Review Article

European Journal of Trauma

Microcirculation Research for Microsurgeons: Basic Concepts and Potential Applications Johannes Frank1, Vijay S. Gorantla2, Gary L. Anderson3, Luis Laurentin-Perez2, Claudio Maldonado2, John H. Barker2 Abstract The primary purpose of this review is to introduce the techniques of modern microcirculation research to reconstructive microsurgeons and trauma surgeons. When applied to clinically relevant situations, these techniques are powerful tools to elucidate the mechanisms regulating success or failure of a reconstructive procedure. Several phenomena at the microcirculatory level can have significant influence on the outcome of free flaps or replants. Most importantly these are ischemia/reperfusion injury, problems with wound healing and failure of downstream microcirculation due to thrombosis at the vascular anastomosis. Traditional research methods to assess free flap or replant failure utilize patency of the repaired vessel as the principal parameter. This all-or-nothing measurement provides little information about the dynamic mechanisms regulating thrombosis and microcirculation in general. Modern microcirculation research techniques, in contrast, permit direct visualization with qualitative and quantitative analysis of microcirculation in vivo. Applying these modern research techniques in model systems that simulate clinical reconstructive microsurgery has the potential to expand our understanding of the relevance of blood flow and its alterations to clinical outcomes. To enable such an understanding, this review focuses on elucidating the basic nomenclature used in microcirculation research in context of microvascular problems clinically encountered by reconstructive and trauma surgeons.

Key Words Microcirculation · Ischemia/reperfusion · Angiogenesis · Wound healing · Thromboembolism · Microvascular surgery Eur J Trauma 2001;27:153–62 DOI 110.1007/s00068-001-1151-2

Introduction Reconstructive microsurgeons and trauma surgeons use highly refined surgical techniques to transplant or replant a variety of different tissues for coverage of large tissue defects or restoration of function. Once a tissue is transplanted into its recipient site and its neurovascular supply is reattached, success is absolutely dependent on continuous arterial inflow and venous outflow through the microvascular anastomoses until neovascularization can be established by peripheral ingrowth of other blood vessels. Many phenomena at the microcirculatory level can significantly influence the outcomes of reconstructive procedures. Firstly, it is now well established that reperfusion of previously ischemic tissue causes injury at the microcirculatory level that is in addition to and distinct from, the injury caused by ischemia alone. Secondly, angiogenesis is an essential component of the vascular union of newly transferred tissue into its recipient bed. Derangement of the process of angiogenesis after reconstruction can result in improper wound healing. Finally, when a replant or free tissue flap shows signs of

Department of Trauma, Hand, and Reconstructive Surgery, University of Frankfurt, Medical School, Frankfurt, Germany, 2 Division of Plastic and Reconstructive Surgery, Department of Surgery, University of Louisville, KY, USA, 3 Department of Physiology and Biophysics, University of Louisville, KY, USA. 1

