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.

Description:
[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° 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 2 O), which have normal boiling point temperatures as low as around minus eighty eight degrees Centigrade (−88° 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 (ΔT) 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. ΔT 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 cryosurgical 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 cryosurgical 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 2 O). 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 sc =−40° 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 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 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. 
         [0022]    Insofar as the supply line is concerned, it is an important aspect of the present invention that the impedance to fluid flow of the primary refrigerant in the supply line be relatively low through the supply tube, as compared with the impedance presented by the capillary tube. Stated differently, it is desirable for the pressure drop, and consequently the temperature reduction, on the primary refrigerant be minimized as it traverses the supply tube. On the other hand, the pressure drop and temperature reduction on the primary refrigerant should be maximized as the refrigerant traverses the capillary tube. Importantly, the physical dimensions of the supply tube, of the capillary tube, and of the catheter tube can be engineered to satisfy these requirements. It is also desirable to engineer the length of the capillary tube so that gases passing from the tip section, back through the return line do not impermissibly warm the capillary tube. By balancing these considerations, the dimensions of the supply line, the tip section and the return line, can all be predetermined. 
         [0023]    As the fluid refrigerant is transferred from its source to the catheter supply line, it passes through the precooler. During this transfer, a control valve(s) is used to establish a working pressure (p w ) for the refrigerant. Also, a pressure sensor is provided to monitor the working pressure on the primary fluid refrigerant before the refrigerant enters the supply line at the proximal end of the catheter. 
         [0024]    On the return side of the system, an exhaust unit is provided for removing the primary fluid from the tip section of the catheter. For the present invention, this exhaust unit consists of a vacuum pump that is attached in fluid communication with the return line at the proximal end of the catheter tube. A pressure sensor is also provided at this point to determine the pressure in the return line at the proximal end of the catheter tube (p r ). 
         [0025]    In accordance with well known thermodynamic principles, when pressures at specific points in a system are known, fluid pressures at various other points in the system can be determined. For the present invention, because the supply line and return line are contiguous and have known dimensions, because “p w ” (working pressure) and “p r ” (return line pressure) can be determined and, further, because the fluid refrigerant experiences a phase change during the transition from p w  to p r , it is possible to calculate pressures on the fluid refrigerant at points between the proximal end of the supply tube (inlet) and the proximal end of the catheter tube (outlet). In particular, it is possible to calculate an outflow pressure (p o ) for the fluid refrigerant as it exits from the distal end of the capillary tube into the tip section of the catheter. 
         [0026]    The outflow pressure (p o ) for the fluid refrigerant can be determined in ways other than as just mentioned above. For one, a pressure sensor can be positioned in the tip section of the catheter near the distal end of the capillary tube to measure the outflow pressure (p o ) directly. Additionally, the system of the present invention can include a temperature sensor that is positioned in the tip section of the catheter to monitor the temperature of the primary fluid refrigerant in the tip section (T t ). Specifically, when this temperature (T t ) is measured as the primary fluid refrigerant is boiling (i.e. as it enters the tip section from the capillary tube), it is possible to directly calculate the outflow pressure (p o ) using well known thermodynamic relationships. 
         [0027]    A computer is used with the system of the present invention to monitor and control the operational conditions of the system. Specifically, the computer is connected to the appropriate sensors that monitor actual values for “p r ” and “p w ”. The values for “p r ” and “p w ” can then be used to determine the outflow pressure “p o ” in the tip section of the catheter (for one embodiment of the present invention, “p o ” is also measured directly). Further, the computer is connected to the control valve to manipulate the control valve and vary the working pressure (p w ) on the primary fluid. At the same time, the computer can monitor the temperature in the tip section of the catheter (T t ) to ensure that changes in the working pressure “p w ” result in appropriate changes in “T t ”. Stated differently, the computer can monitor conditions to ensure that an unwanted increase in “back pressure,” that would be caused by an inappropriate increase in “p w ” does not result in an increase in “T t ”. The purpose here is to maintain the outflow pressure (p o ) in the tip section of the catheter at a desired value (e.g. 15 psia). 
