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  Refrigeration source for a cryoablation catheterUSPTO Application #: 20070277550Title: Refrigeration source for a cryoablation catheter Abstract: An apparatus and method for automatic operation of a refrigeration system to provide refrigeration power to a catheter for tissue ablation or mapping. The primary refrigeration system can be open loop or closed loop, and a precool loop will typically be closed loop. Equipment and procedures are disclosed for bringing the system to the desired operational state, for controlling the operation by controlling refrigerant flow rate, for performing safety checks, and for achieving safe shutdown. The catheter-based system for performing a cryoablation procedure uses a precooler to lower the temperature of a fluid refrigerant to a sub-cool temperature (−40° C.) at a working pressure (400 psi). The sub-cooled fluid is then introduced into a supply line of the catheter. Upon outflow of the primary fluid from the supply line, and into a tip section of the catheter, the fluid refrigerant boils at an outflow pressure of approximately one atmosphere, at a temperature of about −88° C. In operation, the working pressure is computer controlled to obtain an appropriate outflow pressure for the coldest possible temperature in the tip section. (end of abstract) Agent: Baker & Mckenzie LLP Patent Department - Dallas, TX, US Inventors: Hong Li, Ravikumar Kudaravalli USPTO Applicaton #: 20070277550 - Class: 062612000 (USPTO) Related Patent Categories: Refrigeration, Cryogenic Treatment Of Gas Or Gas Mixture, Liquefaction, Natural Gas, Multicomponent Cascade Refrigeration The Patent Description & Claims data below is from USPTO Patent Application 20070277550. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is continuation of application Ser. No. 10/888,804 filed Jul. 9, 2004 which is a continuation of application Ser. No. 10/243,997, which is currently pending and which is a continuation-in-part of application Ser. No. 09/635,108 filed Aug. 9, 2000, now U.S. Pat. No. 6,471,694. The contents of application Ser. Nos. 10/243,997 and 09/635,108 are incorporated herein by reference. FIELD OF THE INVENTION [0002] The present invention pertains generally to systems and methods for implementing cryoablation procedures. More particularly, the present invention pertains to systems and methods that precool a primary fluid to a sub-cooled, fully saturated liquid state, for use in a cryoablation procedure. The present invention is particularly, but not exclusively, useful as a system and method for cooling the distal tip of a cryoablation catheter during cardiac cryoablation therapy to cure heart arrhythmias. The present invention also relates to the field of methods and apparatus used to generate and control the delivery of cryosurgical refrigeration power to a probe or catheter. BACKGROUND OF THE INVENTION [0003] As the word itself indicates, "cryoablation" involves the freezing of material. Of importance here, at least insofar as the present invention is concerned, is the fact that cryoablation has been successfully used in various medical procedures. In this context, it has been determined that cryoablation procedures can be particularly effective for curing heart arrhythmias, such as atrial fibrillation. [0004] It is believed that at least one-third of all atrial fibrillations originate near the ostia of the pulmonary veins, and that the optimal treatment technique is to treat these focal areas through the creation of circumferential lesions around the ostia of these veins. Heretofore, the standard ablation platform has been radiofrequency energy. Radiofrequency energy, however, is not amenable to safely producing circumferential lesions without the potential for serious complications. Specifically, while ablating the myocardial cells, heating energy also alters the extracellular matrix proteins, causing the matrix to collapse. This may be the center of pulmonary vein stenosis. Moreover, radiofrequency energy is known to damage the lining of the heart, which may account for thromboembolic complications, including stroke. Cryoablation procedures, however, may avoid many of these problems. [0005] In a medical procedure, cryoablation begins at temperatures below approximately minus twenty degrees Centigrade (-20.degree. C.). For the effective cryoablation of tissue, however, much colder temperatures are preferable. With this goal in mind, various fluid refrigerants (e.g. nitrous oxide N.sub.2O), which have normal boiling point temperatures as low as around minus eighty eight degrees Centigrade (-88.degree. C.), are worthy of consideration. For purposes of the present invention, the normal boiling point temperature of a fluid is taken to be the temperature at which the fluid boils under one atmosphere of pressure. Temperature alone, however, is not the goal. Specifically, it is also necessary there be a sufficient refrigeration potential for freezing the tissue. In order for a system to attain and maintain a temperature, while providing the necessary refrigeration potential to effect cryoablation of tissue, several physical factors need to be considered. Specifically, these factors involve the thermodynamics of heat transfer. [0006] It