Received: June 18, 2001; accepted: July 3, 2001

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failure, reconstructive surgeons usually look at the site of vascular repair for an occluding thrombus. In case such a thrombus is present, revision of the anastomosis will eventually restore flow. Therefore the ultimate goal of microvascular repair is to restore perfusion of the “downstream” microcirculation. It is in the microcirculation, that problems with ischemia/reperfusion injury, angiogenesis/neovascularization of the wound, and embolic occlusion of small blood vessels come into effect. To study the microvascular pathophysiology underlying these phenomena, several animal and clinical model systems have been developed. However, it has been difficult to obtain a model that yields reproducible results and incorporates the mechanisms regulating hemodynamics of altered microcirculation in the clinical setting. Traditional methods of assessing tissue perfusion (diffusion of dyes, temperature, metabolic changes, oxygen tension, clearance tests) are indirect and can only provide general information indicating the presence or absence of blood flow. These kinds of measurements only allow us to speculate as to the underlying mechanisms responsible for alterations in tissue perfusion and often lead to confusing and contradictory results. In contrast, modern intravital microscopy techniques used by microcirculation researchers enable the microcirculation to be directly viewed and measured in vivo in a real-time fashion. These techniques make it possible to directly view and measure vasospasm, blood flow velocity, generation of thrombosis, effects of emboli, vessel wall leakage, and angiogenesis as they occur in any segment of the microcirculation in a variety of different tissues. The primary purpose of this review is to introduce the techniques of modern microcirculation research to reconstructive and trauma surgeons. This will be done using model systems that study the phenomena mentioned above and simulate the clinical setting. We will focus on microcirculation research as it relates to applications that are relevant to clinical reconstructive microsurgery and trauma surgery. Before getting started, we will discuss the nomenclature that the clinical microvascular surgeon should be familiar with to better understand the pathophysiology of microcirculation. Basic Nomenclature of Microcirculation Anatomic Aspects of Microcirculation While the vessels anastomosed in reconstructive microsurgery procedures are extremely small and require a

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great deal of skill to reattach and/or repair, these vessels are 20–100 times larger than the vessels of the microcirculation. It is thus important for the microsurgeon to understand the nomenclature of the blood vessels comprising the peripheral vasculature. Starting with the smallest arteries (1.0–0.3 mm diameter), if we descend in the direction of blood flow, we encounter several descending orders and sizes of arteries, then arterioles (A1, A2, and A3), and then the precapillary arterioles (A4), before finally reaching the capillaries. The capillaries are the smallest vessels in the microcirculation often measuring less than the diameter of a single red blood cell (0.010–0.007 mm). Continuing downstream from the capillaries, we reach the postcapillary venules (V4). These vessels gradually increase in size as they make their way back to the smallest veins (0.3–1.0 mm diameter) that microsurgeons work on. This anatomic description of the microcirculation is an oversimplification of a complex and dynamic network that microcirculation researchers are just now beginning to understand. Physiologic Aspects of Microcirculation Dynamics of Blood Flow (Rheology). Blood flow (Q) is proportional to the driving pressure (∆P) and inversely proportional to the resistance to flow (R). Thus, Q = ∆P/R. For practical purposes, ∆P can be viewed as the mean arterial blood pressure. Resistance (R) is directly proportional to vessel length and blood viscosity, and inversely proportional to vessel radius according to the following formula: R = 8 ηl/πr4. For practical purposes, R can be viewed as a combination of hematocrit and the collective radius of the arterioles. The velocity of blood flow through a vessel is a function of the viscosity of the blood, the diameter of the vessel, and the force (pressure) exerted to keep the blood flowing. The shear rate is the velocity difference between any two nearby layers of fluid divided by the distance between those layers. Since flow is equal to the cross-sectional area times the bulk velocity, for any given pressure, the velocity will increase in inverse proportion to the square of the vessel radius. Thus, with partial occlusion, the shear rate (centerline velocity/radius) increases in an inverse proportion to the cube of the radius. A narrowing of an anastomosis would increase resistance of that segment in proportion to the fourth power of the radius. In other words, even minimal change in diameter of a blood vessel results in a very big difference in the “downstream” blood flow.