         [0028]    In operation, the sub-cooled primary fluid is introduced into the proximal end of the capillary tube at substantially the working pressure (p w ). The primary fluid then traverses the capillary tube for outflow from the distal end of the capillary tube at the outflow pressure (p o ). Importantly, in the capillary tube the fluid refrigerant is subjected to a pressure differential (Δp). In this case, “Δp” is substantially the difference between the working pressure (p w ) on the primary fluid as it enters the proximal end of the capillary tube (e.g. 300 psi), and a substantially ambient pressure (i.e. p o ) as it outflows from the distal end of the capillary tube (e.g. one atmosphere, 15 psi)(Δp=p w −p o ). In particular, as the pre-cooled primary fluid passes through the capillary tube, it transitions from a sub-cool temperature that is equal to approximately minus forty degrees Centigrade (T sc ≅−40° C.), to approximately its normal boiling point temperature. As defined above, the normal boiling point temperature of a fluid is taken to be the temperature at which the fluid boils under one atmosphere of pressures. In the case of nitrous oxide, this will be a cryoablation temperature that is equal to approximately minus eighty-eight degrees Centigrade (T ca ≅−88° C.). The heat that is absorbed by the primary fluid as it boils, cools the tip section of the catheter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0030]      FIG. 1  is a schematic of a first embodiment of the apparatus of the present invention, using a pressure bottle as the primary refrigerant source; 
           [0031]      FIG. 2  is a schematic of a second embodiment of the apparatus of the present invention, using a compressor as the primary refrigerant source; 
           [0032]      FIG. 3  is a schematic of a third embodiment of the apparatus of the present invention, using two compressors connected in series as the primary refrigerant source; 
           [0033]      FIG. 4  is a schematic of a first embodiment of a control system apparatus according to the present invention, for use with the apparatus shown in  FIG. 1 ; 
           [0034]      FIG. 5  is a schematic of a second embodiment of a control system apparatus according to the present invention, for use with the apparatus shown in  FIG. 2  or  3 ; 
           [0035]      FIG. 6  is a schematic of a parameter display for use with the control equipment of the present invention; and 
           [0036]      FIG. 7  is a flow diagram showing one control sequence for use with the control apparatus of the present invention. 
           [0037]      FIG. 8  is a perspective view of the system of the present invention; 
           [0038]      FIG. 9  is a cross-sectional view of the catheter of the present invention as seen along the line  2 - 2  in  FIG. 8 ; 
           [0039]      FIG. 10  is a schematic view of the computer and its interaction with system components and sensors for use in the control of a cryoablation procedure; 
           [0040]      FIG. 11  is a schematic view of the interactive components in the console of the present invention; 
           [0041]      FIG. 12  is a pressure-temperature diagram (not to scale) graphing an open-cycle operation for a refrigerant fluid in accordance with the present invention; and 
           [0042]      FIG. 13  is a diagram (not to scale) showing the tendency for changes in temperature response to changes of fluid mass flow rate in a catheter environment as provided by the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0043]    According to certain embodiments of the invention, the refrigeration system may be a two stage Joule-Thomson system with a closed loop precool circuit and either an open loop or a closed loop primary circuit. A typical refrigerant for the primary circuit would be R- 508   b , and a typical refrigerant for the precool circuit would be R- 410   a . In the ablation mode, the system may be capable of performing tissue ablation at or below minus 70.degree. C. while in contact with the tissue and circulating blood. In the mapping mode, the system may be capable of mapping by stunning the tissue at a temperature between minus 10.degree. C. and minus 18.degree. C. while in contact with the tissue and circulating blood. These performance levels may be achieved while maintaining the catheter tip pressure at or below a sub-diastolic pressure of 14 psia. 
         [0044]    As shown in  FIG. 1 , one embodiment of the apparatus  10  of the present invention is an open loop system using a pressure bottle for the refrigerant source. Such a system can include a primary refrigerant supply bottle  200 , a primary refrigerant fluid controller  208 , a catheter  300 , a primary refrigerant recovery bottle  512 , a secondary refrigerant compressor  100 , a precool heat exchanger  114 , and various sensors. In certain embodiments, all but the catheter  300  and the precool heat exchanger  114  may be located in a cooling console housing. The precool heat exchanger  114  is connected to the console by flexible lines  121 ,  221 . Pressure of the refrigerant in the primary refrigerant supply bottle  200  is monitored by a primary refrigerant supply pressure sensor  202 . Output of primary refrigerant from the supply bottle  200  is regulated by a pressure regulator  204 , which, in certain embodiments, can receive refrigerant from the bottle  200  at a pressure above 350 psia and regulate it to less than 350 psia. A primary refrigerant relief valve  206  is provided to prevent over pressurization of the primary system downstream of the pressure regulator  204 , for example, above 400 psia. The flow rate of primary refrigerant is controlled by the fluid controller  208 , which can be either a pressure controller or a flow controller. A feedback loop may be provided to control the operation of the fluid controller  208 . The feedback signal for the fluid controller  208  can come from a pressure sensor  310  or a flow sensor  311 , on the effluent side of the catheter  300 , discussed below. 
