A three-dimensional variable geometry countercurrent model for whole limb heat transfer.

A new formulation of the combined macro and microvascular model for heat transfer in a human arm developed in Song et al. [1] is proposed using a recently developed approximate theory for the heat exchange between countercurrent vessels embedded in a tissue cylinder with surface convection [2]. The latter theory is generalized herein to treat an arm with an arbitrary variation in cross-sectional area and continuous bleed off from the axial vessels to the muscle and cutaneous tissue. The local microvascular temperature field is described by a "hybrid" model which applies the Weinbaum-Jiji [3] and Pennes [4] equations in the peripheral and deeper tissue layers, respectively. To obtain reliable end conditions at the wrist and other model input parameters, a plethysmograph-calorimeter has been used to measure the blood flow distribution between the arm and hand circulations, and hand heat loss. The predictions of the model show good agreement with measurements for the axial surface temperature distribution in the arm and confirm the minimum in the axial temperature variation first observed by Pennes [4] for an arm in a warm environment.     

An analytical solution countercurrent heat transfer between parallel vessels with a linear axial temperature gradient

Presented in this paper is a solution for countercurrent heat exchange between two parallel vessels embedded in an infinite medium with a linear temperature gradient along the axes of the vessels. The velocity profile within the vessel is assumed to be parabolic. This solution describes the temperature field within the vessels, as well as in the tissue, and establishes that the intravessel temperature is not uniform, as is generally assumed to be the case. An explicit expression for the intervessel thermal resistance based on the difference between cup-mixed mean temperatures is derived.     

A laboratory exercise using a physical model for demonstrating countercurrent heat exchange

A physical model was used in a laboratory exercise to teach students about countercurrent exchange mechanisms. Countercurrent exchange is the transport of heat or chemicals between fluids moving in opposite directions separated by a permeable barrier (such as blood within adjacent blood vessels flowing in opposite directions). Greater exchange of heat or chemicals between the fluids occurs when the flows are in opposite directions (countercurrent) than in the same direction (concurrent). When a vessel loops back on itself, countercurrent exchange can occur between the two arms of the loop, minimizing loss or uptake at the bend of the loop. Comprehension of the physical principles underlying countercurrent exchange helps students to understand how kidneys work and how modifications of a circulatory system can influence the movement of heat or chemicals to promote or minimize exchange and reinforces the concept that heat and chemicals move down their temperature or concentration gradients, respectively. One example of a well-documented countercurrent exchanger is the close arrangement of veins and arteries inside bird legs; therefore, the setup was arranged to mimic blood vessels inside a bird leg, using water flowing inside tubing as a physical proxy for blood flow within blood vessels.     

Readdressing the issue of thermally significant blood vessels using a countercurrent vessel network

A physiologically realistic arterio-venous countercurrent vessel network model consisting of ten branching vessel generations, where the diameter of each generation of vessels is smaller than the previous ones, has been created and used to determine the thermal significance of different vessel generations by investigating their ability to exchange thermal energy with the tissue. The temperature distribution in the 3D network (8178 vessels; diameters from 10 to 1000 microm) is obtained by solving the conduction equation in the tissue and the convective energy equation with a specified Nusselt number in the vessels. The sensitivity of the exchange of energy between the vessels and the tissue to changes in the network parameters is studied for two cases; a high temperature thermal therapy case when tissue is heated by a uniformly distributed source term and the network cools the tissue, and a hypothermia related case, when tissue is cooled from the surface and the blood heats the tissue. Results show that first, the relative roles of vessels of different diameters are strongly determined by the inlet temperatures to those vessels (e.g., as affected by changing mass flow rates), and the surrounding tissue temperature, but not by their diameter. Second, changes in the following do not significantly affect the heat transfer rates between tissue and vessels; (a) the ratio of arterial to venous vessel diameter, (b) the diameter reduction coefficient (the ratio of diameters of successive vessel generations), and (c) the Nusselt number. Third, both arteries and veins play significant roles in the exchange of energy between tissue and vessels, with arteries playing a more significant role. These results suggest that the determination of which diameter vessels are thermally important should be performed on a case-by-case, problem dependent basis. And, that in the development of site-specific vessel network models, reasonable predictions of the relative roles of different vessel diameters can be obtained by using any physiologically realistic values of Nusselt number and the diameter reduction coefficient.     

Algorithm for recipient vessel selection in free tissue transfer to the lower extremity.