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Fluids can be classified into two general categories, Newtonian (after Sir Isaac Newton) and non-Newtonian. Newtonian fluids like plasma, urine and water have constant viscosity that does not change with shear rate or time. Non-Newtonian fluids like blood (has red blood cells that can stick together) decrease their viscosity with increases in shear rate. Since the shear rate of blood is proportional to the flow velocity, higher flow velocities tend to lower the viscosity. The hematocrit of the blood can also have a profound effect on the viscosity of the blood. One consequence of the non-Newtonian properties of blood is that reduction in hematocrit can often result in reductions in viscosity, which reduces the resistance to blood flow and increases net blood flow. In fact, patients with limited peripheral perfusion (those in shock after severe trauma) can often improve blood flow in distal tissues by isovolemic hemodilution that results in a significantly reduced resistance to blood flow [1]. For example, a reduction in the concentration of red blood cells after severe hemorrhage by one half reduces oxygen transport by only 10% [2]. This is due to the fact that the cardiac output increases to compensate for lower oxygen carrying capacity by the blood and there is also an associated increase in tissue blood flow due to reasons explained above. Determinants of Blood Flow. As discussed above, the two most important factors that determine the amount of blood flowing through any region of a tissue are the perfusion pressure and the resistance to flow (Q = ∆P/R). The pressure difference between the artery and vein is the actual perfusion pressure. The resistance is made up of both geometric and rheologic factors [3]. The geometric factors are the most important determinants of blood flow. The three factors, which constitute the geometric contribution to resistance, are the length, number and radius of microvessels. Since the small arteries and arterioles are the major sites of resistance to flow, it is the change in radius of these vessels that will alter overall resistance. The length of individual vessels generally does not change except during growth. Loss of arterioles, which is called rarefaction, can cause high resistance without necessitating a vasoconstriction. The main microvascular concern to the reconstructive microsurgeon is the radii of arterioles because these vessels are of greatest importance in determining resistance and blood flow. The rheologic factors that affect resistance include contributions from four elements of blood: white blood

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cells, plasma proteins, platelets, and red blood cells [3]. White blood cells can be important in determining the flow through individual capillaries. If white blood cells become activated or less deformable, as occurs following periods of ischemia, they may be unable to traverse capillaries, and subsequently will block perfusion of those capillaries. Such a capillary occlusion can interfere with efficient transfer of a substrate such as oxygen from the blood to the tissues. Therefore, white blood cell obstruction of capillaries may be a major mechanism of tissue injury and destruction during tissue ischemia following trauma or during reconstructive procedures. If the protein concentration of plasma rises (as after severe muscle crush injury resulting in rhabdomyolysis), the blood viscosity can increase sufficiently to elevate resistance [4]. Platelets can also lead to a decrease in flow if they have increased aggregability, whether chronic or acute [5]. Lastly, the deformability, size, shape, concentration, and aggregability of red blood cells can significantly influence blood viscosity. Platelet and red blood cell characteristics are altered in syndromes like disseminated intravascular coagulation (DIC) that are common in trauma practice. Determinants of Tissue Oxygen Delivery. The most important factors determining diffusion of oxygen from blood to tissue are the rate of blood flow, the number of perfused capillaries, the tissue mitochondrial density and distribution, and lastly the oxygen diffusion coefficient of the perfused tissue. The exchange of oxygen and various substrates depends on the rate of blood flow and on the distribution of that flow within the microcirculation. Even in the presence of normal blood flow, oxygen delivery can be severely limited if only a few capillaries are receiving the blood. For example in the case of DIC after severe crush injury, microemboli may block capillaries in some tissue beds. In this case, even a very high blood flow may not be effective at exchanging substrates or delivering oxygen if all the red blood cells flow through only a few capillaries. The number and location of tissue mitochondria are important for tissue oxygenation because, 90% of oxygen used by tissues is consumed by mitochondria for oxidative reactions. In skeletal muscle most mitochondria are clustered near the contractile machinery [6]. The diffusion coefficient of oxygen depends on the capillary versus tissue oxygen difference. Higher diffusion coefficients can increase the flux of oxygen from a capillary to mitochondria. Highest diffusion occurs