         [0045]    A primary refrigerant high pressure sensor  210  is provided downstream of the fluid controller  208 , to monitor the primary refrigerant pressure applied to the precool heat exchanger  114 . The high pressure side  212  of the primary loop passes through the primary side of the cooling coil of the precool heat exchanger  114 , then connects to a quick connect fitting  304  on the precool heat exchanger  114 . Similarly, the low side quick connect fitting  304  on the precool heat exchanger  114  is connected to the low pressure side  412  of the primary loop, which passes back through the housing of the precool heat exchanger  114 , without passing through the cooling coil, and then through the flow sensor  311 . The catheter tip pressure sensor  310  monitors catheter effluent pressure in the tip of the catheter  300 . The control system maintains catheter tip pressure at a sub-diastolic level at all times. 
         [0046]    The low pressure side  412  of the primary loop can be connected to the inlet  402  of a vacuum pump  400 . A primary refrigerant low pressure sensor  410  monitors pressure in the low side  412  of the primary loop downstream of the precool heat exchanger  114 . The outlet  404  of the vacuum pump  400  can be connected to the inlet  502  of a recovery pump  500 . A 3 way, solenoid operated, recovery valve  506  is located between the vacuum pump  400  and the recovery pump  500 . The outlet  504  of the recovery pump  500  is connected to the primary refrigerant recovery bottle  512  via a check valve  508 . A primary refrigerant recovery pressure sensor  510  monitors the pressure in the recovery bottle  512 . A 2 way, solenoid operated, bypass valve  406  is located in a bypass loop  407  between the low side  412  of the primary loop upstream of the vacuum pump  400  and the high side  212  of the primary loop downstream of the fluid controller  208 . A solenoid operated bypass loop vent valve  408  is connected to the bypass loop  407 . 
         [0047]    In the catheter  300 , the high pressure primary refrigerant flows through an impedance device such as a capillary tube  306 , then expands into the distal portion of the catheter  300 , where the resultant cooling is applied to surrounding tissues. A catheter tip temperature sensor  307 , such as a thermocouple, monitors the temperature of the distal portion of the catheter  300 . A catheter return line  308  returns the effluent refrigerant from the catheter  300  to the precool heat exchanger  114 . The high and low pressure sides of the catheter  300  are connected to the heat exchanger quick connects  304  by a pair of catheter quick connects  302 . As an alternative to pairs of quick connects  302 ,  304 , coaxial quick connects can be used. In either case, the quick connects may carry both refrigerant flow and electrical signals. 
         [0048]    In the precool loop, compressed secondary refrigerant is supplied by a precool compressor  100 . An after cooler  106  can be connected to the outlet  104  of the precool compressor  100  to cool and condense the secondary refrigerant. An oil separator  108  can be connected in the high side  117  of the precool loop, with an oil return line  110  returning oil to the precool compressor  100 . A high pressure precooler pressure sensor  112  senses pressure in the high side  117  of the precool loop. The high side  117  of the precool loop is connected to an impedance device such as a capillary tube  116  within the housing of the precool heat exchanger  114 . High pressure secondary refrigerant flows through the capillary tube  116 , then expands into the secondary side of the cooling coil of the precool heat exchanger  114 , where it cools the high pressure primary refrigerant. The effluent of the secondary side of the precool heat exchanger  114  returns via the low side  118  of the precool loop to the inlet  102  of the precool compressor  100 . A low pressure precooler pressure sensor  120  senses pressure in the low side  118  of the precool loop. 
