There has been an increased awareness and interest in reperfusion injury or the no-reflow phenomenon. Unfortunately, the pathogenesis of this phenomenon is still not completely understood. Many hypotheses have been postulated, such as vasospasm, oxygen free radicals, platelet-plugging microthrombus formation, and changes in the vascular permeability leading to the interstitial edema. The direct observation of these highly dynamic mechanisms is important since it may explain many of the clinical events happening during postoperative follow-up in many microvascular and vascular surgery procedures.
All important parameters involved in ischemia/reperfu-sion injury can be evaluated using the cremaster muscle flap. They include measurements of vasospasm, vessel diameters, thrombus formation, presence of microembolism, functional capillary density, leukocytes-endothelial interaction, and microvascular leakage. Furthermore, the cremaster chamber model allows for chronic observation of the microcircula-tory events, which is critical for monitoring consequences of ischemia/reperfusion.
Technique. The cremaster muscle flap dissection described earlier can be modified for application to the ischemia/reperfusion injury studies.
In order to match the events of reperfusion injury associated with the vasospastic phenomenon and decrease in capillary perfusion, the cremaster flap has been applied as a microsurgery ischemia model. Technically, the cremaster vascular pedicle is transected and re-anastomosed, and the flap is further assessed by microcirculatory monitoring An important observation from this ischemia model was the description of concept of the cremaster flap risk zones by simultaneous assessment of the site of vascular anastomosis and peripheral microcirculation. The zones were divided into the upstream risk zone (anastomotic site) and a downstream risk zone (the microcirculation) . The events monitored within these zones were directly related: A process occurring within the upstream zone such as thrombus formation was influencing events at the downstream zone, causing decrease in capillary perfusion. Other observations from this model have shown that the ischemia time that is the period needed to perform the microvascular anastomosis induced a profound vasospasm peripherally, specifically in the third-order arterioles of the cremaster flap. Some authors suggested that this mechanism is most probably related to microcirculatory response to anesthetic drugs, poor micro-surgical technique of vessel repair, and tissue hypoxia and acidosis.
The cremaster flap can be also used as a clamp-ischemia model. In this model homodynamic changes occurring at the microcirculatory level following clamp application to the flap feeding artery can be compared to the ischemia-induced events during free tissue transfer procedures. The technique involves clamping the cremaster vascular pedicle for a different periods ranging from 2 to 6 hours with subsequent monitoring of the microcirculatory conditions. This was specifically of interest in studies related to ischemic preconditioning and its effects on the microcirculation. This model is also applicable for simulation of reduced flow states, in which instead of clamp application a silk suture is placed around the pedicle, reducing blood flow to the flap.
The response of leukocyte-endothelial interactions to ischemia/reperfusion injury has been extensively studied. Upon reperfusion, a cascade of events takes place, leading to activation, adherence, and transmigration of the leukocytes, ultimately resulting in decreased blood flow, increased capillary permeability, and subsequent tissue damage and microcirculatory failure. Substances such as anti-ICAM and anti-PECAM antibodies, growth factors, interferon gamma, dehydroepiandrosterone, and cyclo-sporin and many more have been tested in our laboratory in different experimental scenarios of tissue ischemia, reperfusion, denervation, and preconditioning. The ability to reduce the effects of the ischemia/reperfusion injury after administration of different pharmacological agents has opened new horizons in the field of reconstructive microsurgery leading to better understanding and improvement of the free tissue transfer flap survival.
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