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when oxygen delivery is high (high blood flow) and mitochondrial oxygen usage is high (high metabolism). Understanding the factors controlling blood flow and substrate delivery allows the microsurgeon to understand the fluid dynamics at an anastomosis as well as the entire microcirculation. With this knowledge it is easier for the reconstructive surgeon to review microcirculation research in the context of ischemia/reperfusion injury, angiogenesis and wound healing, and the microcirculatory effects of microvascular surgery. Ischemia/Reperfusion Injury Ischemia/reperfusion injury represents a condition where reperfusion of a tissue, which had been ischemic, results in an additional injury, one distinct from that caused by the ischemia alone. Reperfusion injury [7] is a phenomenon of great importance to surgical specialties like vascular trauma and reconstructive surgery. Despite advances in microsurgical techniques during the last 2 decades, reperfusion injuries are still responsible for up to 30% of failures in replantation [8] and up to 10% of failures in free tissue transfers [9]. Apart from inadequate blood vessel repair, postischemic deterioration of the nutritive microcirculation distal to the anastomoses has been proposed to be most often the cause for failure, threatening the success of the reconstructive intervention. After massive tissue trauma, reperfusion may have local as well as systemic consequences with loss of function and viability of the perfused organ and development of a systemic inflammatory response syndrome (SIRS) with the possibility of a multiple organ dysfunction syndrome (MODS) [10–12]. Microcirculatory Consequences of Ischemia/ Reperfusion Injury Both the ischemic insult and the reperfusion-associated events play a role in the manifestation of postischemic tissue injury. Although mandatory for the survival of free flaps and replants, reperfusion and reoxygenation of skeletal muscle [13] and skin are paradoxically associated with a variety of events contributing to microvascular reperfusion injury. The reintroduction of molecular oxygen during reperfusion initiates the release of reactive oxygen metabolites [14, 15], which increase postischemic tissue injury by activating white blood cells [16]. Reperfusion-induced activation of white blood cells will result in the additional formation of reactive oxygen metabolites, and both oxygen radicals

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and leukocytes promote the release of aggressive mediators, such as leukotrienes and platelet activating factor, which in turn have the potential to chemotactically attract more leukocytes to the postischemic tissue. Thus, the sequence of these pathophysiologic events results in a self-sustaining vicious circle. Interruption of this vicious circle with antioxidative metabolites and monoclonal antibodies is the focus of a vast amount of microcirculation research. To improve survival rates of free flaps and replants, research into microcirculatory mechanisms involved in ischemia/reperfusion injury has been possible using animal models. Traditional methods to study ischemia/reperfusion injury (xenon clearance technique [17], fluorometric quantification of dye delivery [18], photoplethysmography, transcutaneous oxygen measurements [19], thermography [20], radioactive-labeled microspheres [21], and laser Doppler flowmetry) indirectly assess microvascular perfusion of skin and striated muscle, but do not allow for distinct analyses of microcirculatory and microhemodynamic mechanisms within individual segments of the microvasculature, i.e., terminal arterioles, nutritive capillaries, and postcapillary venules. Furthermore, these techniques do not allow for the analysis of individual cellular mechanisms, including cell-cell interaction among white blood cells, platelets, and endothelial cells. In contrast, modern microcirculation techniques (intravital microscopy) represent the only method for quantitative and functional analysis of microhemodynamic parameters of skin and skeletal muscle microcirculation (functional capillary density, red blood cell velocity, vessel diameter, microvascular hematocrit, changes in vascular permeability, loss of endothelial integrity, cellular transport, and cell viability). In the experimental setting, in vivo microscopy represents an ideal tool to study individual mechanisms involved in the microcirculatory manifestation of ischemia/reperfusion injury in skeletal muscle and skin. Skeletal muscle microcirculation can be studied using chambers implanted on the backs of hamsters [22], mice [23], and rats. Skin microcirculation can be studied using the hairless mouse ear, or the dorsal skin of the hairless mouse [24]. These studies in different models have paved the way for therapeutic efforts to prevent reperfusion injury. Hypertonic solutions, isovolemic hemodilution, oxygen scavengers or the blockade of specific integrins, which mediate intercellular adhesion, can decrease