         [0049]    Instead of using primary refrigerant supply and return bottles, the apparatus can use one or more primary compressors in a closed loop system.  FIG. 2  shows a second embodiment of the apparatus of the present invention, with a single compressor system. This embodiment would be appropriate in applications where the high side and low side pressures can be adequately controlled with a single compressor. In the apparatus  10 ′ of this type of system, the low side  622  of the primary loop conducts the effluent of the catheter  300  to the inlet  602  of a primary refrigerant compressor  600 . The compressor  600  compresses the primary refrigerant, and returns it from the compressor outlet  604  via the high side  612  of the primary loop to the primary side of the precool heat exchanger  114 . A primary refrigerant high pressure sensor  614  is provided in the high side  612  of the primary loop, to monitor the primary refrigerant pressure applied to the precool heat exchanger  114 . A primary refrigerant high pressure flow sensor  312  can be provided in the high side  612  of the primary loop. A primary refrigerant low pressure sensor  610  monitors pressure in the low side  622  of the primary loop downstream of the precool heat exchanger  114 . A primary loop filter  608  can be provided in the low side  622  of the primary loop. A 2 way, solenoid operated, primary refrigerant charge valve  626  and a primary refrigerant reservoir  628  can be provided in the low side  622  of the primary loop. A high pressure after-cooler  605  can be provided downstream of the primary refrigerant compressor  600 . 
         [0050]    As further shown in  FIG. 2 , a 2 way, solenoid operated, primary loop bypass valve  606  is located in a bypass loop  607  between the low side  622  of the primary loop upstream of the compressor  600  and the high side  612  of the primary loop downstream of the compressor  600 . Opening of the primary loop bypass valve  606  can facilitate startup of the primary compressor  600 . A precool loop filter  101  can be provided in the low side  118  of the precool loop. Further, a 2 way, solenoid operated, precool loop bypass valve  111  is located in a bypass loop  119  between the low side  118  of the precool loop upstream of the compressor  100  and the high side  117  of the precool loop downstream of the compressor  100 . Opening of the precool loop bypass valve  111  can facilitate startup of the precool compressor  100 . 
         [0051]    A purification system  900  can be provided for removing contaminants from the primary refrigerant and the secondary refrigerant. Solenoid operated 3 way purification valves  609 ,  611  are provided in the high side and low side, respectively, of the primary loop, for selectively directing the primary refrigerant through the purification system  900 . Similarly, solenoid operated 3 way purification valves  115 ,  113  are provided in the high side and low side, respectively, of the precool loop, for selectively directing the secondary refrigerant through the purification system  900 . 
         [0052]    The remainder of the precool loop, the precool heat exchanger  114 , and the catheter  300  are the same as discussed above for the first embodiment. 
         [0053]    In applications where separate low side and high side pressure control is required, but where a closed loop system is desired, a two compressor primary system may be used.  FIG. 3  shows a third embodiment of the apparatus of the present invention, with a dual compressor system. In the apparatus  10 ″ of this type of system, the low side  622  of the primary loop conducts the effluent of the catheter  300  to the inlet  616  of a low side primary refrigerant compressor  618 . The low side compressor  618  compresses the primary refrigerant, and provides it via its outlet  620  to the inlet  602  of a high side primary refrigerant compressor  600 . A low pressure after-cooler  623  can be provided downstream of the low side compressor  618 . The high side compressor  600  further compresses the primary refrigerant to a higher pressure and returns it via its outlet  604  and via the high side  612  of the primary loop to the primary side of the precool heat exchanger  114 . A primary refrigerant high pressure sensor  614  is provided in the high side  612  of the primary loop, to monitor the high side primary refrigerant pressure upstream of the precool heat exchanger  114 . A primary refrigerant low pressure sensor  610  monitors pressure in the low side  622  of the primary loop downstream of the precool heat exchanger  114 . A primary refrigerant intermediate pressure sensor  624  monitors pressure between the outlet  620  of the low side compressor  618  and the inlet  602  of the high side compressor  600 . The high side compressor  600  and the low side compressor  618  are separately controlled, using feedback from the catheter tip pressure sensor  310  and/or the flow sensors  311 ,  312 . 
         [0054]    As further shown in  FIG. 3 , a 3 way, solenoid operated, bypass valve  606 ′ is located in a bypass loop  607  between the low side  622  of the primary loop upstream of the low side compressor  618  and the high side  612  of the primary loop downstream of the high side compressor  600 . A third port is connected between the high side and low side compressors. The precool loop, the precool heat exchanger  114 , and the catheter  300  are the same as discussed above for the first and second embodiments. 