is well known that when a fluid boils (i.e. changes from a liquid state to a gaseous state) a significant amount of heat is transferred to the fluid. With this in mind, consider a liquid that is not boiling, but which is under a condition of pressure and temperature wherein effective evaporation of the liquid ceases. A liquid in such condition is commonly referred to as being "fully saturated". It will then happen, as the pressure on the saturated liquid is reduced, the liquid tends to boil and extract heat from its surroundings. Initially, the heat that is transferred to the fluid is generally referred to as latent heat. More specifically, this latent heat is the heat that is required to change a fluid from a liquid to a gas, without any change in temperature. For most fluids, this latent heat transfer can be considerable and is subsumed in the notion of wattage. In context, wattage is the refrigeration potential of a system. Stated differently, wattage is the capacity of a system to extract energy at a fixed temperature. [0007] An important consideration for the design of any refrigeration system is the fact that heat transfer is proportional to the difference in temperatures (.DELTA.T) between the refrigerant and the body that is being cooled. Importantly, heat transfer is also proportional to the amount of surface area of the body being cooled (A) that is in contact with the refrigerant. In addition to the above considerations (i.e. .DELTA.T and A); when the refrigerant is a fluid, the refrigeration potential of the refrigerant fluid is also a function of its mass flow rate. Specifically, the faster a heat-exchanging fluid refrigerant can be replaced (i.e. the higher its mass flow rate), the higher will be the refrigeration potential. This notion, however, has it limits. [0008] As is well known, the mass flow rate of a fluid results from a pressure differential on the fluid. More specifically, it can be shown that as a pressure differential starts to increase on a refrigerant fluid in a system, the resultant increase in the mass flow rate of the fluid will also increase the refrigeration potential of the system. This increased flow rate, however, creates additional increases in the return pressure that will result in a detrimental increase in temperature. As is also well understood by the skilled artisan, this effect is caused by a phenomenon commonly referred to as "back pressure." Obviously, an optimal operation occurs with the highest mass flow rate at the lowest possible temperature. [0009] In light of the above, it is an object of the present invention to provide an open-cycle, or closed-cycle, refrigeration system for cooling the tip of a cryoablation catheter that provides a pre-cooling stage in the system to maximize the refrigeration potential of the refrigerant fluid at the tip of the catheter. Another object of the present invention is to provide a refrigeration system for cooling the tip of a cryoablation catheter that substantially maintains a predetermined pressure at the tip of the catheter to maximize the refrigeration potential of the refrigerant fluid at the tip. Still another object of the present invention is to provide a refrigeration system for cooling the tip of a cryoablation catheter that provides the maximum practical surface area for the tip that will maximize the ablation potential of the refrigerant fluid. Also, it is an object of the present invention to provide a refrigeration system for cooling the tip of a cryoablation catheter that is relatively easy to manufacture, is simple to use, and is comparatively cost effective. SUMMARY OF THE PREFERRED EMBODIMENTS [0010] In a cryosurgical system, contaminants such as oil, moisture, and other impurities are often deposited in the impedance tubing or other restriction through which the refrigerant is pumped. In the impedance tubing, the temperature is very low, and the flow diameter is very small. Deposit of these impurities can significantly restrict the flow of the cooling medium, thereby significantly reducing the cooling power. [0011] A cryosurgical catheter used in a cardiac tissue ablation process should be able to achieve and maintain a low, stable, temperature. Stability is even more preferable in a catheter used in a cardiac signal mapping process. When the working pressure in a cryosurgery system is fixed, the flow rate can vary significantly when contaminants are present, thereby varying the temperature to which the probe and its surrounding tissue can be cooled. For a given cryosurgery system, there is an optimum flow rate at which the lowest temperature can be achieved, with the highest possible cooling power. Therefore, maintaining the refrigerant flow rate at substantially this optimum level is beneficial. [0012] In either the ablation process or the mapping process, it may be beneficial to monitor the flow rates, pressures, and temperatures, to achieve and maintain the optimum flow rate. Further, these parameters can be used to more safely control the operation of the system. [0013] A cryosurgical system which is controlled based only upon monitoring of the refrigerant pressure and catheter temperature may be less effective at maintaining the optimum flow rate, especially when contaminants are present in the