in a decrease in interstitial pressure, which helps the capillaries reopen and restores the flow around(12). Drainage of fluid also reduce bacteria colonization, avoiding wound infections. An animal based study pointed out that the application of NPWT in patients who had a higher risk of incision dehiscence, nonunion and infection could reduce the occurrence of postoperative complications and help incision healing(13). Another randomized controlled study(18) has confirmed on patients that an increase in skin perfusion could be noted on day 5 of NPWT treatment, allowing earlier removal of drains and enhancing the skin perfusion on the repaired skin, which significantly reduced the amount of fluid collected, and more importantly, complications and days of hospital stay.
Techniques regarding NPWT have been developing, though the way of applying is still unsettled, especially on its mode. Series of trials have confirmed that the external pressure exerted by NPWT can mechanically squeeze the stagnated venous blood and tissue fluid to relieve congestion and edema, but may also cause arterial ischemia(19). These findings arose novel ways of application, such as lower magnitude(20), delayed setting(21), sparing the pedicle part(22), etc. In our cases, we choose to apply the NPWT over the congested area, sparing the pedicle area as an observation window to monitor the skin color and temperature, so that we could notice if any other complications happened such as artery insufficiency. As for NPWT modes, according to literatures, continuous mode still remains priority in the application of NPWT considering wound therapy. However, Morykwas and Argenta found an increase in granulation tissue formation in both continuous and intermittent application but a more rapid deposition in the latter one(p ≤ 0.01) (3), although in an animal based study in 2013, Kangwoo N Lee, et al found all NPWT modes despite of continuous, intermittent or cyclic showed more reduction in wound volume compared with the no-pressure group and there was no statistical significance (23). In 2018, A. Sogorski, et al supplemented this conclusion by measuring local pressure to the underlying tissue and microcirculatory changes such as blood flow, oxygen saturation, relative hemoglobin content and red blood cell velocity on 2 intermittent suctions. Their results supported that repetitive triggering brought by intermittent NPWT mode could enhance the fundamental physiologic response to induced shear-stress, which promoted the release of growth factors(24) and therefore increase granulation tissue formation, compared to continuous mode with single triggering(12, 25). Hence, it is obvious that intermittent mode is taking over more stage on wound therapy. It contains a suction part along with a release part and repeats as settled. In the suction part, drainage by NPWT appears as one of the major effective factors among others while in the subsequent part, releasing of tissue allows oxygen-rich blood from artery to nourish the flap. A recent study showed that blood flow rose over time in intermittent mode and even sustained after the last suction ended. Researchers assumed that a compressed vasculature post-occlusive reactive hyperemia (PORHA), also known as reactive hyperemia might be an explanation for the increase in perfusion after suction was turned off(25, 26). Therefore, in our cases, we tried to apply NPWT on perforator flaps with venous congestion in an intermittent mode, which turned out to be an effective treatment.
Many scholars have found that NPWT has a good therapeutic effect on venous congestion after flap operation. Vaienti, et al applied NPWT on pedicle and free flaps for reconstruction of lower limbs after trauma. After NPWT removal, viable granulation tissue was observed and the wound could be treated with a partial- thickness skin graft in all cases(27). Qiu, et al used NPWT as a salvage method on pedicle and free-flaps on multiple reconstructions including defects after tumor, defects with prosthesis or bone exposure, and chronic wounds. They removed the furthest stitches and applied NPWT on a raw surface. As a result, all flaps survived(20). Uygur, et al saved an anterolateral thigh fasciocutaneous flap with two cycles of VAC therapy–an early version of NPWT, in which case venous congestion disappeared mostly 72 hours later(22). However, none of the above have ever reported NPWT application along with mesh incisions on the perforator flap with venous congestion. Referring to literatures reported, we decided to apply NPWT on flaps threatened by venous congestion, as soon as a sign of compromise was noted and right after exploration was done. At present, the selection of NPWT pressure is still not unified, ranging from − 50 mmHg to -150 mmHg, and the treatment mode is mainly continuous while some researchers also use intermittent treatment mode(11). Our current plan is intermittent treatment with a -125 mmHg pressure. The choice of treatment mode and pressure should be further studied. And as a salvage treatment, considering flap perfusion under negative pressure might increase around 5 days, we decided to apply NPWT for at least 5 days, also depending on the improvement of congestion.
As a salvage treatment for post-surgery complications including flap venous congestion, NPWT plays an important role with a high success rate of 96.9%(11). Although, we still consider prevention as the first priority. Before surgery, radiological examination like CTA or MRA is recommended if possible. Studies have shown that preoperative CTA can identify atypical venous connections between deep and superficial systems that increase the risk of postoperative congestion five-fold(28). Identifying atypical venous connections maximizes the chances of flap survival and minimizes complications. Besides, comorbidities like diabetes, hypertension disease, peripheral vascular disease, hypoproteinemia, anemia, etc. should be evaluated before. During the operation, surgeons should dissect the vascular pedicle completely and avoid pulling or shearing during the transfer and rotation of the flap, which may affect the blood supply of the flap. Also, precise operation is needed, to reduce damage around the vascular pedicle. When the surgery is finished, doctors and nurses should provide expert intensive care in the early postoperative period: monitor if any oozing of dusky blood occurs, notice any changes in skin color and temperature, check the blood flow by puncture tests, and take photos regularly if possible(20).
Current diagnosis of venous congestion mainly depends on clinical evidence including cyanotic skin color, cool temperature(༞2ºC of difference compared to control), rapid capillary refill, increased tissue turgor, and rapid dark bleeding in response to a puncture(5, 6). Once a venous issue occurs, early treatment is of great urgency. Doctors should exclude mechanical obstruction such as hematoma under the flap and venous thrombosis or any other possible factors by surgical intervention. If the obstruction is not relieved, the blood supply of the skin flap is difficult to recover. We have also tried to use NPWT in patients with failed recanalization of venous thrombosis, but none of them were successful. However, contraindications of surgical intervention do exist at some point. Examples are, venous compromise after propeller flap reconstruction with sizeable perforators; venous congestion after dividing the skin bridge in pedicle flaps; venous congestion after non-physiological flap reconstructions; and other patient conditions for which surgical intervention is contraindicated, such as patient’s severe comorbidities, refusal of further surgeries, or tortuous venous anatomy(20). In our cases, we used a mesh incision as an exploration and observation method. Short cuts through full-thickness skin created an observation window from which we could carefully determine whether the blood color was bright red or dull and the fat tissue necrosis or not to evaluate the circulation under the skin. This procedure is similar to blood-letting method, which has been a mature treatment for venous stasis(11). We combined this method with NPWT here was because besides checking the blood circulation, this kind of mesh incision that aimed for blood-letting also had a function of skin tension releasing. It allowed off-loading releasing without removing the suture between flap and recipient site, which ensured the fixation of flap and accelerated healing, shortening hospital stay. It increased the area of fluid drainage, helping release of tissue edema, moreover, it also ensured a sufficient contact area between NPWT device and wound bed, which was vital since we expected the negative pressure could transmit into the deep structure to fully develop it capacity.
In conclusion, intermittent NPWT can be used as a non-surgical treatment for patients with venous congestion with a high success rate and few complications. A mesh incision before may help with observing and tension releasing. We believe that early application of NPWT along with a mesh incision may improve the survival rate of the flap. However, patient’s discomfort should be taken into consider since it may influence patient’s compliance for the treatment, thereby affect the efficacy of this method. It is also necessary to determine the effectiveness in further clinical practices such as different implementations on specific kinds of flaps. Moreover, mechanism in this process, contraindications and personalized mode needs further study.