The proper selection of a recipient vessel is essential for the success of free tissue transfer, especially when the transfer is to the lower extremity. However, a general agreement on which vessel to use has not been reached yet. Conflicting data have been reported on the survival and outcome of the transferred flaps, depending on the vessel used or the location of anastomosis. The aim of this study was to identify the patterns and problems in the selection of recipient vessels for free tissue transfer to the lower extremity and to establish a general guideline for proper selection. From September of 1990 to December of 1997, 50 consecutive, microvascular, free tissue transfers were performed on the lower extremity. The causes requiring soft-tissue coverage included trauma (25), unstable scar (11), chronic osteomyelitis (7), and tumors (7). The mean follow-up period was 22.4 months (range, 2 to 41 months). In 25 cases, the posterior tibial vessel was used as the recipient vessel. The microvascular anastomosis was done proximal to the zone of injury in 45 cases. The two most important factors in the selection of a recipient vessel are the site of injury and the vascular status of the lower extremity. Less important factors include the flap to be used, method, and site of microvascular anastomosis. All the currently feasible options for recipient vessels are included, and the opinions of other surgeons are reviewed. A general guideline is established, and an algorithm for the proper selection of a recipient vessel is proposed. This algorithm is a fast and convenient guide for evaluating the wound and planning the free flap to the lower extremity.     

Effect of variable heat transfer coefficient on tissue temperature next to a large vessel during radiofrequency tumor ablation

Background  

One of the current shortcomings of radiofrequency (RF) tumor ablation is its limited performance in regions close to large blood vessels, resulting in high recurrence rates at these locations. Computer models have been used to determine tissue temperatures during tumor ablation procedures. To simulate large vessels, either constant wall temperature or constant convective heat transfer coefficient (h) have been assumed at the vessel surface to simulate convection. However, the actual distribution of the temperature on the vessel wall is non-uniform and time-varying, and this feature makes the convective coefficient variable.     

Conditions for initiating heat stress in Pseudomonas geniculata

The heating of Pseudomonas geniculata 338 at an elevated temperature causes a heat stress in the culture. The extent of the stress depends on the temperature and duration of heating. The incubation of the bacterium at 40 and 45 degrees C did not inhibit its growth after 30 min of heating, and no essential quantities of intracellular compounds absorbing at 260 nm were lost (E260 increased by 12-19%). When the bacterium was heated at 50 degrees C for the same period of time, a three-hour lag-phase appeared during the subsequent cultivation of the bacterium whereas. E260 rose by a factor of 1.7. The resistance of the bacterium to heating depended on the physiological state of the culture: cells at the logarithmic growth phase were most susceptible to heating while the bacterium became more resistant to heating in the course of aging. The addition of NaCl at a concentration of 1.5% or of 10(-3)-10(-4) M EDTA to the reparation medium makes it possible to estimate the population of bacterial cells in the state of stress.     

Conditions for conjugative transposon transfer in Lactococcus lactis

Three different techniques for bacterial mating were applied to wild type and culture collection strains of Lactococcus lactis harbouring transposons: direct plate conjugation, filter mating and mating on milk agar. Efficiencies and frequencies of transfer were compared. Transconjugants were characterized by marker properties and molecular assays. Transposon-coded Suc+ Nis+ phenotype as well as Suc+ Bac+ Nis- phenotype were transferred with frequencies ranging between 10-9 and 10-6. Milk agar plate mating was the best technique for obtaining gene transfer events involving wild type lactococci.     

A prototype segmental model for blood flow and heat transfer in the limb.

A general modeling technique for characterizing the blood flow and heat tranfer properties in the human limb is reported in this paper. The basic idea is to take the segmental approach so that a lumped model for each segment can be constructed. Consequently, a prototype segmental computer model is proposed which describes, in general terms, the interrelationships between the circulatory system and the thermal system of the limb. Simulation study of digital response to hand cooling is made and the results agree very well with the experimental data.     

Numerical simulation for heat transfer in prostate cancer cryosurgery

A comprehensive computational framework to simulate heat transfer during the freezing process in prostate cancer cryosurgery is presented. Tissues are treated as nonideal materials wherein phase transition occurs over a temperature range, thermophysical properties are temperature dependent and heating due to blood flow and metabolism are included. Boundary conditions were determined at the surfaces of the commercially available cryoprobes and urethral warmer by experimental study of temperature combined with a mathematical optimization process. For simulations, a suitable computational geometry was designed based on MRI imaging data of a real prostate. An enthalpy formulation-based numerical solution was performed for a prescribed surgical protocol to mimic a clinical freezing process. This computational framework allows for the individual planning of cryosurgical procedures and objective assessment of the effectiveness of prostate cryosurgery.