refrigerant. Further, a system in which only the refrigerant pressure is monitored may not have effective safety control, such as emergency shut down control. [0014] It may also be more difficult to obtain the necessary performance in a cryosurgery catheter in which only a single compressor is used as a refrigeration source. This is because it can be difficult to control both the low and high side pressures at the most effective levels, with any known compressor. Therefore, it can be beneficial to have separate low side and high side pressure control in a cryosurgical system. [0015] Finally, it is beneficial to have a system for monitoring various parameters of data in a cryosurgery system over a period of time. Such parameters would include catheter temperature, high side refrigerant pressure, low side refrigerant pressure, and refrigerant flow rate. Continuous historical and instantaneous display of these parameters, and display of their average values over a selected period of time, can be very helpful to the system operator. [0016] The present invention provides methods and apparatus for controlling the operation of a cryosurical catheter refrigeration system by monitoring pressures, temperature, and/or flow rate, in order to automatically maintain a stable refrigerant flow rate at or near an optimum level for the performance of crysurgical tissue ablation or mapping. Different refrigerant flow rates can be selected as desired for ablation or mapping. Flow rate, pressures, and temperature can be used for automatic shut down control. Refrigerant sources which provide separate high side and low side pressure controls add to the performance of the system. Continuous displays of temperature, high side refrigerant pressure, low side refrigerant pressure, and refrigerant flow rate are provided to the operator on a single display, to enhance system efficiency and safety. [0017] A refrigeration system (open-cycle, or closed-cycle) for cooling the tip of a cryoablation catheter includes a source for a primary fluid refrigerant, such as nitrous oxide (N.sub.2O). Initially, the primary fluid is held under pressure (e.g. 750 psia) at ambient temperature (e.g. room temperature). A pressure regulator is connected in fluid communication with the primary fluid source for reducing the pressure on the primary fluid down to a working pressure (e.g. approximately 400 psia). During this pressure reduction to the working pressure, the primary fluid remains at substantially the ambient temperature. [0018] After pressure on the primary fluid has been reduced to the working pressure, a precooler is used to pre-cool the primary fluid from the ambient temperature. This is done while substantially maintaining the primary fluid at the working pressure. Importantly, at the precooler, the primary fluid is converted into a fully saturated liquid which has been pre-cooled to a sub-cool temperature. As used here, a sub-cool temperature is one that is below the temperature at which, for a given pressure, the fluid becomes fully saturated. For example, when nitrous oxide is to be used, the preferred sub-cool temperature will be equal to approximately minus forty degrees Centigrade (T.sub.sc=-40.degree. C.). [0019] Structurally, the precooler is preferably a closed-cycle refrigeration unit that includes an enclosed secondary fluid (e.g. a freon gas). Additionally, the precooler includes a compressor for increasing the pressure on the secondary fluid to a point where the secondary fluid becomes a liquid. Importantly, for whatever secondary fluid is used, it should have a normal boiling point that is near to the preferred sub-cool temperature of the primary fluid (T.sub.sc). The secondary fluid is then allowed to boil, and to thereby pre-cool the primary fluid in the system to its sub-cool temperature (T.sub.sc). As a closed-cycle unit, the secondary fluid is recycled after it has pre-cooled the primary fluid. [0020] The cryoablation catheter for the system of the present invention essentially includes a capillary tube that is connected with, and extends coaxially from a supply tube. Together, the connected supply and capillary tubes are positioned in the lumen of a catheter tube and are oriented coaxially with the catheter tube. More specifically, the supply tube and the capillary tube each have a distal end and a proximal end and, in combination, the proximal end of the capillary tube is connected to the distal end of the supply tube to establish a supply line for the catheter. [0021] For the construction of the cryoablation catheter, the supply tube and the capillary tube are concentrically (coaxially) positioned inside the lumen of the catheter tube. Further, the distal end of the capillary tube (i.e. the distal end of the supply line) is positioned at a closed-in tip section at the distal end of the catheter tube. Thus, in addition to the supply line, this configuration also defines a return line in the lumen of the catheter tube that is located between the inside surface of that catheter tube and the supply line. In particular, the return line extends from the tip section at the distal end of the catheter tube, back to the proximal end of the catheter tube. Continue reading... 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