         [0055]      FIG. 4  shows a control diagram which would be suitable for use with the apparatus shown in  FIG. 1 . A computerized automatic control system  700  is connected to the various sensors and control devices to sense and control the operation of the system, and to provide safety measures, such as shut down schemes. More specifically, on the sensing side, the low pressure precool sensor  120  inputs low side precool pressure PA, the high pressure precool sensor  112  inputs high side precool pressure PB, the primary supply pressure sensor  202  inputs supply bottle pressure P 1 , the primary recovery pressure sensor  510  inputs recovery bottle pressure P 2 , the high pressure primary sensor  210  inputs high side primary pressure P 3 , the low pressure primary sensor  410  inputs low side primary pressure P 4 , the catheter tip pressure sensor  310  inputs catheter tip pressure P 5 , the temperature sensor  307  inputs catheter tip temperature T, and the flow sensor  311  inputs primary refrigerant flow rate F. Further, on the control side, the control system  700  energizes the normally closed bypass valve  406  to open it, energizes the normally open vent valve  408  to close it, and energizes the recovery valve  506  to connect the vacuum pump outlet  404  to the recovery pump inlet  502 . Finally, the control system  700  provides a pressure set point SPP or flow rate set point SPF to the fluid controller  208 , depending upon whether it is a pressure controller or a flow controller. 
         [0056]      FIG. 5  shows a control diagram which would be suitable for use with the apparatus shown in  FIG. 2  or  FIG. 3 . A computerized automatic control system  700  is connected to the various sensors and control devices to sense and control the operation of the system, and to provide safety measures, such as shut down schemes. More specifically, on the sensing side, the low pressure precool sensor  120  inputs low side precool pressure PA, the high pressure precool sensor  112  inputs high side precool pressure PB, the high pressure primary sensor  614  inputs high side primary pressure P 3 , the low pressure primary sensor  610  inputs low side primary pressure P 4 , the catheter tip pressure sensor  310  inputs catheter tip pressure P 5 , the temperature sensor  307  inputs catheter tip temperature T, and the flow sensors  311 ,  312  input primary refrigerant flow rate F. Further, on the control side, the control system  700  energizes the normally closed primary loop bypass valve  606 ,  606 ′ to open it, and the control system  700  energizes the normally closed precool loop bypass valve  111  to open it. The control system  700  also energizes the primary loop purification valves  609 ,  611  to selectively purify the primary refrigerant, and the control system  700  energizes the precool loop purification valves  113 ,  115  to selectively purify the secondary refrigerant. Finally, the control system  700  provides a minimum high side pressure set point PL 2  to the controller  601  of the primary compressor  600  in the system shown in  FIG. 2 . Alternatively, in the system shown in  FIG. 3 , the control system  700  provides a minimum high side pressure set point PL 2 B to the controller  601  of the high side primary compressor  600 , and the control system  700  provides a maximum low side pressure set point PL 2 A to the controller  619  of the low side primary compressor  618 . 
         [0057]    A numeric digital display, or a graphical display similar to that shown in  FIG. 6 , is provided on the cooling console to assist the operator in monitoring and operating the system. For example, on a single graphical display, graphs can be shown of catheter tip temperature T, high side primary pressure P 3 , low side primary pressure P 4 , and primary flow rate F, all versus time. Further, on the same display, the operator can position a vertical cursor at a selected time, resulting in the tabular display of the instantaneous values of T, P 3 , P 4 , and F, as well as the average, maximum, and minimum values of these parameters. 
         [0058]    The present invention will now be further illustrated by describing a typical operational sequence of the open loop embodiment, showing how the control system  700  operates the remainder of the components to start up the system, to provide the desired refrigeration power, and to provide system safety. The system can be operated in the Mapping Mode, where the cold tip temperature might be maintained at minus 10 C., or in the Ablation Mode, where the cold tip temperature might be maintained at minus 65 C. Paragraphs are keyed to the corresponding blocks in the flow diagram shown in  FIG. 7 . Suggested exemplary Pressure Limits used below could be PL 1 =160 psia; PL 2 =400 psia; PL 3 =500 psia; PL 4 =700 psia; PL 5 =600 psia; PL 6 =5 psia; PL 7 =diastolic pressure; PL 8 =375 psia; and PL 9 =5 psia. Temperature limits, flow limits, procedure times, and procedure types are set by the operator according to the procedure being performed. 
         [0059]    Perform self tests (block  802 ) of the control system circuitry and connecting circuitry to the sensors and controllers to insure circuit integrity. 