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microvascular injury by directly influencing those microhemodynamic parameters mentioned above [25–28]. Unlike with other traditional research methods, the use of microcirculatory models that give highquality in vivo images of the microvasculature will help to define further the dynamic effects of ischemia/reperfusion. Additionally, they will provide a direct method to evaluate the effects of new agents with beneficial potential on the microcirculation. Wound Healing and Angiogenesis Physiologically, wound healing is the phase when transplanted or replanted tissue establishes vascular reconnection to its new bed. For optimal wound healing to occur, multiple local and systemic conditions must be met. To any surgeon, wound healing is a critical indicator of postoperative success. Normal repair is a coordinated cellular and biochemical event including destruction and repair. It involves a variety of different cells, proteins, chemoattractants, proteinases, and growth factors. It is a multistep process consisting of a sequence of events including coagulation, inflammation, fibroplasia, matrix deposition, epithelialization, contraction, and angiogenesis. Angiogenesis or neovascularization is the process of new vessel formation that is essential to a wide variety of physiologic and pathologic processes including wound healing. The mechanisms regulating new vessel growth [29] are the focus of a vast amount of research in the fields of cancer prevention, wound healing, drug delivery etc. At the present time though, our knowledge of such regulation of new vessel growth is still poor. Microcirculatory Consequences of Impaired Angiogenesis and Wound Healing Nonhealing wounds and altered wound healing are common problems in many specialties including reconstructive and trauma surgery, constituting a great and persisting challenge to modern surgery [30]. Indeed, problem wound situations are often the basis for which reconstructive microsurgeons are called upon for assistance in coverage. A wound’s ability to resist invading microorganisms is partly determined by its ability to form new blood vessels (neovascularization) and epithelium (epithelialization). The new blood vessels supply the wound with oxygen, nutrients, white blood cells, and immunoglobulins and are the conduit for discarding the wastes. The newly formed epithelium has the important barrier function of protecting the wound

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from contamination. Most microcirculation research has focused on wound epithelialization and angiogenesis (neovascularization) [31]. In fact, the aspect of healing which is most poorly understood is angiogenesis [32, 33]. Traditional models in which angiogenesis [34] has been studied using subjective measuring methods include the avascular cornea of the rabbit [35], the rabbit ear chamber [36], and the chick chorioallantoic membrane assay (CAM) [37]. These techniques allow semiquantitative gross ranking of new vessel growth and are adequate in certain instances. More demanding preparations like the skinfold chamber models in rodents (hamster, rat, mouse) have been used via intravital microscopy techniques to study angiogenesis continuously [38–40]. Unfortunately these preparations are dependent on the injection of intravascular dyes (e.g., fluorescein) to enhance the contrast of the vessels and cause dye leakage especially after applications of growth factors. Aside from these in vivo models perhaps the most widely used are those employing histological fixation and quantification [41]. These methods are objective and accurate. However, serial quantitative analysis of the same angiogenic process has been difficult using any of these preparations. This is important because angiogenesis is a highly dynamic process and animal models in which it can be continuously observed and measured in the same wound throughout the healing process could be useful. Modern microcirculation techniques (Figure 1) have revolutionized the study of angiogenesis and wound healing. The hairless mouse ear wound model is a useful tool for continuous, quantitative measurements in vivo of two essential components of wound healing, i.e., epithelialization and angiogenesis, throughout the healing process [42–44]. Many kinds of healing impairments can be simulated in the hairless mouse thus making it possible to measure the effects of both pathology and subsequent treatments on these components [45]. This model has also been used to study burn injury, reperfusion injury, and flap necrosis. Local factors such as infection, venous insufficiency, foreign bodies and ischemia that are common causes of delayed healing can be reproduced in this model. The hairless mouse model also provides an in vivo system in which the angiogenic potency of different growth factors can be analyzed, quantified and compared with each other. Using this model, quantification of angiogenesis resulting from different concentrations and varying dosing regimens is

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Monitor

VCR

Camera PC Microscope Mouse ear

Filter

Light source

Figure 1. The hairless mouse ear can be utilized as a model employing intravital microscopy to study the microcirculation in skin. In this case, angiogenesis is being documented after local injection of growth factors.

