50. Isolated Support With an Extracorporeal Perfusion System Deters Ischemia-related Metabolic Derangement of a Rat Fasciocutaneous Free Flap (2022)

Purpose: Machine perfusion of vascularized tissues can positively impact the future of microsurgery by enabling isolated perfusion of composite tissues such as free flaps. The goal of perfusion in this setting is to allow for preservation of tissues to extend the ischemia time for autologous or allogeneic flaps and temporarily perfuse until fully supported by neovascularization. For different tissue types and components, an improved understanding of the type and level of support is needed to develop an optimal perfusion system. Although they are more cost-effective, rodent free flaps have been employed with limited success in this setting due to challenges with small vessels and relatively delicate tissues. This study aimed to establish a rodent model of machine perfusion in a fasciocutaneous free flap to serve as an affordable testbed and determine the potential of the developed protocol to deter ischemia-related metabolic derangement. Methods: A 2x3 cm rat epigastric fasciocutaneous free flap was harvested based on the superficial inferior epigastric vessels and the vessels were cannulated. The flap was transferred to a closed circuit that provides circulatory support via a peristaltic pump and respiratory support via a custom gas exchanger. Flaps were kept within an enclosed chamber to maintain temperature and humidity. Fresh, heparinized whole rat blood (~40 mL) from donor rats was passed through a leukocyte filter and used as the perfusate within the circuit. Outflow from the flap vein was connected to a small compliant reservoir such that the same volume of blood was recirculated through the flap during 8 hours of support. Blood flow rate was adjusted to maintain arterial-like perfusion pressures. Continuous papaverine infusion (1 mg/hr) was used to mitigate vascular spasm and improve maintenance of flow stability. Blood samples were drawn during support for measurement of gases and metabolites. Extracellular tissue lactate and glucose levels were characterized with a custom microdialysis probe placed in the flap tissue. Lactate to glucose ratio (L/G) was used as an indicator of tissue metabolism and compared with warm ischemic, cold ischemic and anastomosed free flap controls at the same timepoints. Results: Maintenance of physiologic arterial pressures (85-100 mmHg) resulted in average pump flow rates of 300-450 uL/min with minimal flap bleeding. Blood-based measurements showed maintained glucose and oxygen consumption throughout support, indicating sustained metabolic activity. Average normalized L/G for the perfused flaps was 5- to 32-fold lower than that for the warm ischemic flap controls during hours 2-8 (p<0.05). Conclusion: We developed a rat model of extended machine perfusion of a fasciocutaneous free flap. Ex vivo machine perfusion maintained stable perfusion and tissue metabolic activity out to 8 hours of support. This model can be used to further assess critical elements of support in this setting as well as explore other novel therapies and technologies to improve free tissue transfer.