         [0060]    Read and store supply cylinder pressure P 1 , primary low pressure P 4 , and catheter tip pressure P 5  (block  804 ). At this time, P 4  and P 5  are at atmospheric pressure. If P 1  is less than Pressure Limit PL 2  (block  808 ), display a message to replace the supply cylinder (block  810 ), and prevent further operation. If P 1  is greater than PL 2 , but less than Pressure Limit PL 3 , display a message to replace the supply cylinder soon, but allow operation to continue. 
         [0061]    Read precool charge pressure PB and recovery cylinder pressure P 2  (block  806 ). If PB is less than Pressure Limit PL 1  (block  808 ), display a message to service the precool loop (block  810 ), and prevent further operation. If P 2  is greater than Pressure Limit PL 4  (block  808 ), display a message to replace the recovery cylinder (block  810 ), and prevent further operation. If P 2  is less than PL 4 , but greater than Pressure Limit PL 5 , display a message to replace the recovery cylinder soon, but allow operation to continue. 
         [0062]    Energize the bypass loop vent valve  408  (block  812 ). The vent valve  408  is a normally open two way solenoid valve open to the atmosphere. When energized, the vent valve  408  is closed. 
         [0063]    Start the precool compressor  100  (block  814 ). Display a message to attach the catheter  300  to the console quick connects  304  (block  816 ). Wait for the physician to attach the catheter  300 , press either the Ablation Mode key or the Mapping Mode key, and press the Start key (block  818 ). Read the catheter tip temperature T and the catheter tip pressure P 5 . At this time, T is the patient&#39;s body temperature and P 5  is atmospheric pressure. 
         [0064]    Energize the bypass loop valve  406 , while leaving the recovery valve  506  deenergized (block  820 ). The bypass valve  406  is a normally closed 2 way solenoid valve. Energizing the bypass valve  406  opens the bypass loop. The recovery valve  506  is a three way solenoid valve that, when not energized, opens the outlet of the vacuum pump  400  to atmosphere. Start the vacuum pump  400  (block  822 ). These actions will pull a vacuum in the piping between the outlet of the fluid controller  208  and the inlet of the vacuum pump  400 , including the high and low pressure sides of the catheter  300 . Monitor P 3 , P 4 , and P 5  (block  824 ), until all three are less than Pressure Limit PL 6  (block  826 ). 
         [0065]    Energize the recovery valve  506  and the recovery pump  500  (block  828 ). When energized, the recovery valve  506  connects the outlet of the vacuum pump  400  to the inlet of the recovery pump  500 . De-energize the bypass valve  406 , allowing it to close (block  830 ). Send either a pressure set point SPP (if a pressure controller is used) or a flow rate set point SPF (if a flow controller is used) to the fluid controller  208  (block  832 ). Where a pressure controller is used, the pressure set point SPP is at a pressure which will achieve the desired refrigerant flow rate, in the absence of plugs or leaks. The value of the set point is determined according to whether the physician has selected the mapping mode or the ablation mode. These actions start the flow of primary refrigerant through the catheter  300  and maintain the refrigerant flow rate at the desired level. 
         [0066]    Continuously monitor and display procedure time and catheter tip temperature T (block  834 ). Continuously monitor and display all pressures and flow rates F (block  836 ). If catheter tip pressure P 5  exceeds Pressure Limit PL 7 , start the shutdown sequence (block  840 ). Pressure Limit PL 7  is a pressure above which the low pressure side of the catheter  300  is not considered safe. 
         [0067]    If F falls below Flow Limit FL 1 , and catheter tip temperature T is less than Temperature Limit TL 1 , start the shutdown sequence (block  840 ). Flow Limit FL 1  is a minimum flow rate below which it is determined that a leak or a plug has occurred in the catheter  300 . FL 1  can be expressed as a percentage of the flow rate set point SPF. Temperature Limit TL 1  is a temperature limit factored into this decision step to prevent premature shutdowns before the catheter  300  reaches a steady state at the designed level of refrigeration power. So, if catheter tip temperature T has not yet gone below TL 1 , a low flow rate will not cause a shutdown. 
         [0068]    If P 3  exceeds Pressure Limit PL 8 , and F is less than Flow Limit FL 2 , start the shutdown sequence (block  840 ). PL 8  is a maximum safe pressure for the high side of the primary system. Flow Limit FL 2  is a minimum flow rate below which it is determined that a plug has occurred in the catheter  300 , when PL 8  is exceeded. FL 2  can be expressed as a percentage of the flow rate set point SPF. 