possible. It thus could allow appropriate use of growth factors in the clinical setting where wound healing is a problem. The implications for research in healing of infected and postoperative wounds after trauma or reconstructive surgery with this model are obvious [46]. For example, stimulation of wound epithelialization would allow more rapid skin graft reharvesting from the donor site; and enhanced reepithelialization in burns would allow for more rapid rehabilitation and discharge from hospital. Microcirculatory Consequences of Microvascular Surgery Vascular compromise at or near the site of vessel repair is a common cause of failure of the great majority of free flaps [47, 48] or replants [49, 50]. This may be due to vasospasm or external compression by hematoma or edema, however overall, in the clinical setting, the most important cause is intraluminal thrombosis. The incidence of thrombotic anastomotic failure is much higher than is represented by studies in the literature, which report survival after reconstructive/trauma surgery.

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This is because of the high number of potential failures that are salvaged by revision surgery. Nevertheless, it is important to remember that the success of such secondary microsurgery is inversely related to the time interval between the onset of tissue ischemia and its reversal [51]. Thus, the occurrence of any of the above factors after trauma or reconstructive surgery could spell disaster to the procedure, resulting in tissue or limb loss necessitating resection or amputation. Effective prevention of these factors relies on an improved knowledge of microvascular pathophysiology obtained through research in animal models. Traditional models to measure vascular thrombosis have focused on presence or absence of patency following thrombogenic injury to the vessel. These models of thrombosis have included the insertion into the vessel lumen of cotton thread [52], metallic wire [53], and polyethylene tubing [54], the injection of enzymes, chemical and sclerosing agents [55, 56], and the application of electric current [57], ligatures, laser, clamps [58] and constricting vascular sleeves. Though anastomotic patency is essential for flap or replant survival, it is only the final step in a complex cascade of events. Little is known about the “downstream” microcirculatory consequences of vascular compromise at the site of anastomosis. In such a situation, animal models to directly visualize and quantitatively analyze thrombus formation at an arterial anastomosis on a continuous basis would prove useful. Modern microcirculatory research employs several animal models that allow direct visualization and measurement of microcirculation. These include, among others, the hamster cheek pouch [59], the rabbit ear chamber, the cat tenuissimus muscle [60], and the rat cremaster muscle preparation [61] (Figure 2). A modification of the rat cremaster model [62] has been used in our laboratories to simulate conditions of free tissue transfer (Figure 3). Such a model facilitates the study of changes in “downstream” microcirculation following an “upstream” thrombogenic microvascular injury [63], thus improving our understanding of the factors influencing the outcome after microvascular surgery (Figure 4). The optimal treatment modality to achieve nonfailing flaps or replants will be a surgical manipulation or pharmacologic treatment, which will prevent thrombus formation upstream while simultaneously protecting the downstream microcirculation from ischemic injury. The potential for the use of direct observation and

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Systemic pressure (carotid)

Cremaster inflow pressure (femoral)

Temperature probes: cremaster surface and core (rectal)

Figure 2. The rat cremaster muscle is employed to study skeletal muscle microcirculation. Systemic blood pressure and cremaster perfusion can be measure and controlled in this preparation.

Pedicle pressure

Iliac artery and vein

Genitofemoral nerve

Pudic – epigastric artery and vein Cremaster muscle

Figure 3. The isolated rat cremaster muscle simulates a free flap and can be used to study the effects of emboli originating from an anastomosis of the iliac artery. Ischemia and reperfusion injury also can be studied.

