Temperature and Heat Stress of Vessels during Cold Perfusion of Kidney

The first step of organ cryopreservation is a cryoprotectant perfusion, which can speed cooling. The temperature of the kidney decreased from 37°C to about 0°C by perfusion. During the kidney perfusion process, the cryoprotectant enters the kidneys through the renal artery and leaves the kidneys through the renal vein after passing through a series of capillaries. In cold perfusion vessels with a diameter larger than 0.3 mm must be treated individually. The obvious temperature change in the larger vessels will bring in the displacement causing by the thermal contraction. This paper is dedicated to present a comprehensive investigation on the thermal effects of larger blood vessels during cold perfusion including temperature change and corresponding heat stress. A structural model of heat transfer in kidney is developed using currently available anatomical and physiological data. To characterize the effect of thermally significant blood vessels on heat transfer inside the tissues during cold perfusion, the cryoprotectant in the blood vessel was controlled by the energy equation and Navier-Stokes equations. The tiny capillaries and its surrounding biological tissue were treated as the porous media following Darcy's Law. The controlling equations were numerically solved by CFD software. The numerical simulation for the coupled transient thermal field and stress field is carried out by sequentially thermal-structural coupled method based on ANSYS to evaluate the stress fields and of deformations which are established in the blood vessel and tissue. The results indicated that the thermal effects of large blood vessels could remarkably affect the temperature distribution of cold perfusion. And the heat stress obvious changed during cooling, especially for the vein. The maximal heat stress occurred at the export of the vein. This position may be the keys to avoid stress injury during perfusion. This paper provides a guideline to optimize the cold perfusion process from the biomechanics effect. INTRODUCTION Organ transplantation has been one of the most significant advances in medicine in the latter half of the 20th century and remains in many cases the only effective therapy for end-stage organ failure [1]. Hypothermia is employed for organ preservation to reduce the kinetics of metabolic activities. Cooling is induced by a brief flush with a chilled preservation solution after the organ is removed from the donor. Kazuhiro et al. [2] pointed out that the initial flush is equally as important as the storage itself. The improvement of this process could minimize preservation injury and supply the organ of high quality and efficacy [3,4]. Therefore, the demand for a detailed understanding of the temperature history of the organ in order to monitor the extent of cryopreservation is compelling. During the kidney perfusion process, the cryoprotectant enters the kidneys through the renal artery and leaves the kidneys through the renal vein after passing through a series of capillaries, which will decrease the organ temperature and produce temperature gradient in biological tissue. This is a typical coupled heat transfer problems. In the bio-heat transfer calculation, the vessels with a diameter larger than 0.3 mm must be treated individually. The thermal effect of the vessels and tissue should be analyzed to provide information to clinicians for better predicting hypothermic perfusion process and making more reliable treatment decisions. In recent years, many researchers have done numerical simulation and experimental analysis on thermal stress and fracture problems in freezing preservation of organs [6-9].While the biomechanical effect during hypothermic perfusion did not attract enough attention. In the recent, kidney transplantation is one of the most common and successful organ transplantation [5]. In this paper, we focus on the initial flushing of kidney before storage. This paper is dedicated to present a comprehensive investigation on the thermal effects on the temperature profile