         [0069]    If P 4  is less than Pressure Limit PL 9 , and F is less than Flow Limit FL 3 , start the shutdown sequence (block  840 ). PL 9  is a pressure below which it is determined that a plug has occurred in the catheter  300 , when flow is below FL 3 . FL 3  can be expressed as a percentage of the flow rate set point SPF. 
         [0070]    An exemplary shutdown sequence will now be described. Send a signal to the fluid controller  208  to stop the primary refrigerant flow (block  840 ). Energize the bypass valve  406  to open the bypass loop (block  842 ). Shut off the precool compressor  100  (block  844 ). Continue running the vacuum pump  400  to pull a vacuum between the outlet of the fluid controller  208  and the inlet of the vacuum pump  400  (block  846 ). Monitor primary high side pressure P 3 , primary low side pressure P 4 , and catheter tip pressure P 5  (block  848 ) until all three are less than the original primary low side pressure which was read in block  804  at the beginning of the procedure (block  850 ). Then, de-energize the recovery pump  500 , recovery valve  506 , vent valve  408 , bypass valve  406 , and vacuum pump  400  (block  852 ). Display a message suggesting the removal of the catheter  300 , and update a log of all system data (block  854 ). 
         [0071]    Similar operational procedures, safety checks, and shutdown procedures would be used for the closed loop primary system shown in  FIG. 2  or  FIG. 3 , except that the primary compressor  600  or compressors  600 ,  618  would provide the necessary primary refrigerant flow rate in place of the supply and recovery cylinders, the fluid controller, and the vacuum and recovery pumps. As with the open loop system, the closed loop system can be operated in the Mapping Mode, where the cold tip temperature might be maintained at minus 10 C., or in the Ablation Mode, where the cold tip temperature might be maintained at minus 65 C. As a first option to achieve the desired cold tip temperature, the precool bypass valve  111  can be adjusted to control the liquid fraction resulting after expansion of the secondary refrigerant, thereby adjusting the refrigeration capacity. Under this option, primary refrigerant high and low pressures are kept constant. As a second option, or in combination with the first option, primary refrigerant flow rate can be by means of operating controllers  601 ,  619  on the primary compressors  600 ,  618  to maintain a high pressure set point SPP which will achieve the desired flow rate, resulting in the desired cold tip temperature. 
         [0072]    A Service Mode is possible, for purification of the primary and secondary refrigerants. In the Service Mode, the normally open bypass valves  111 ,  606  are energized to close. The primary loop purification valves  609 ,  611  are selectively aligned with the purification system  900  to purify the primary refrigerant, or the precool loop purification valves  113 ,  115  are selectively aligned with the purification system  900  to purify the secondary refrigerant. 
         [0073]    In either the Mapping Mode or the Ablation Mode, the desired cold tip temperature control option is input into the control system  700 . Further, the type of catheter is input into the control system  700 . The normally closed charge valve  626  is energized as necessary to build up the primary loop charge pressure. If excessive charging is required, the operator is advised. Further, if precool loop charge pressure is below a desired level, the operator is advised. 
         [0074]    When shutdown is required, the primary loop high side purification valve  609  is closed, and the primary loop compressors  600 ,  618  continue to run, to draw a vacuum in the catheter  300 . When the desired vacuum is achieved, the primary loop low side purification valve  611  is closed. This isolates the primary loop from the catheter  300 , and the disposable catheter  300  can be removed. 
         [0075]    Referring to  FIG. 8 , a system for performing cryoablation procedures is shown and generally designated  910 . As shown, the system  910  includes a cryoablation catheter  912  and a primary fluid source  914 . Preferably, the primary fluid is nitrous oxide (N 2 O) and is held in source  914  at a pressure of around 750 psig.  FIG. 8  also shows that the system  910  includes a console  916  and that the console  916  is connected in fluid communication with the primary fluid source  914  via a fluid line  918 . Console  916  is also connected in fluid communication with the catheter  912  via a fluid line  920 . Further, the console  916  is shown to include a precooler  922 , an exhaust unit  924 , and a computer  926 . 