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measurements of phenomena on both these upstream and downstream risk zones is tremendous. Using these techniques, the effects of various treatment modalities can be quantitatively and qualitatively assessed both at the upstream and downstream risk zones. Perhaps most importantly, using these direct techniques, more can be ascertained about the mechanisms regulating these complex processes. Armed with this information, we will be better prepared to prevent free flap and replant failure. For example: The upstream arterial microanastomosis can produce platelet emboli, which pass into the downstream microcirculation [64]. The number will increase with poor surgical technique. These emboli produce perturbations and are harmful to the reperfused tissue by reducing the number of perfused capillaries and causing arteriole constriction via thromboxane (Tx) A2 released from activated platelets. The reduction of blood flow across the arterial anastomotic site decreases the number of platelet emboli generated. Using this model, heparin was shown to reduce the number of visible platelet emboli without any improvement of capillary perfusion. In addition, dietary cod liver oil was shown to have no effect on the number of emboli but did preserve normal capillary perfusion [65]. These effects were also demonstrated with medications such as Ridogrel, a TxA2 synthetase inhibitor [66]. Other problems associated with reperfusion injury like edema formation also can be studied with these models (Figure 5). This model thus has a broad range of potential applications, not only in microvascular surgery research but also in thrombosis research in general. Overall modern microcirculation models have a broad range of potential applications and enable an understanding of pathophysiologic mechanisms of thromboembolic injuries [67, 68], which might lead to improved pharmacologic control of these mechanisms. Conclusions Success rates in microvascular surgery have greatly improved in the past three decades due to a better understanding of vascular anatomy, improved pre- and postoperative care, better technical skills and instrumentation, and advances in the understanding of microcirculatory pathophysiology of reperfused tissue. In reconstructive or trauma surgery, the success of a free flap transfer or replantation relies absolutely on restoration of blood flow (both inflow and outflow) through microvascular anastomoses until neovascular-

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ization is established. If the technique used is good and done expeditiously, the risk of occlusive thrombosis at the anastomotic site as well as that of ischemia/reperfusion injury is minimized. Therefore, the factors contributing to successful clinical outcome of a microvascular reconstructive procedure are a patent anastomosis (lack of thrombosis), early revascularization (lack of ischemia/reperfusion injury), and good neovascularization in the wound (early angiogenesis). The importance of these facFigure 4. The intravital microscopy system used to monitor thrombosis and emboli in the microcirculation. tors is aptly summarized by Sir Harold Gillies who said: “Reconstructive surgery is a battle between beauty and blood supply.” For the clinician, knowledge of microvascular pathophysiology is essential for predicting the outcome of reconstructive procedures and preventing complications. Free flap and composite tissue transplantation have added new dimensions to trauma reconstructive microsurgery. The microcirculation is a highly dynamic environment of vessels in a constant state of change. Only through direct observation of the microcirculation can one understand rather than speculate as to the mechanisms causing free flap or replant failure at this level. Using modern microcirculatory models and techniques, one can directly observe and measure the microcirculation (vasospasm, vessel diameters, presence of thrombi, microvascular leakage, and neovascularization). The ability to monitor changes in microcirculatory parameters should help to pave the way toward a greater understanding of the pathophysiologic mechanisms of microvascular injury. A better understanding of the mechanisms involved in thrombosis and the changes that occur in the microcirculation downstream from the anastomosis is thus vital to improve pharmacologic research, treatment and clinical practice. This would in turn lead to improved surgical/pharmacologic prevention for clinical success. The models and techniques discussed in this review have opened whole new avenues of Figure 5. View of the microcirculation of the rat cremaster muscle research into mechanisms of surgical failure after produced by intravital fluorescence microscopy. Fluorescein isothiomicrovascular surgery. The implications of such recyanate-labeled albumin is seen in the microcirculation (top panel) search are indeed tremendous for the reconstructive or and leaking from venules after histamine application (bottom pantrauma surgeon. el).

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Correspondence Address John H. Barker, MD, PhD Plastic Surgery Research 320 MDR Building 511 South Floyd Street University of Louisville Louisville, KY 40292 USA Phone (+1/502) 852-0166, Fax -4675 e-mail: [email protected]

European Journal of Trauma 2001 · No. 4 © Urban & Vogel