         [0076]    In detail, the components of the catheter  912  will be best appreciated with reference to  FIG. 9 . There, it will be seen that the catheter  912  includes a catheter tube  928  that has a closed distal end  930  and an open proximal end  932 . Also included as part of the catheter  912 , are a supply tube  934  that has a distal end  936  and a proximal end  938 , and a capillary tube  940  that has a distal end  942  and a proximal end  944 . As shown, the distal end  936  of supply tube  934  is connected with the proximal end  944  of the capillary tube  940  to establish a supply line  946 . Specifically, supply line  946  is defined by the lumen  948  of supply tube  934  and the lumen  950  of capillary tube  940 . It is an important aspect of the system  910  that the diameter (i.e. cross section) of the supply tube  934  is greater than the diameter (i.e. cross section) of the capillary tube  940 . The consequence of this difference is that the supply tube  934  presents much less impedance to fluid flow than does the capillary tube  940 . In turn, this causes a much greater pressure drop for fluid flow through the capillary tube  940 . As will be seen, this pressure differential is used to advantage for the system  910 . 
         [0077]    Still referring to  FIG. 9 , it is seen that the supply line  946  established by the supply tube  934  and capillary tube  940 , is positioned coaxially in the lumen  952  of the catheter tube  928 . Further, the distal end  942  of the capillary tube  940  (i.e. also the distal end of the supply line  946 ) is displaced from the distal end  930  of catheter tube  928  to create an expansion chamber  954  in the tip section  956  of the catheter  912 . Additionally, the placement of the supply line  946  in the lumen  952  establishes a return line  958  in the catheter  912  that is located between the supply line  946  and the wall of the catheter tube  928 . 
         [0078]    Optionally, a sensor  960  can be mounted in expansion chamber  954  (tip section  956 ). This sensor  960  may be either a temperature sensor or a pressure sensor, or it may include both a temperature and pressure sensor. In any event, if used, the sensor  960  can be of a type well known in the art for detecting the desired measurement. Although  FIG. 9  shows both a pressure sensor  962  and a valve  964  positioned at the proximal end  938  of the supply tube  934 , this is only exemplary as the sensor  962  and valve  964  may actually be positioned elsewhere. The import here is that a pressure sensor  962  is provided to monitor a working fluid pressure, “p w ,” on a fluid refrigerant (e.g. N 2 O). In turn, this pressure “p w ” is controlled by a valve  964  as it enters the inlet  966  of the supply line  946 . Further,  FIG. 9  shows that a pressure sensor  968  is provided to monitor a return pressure “p r ” on the fluid refrigerant as it exits from the outlet  970  of the return line  958 . 
         [0079]      FIG. 10  indicates that the various sensors mentioned above are somehow electronically connected to the computer  926  in console  916 . More specifically, the sensors  960 ,  962  and  968  can be connected to computer  926  in any of several ways, all known in the pertinent art. Further,  FIG. 10  indicates that the computer  926  is operationally connected with the valve  964 . The consequence of this is that the computer  926  can be used to control operation of the valve  964 , and thus the working pressure “p w ”, in accordance with preprogrammed instructions, using measurements obtained by the sensors  960 ,  962  and  968  (individually or collectively). 
         [0080]    A schematic of various components for system  910  is presented in  FIG. 11  which indicates that a compressor  972  is incorporated as an integral part of the precooler  922 . More specifically, the compressor  972  is used to compress a secondary fluid refrigerant (e.g. Freon) into its liquid phase for subsequent cooling of the primary refrigerant in the precooler  922 . For purposes of the present invention, the secondary fluid refrigerant will have a normal boiling point that is at a temperature sufficiently low to take the primary fluid refrigerant to a sub-cool condition (i.e. below a temperature where the primary fluid refrigerant will be fully saturated). For the present invention, wherein the primary fluid refrigerant is nitrous oxide, the temperature is preferably around minus forty degrees Centigrade (T sc =−40° C.). 
         [0081]    The operation of system  910  will be best appreciated by cross referencing  FIG. 11  with  FIG. 12 . During this cross referencing, recognize that the alphabetical points (A, B, C, D and E), shown relative to the curve  974  in  FIG. 12 , are correspondingly shown on the schematic for system  910  in  FIG. 11 . Further, appreciate that curve  974 , which is plotted for variations of pressure (P) and temperature (T), represents the fully saturated condition for the primary fluid refrigerant (e.g. nitrous oxide). Accordingly, the area  976  represents the liquid phase of the refrigerant, and area  978  represents the gaseous phase of the refrigerant. 
         [0082]    Point A ( FIG. 11  and  FIG. 12 ) represents the primary fluid refrigerant as it is drawn from the fluid source  914 , or its back up source  914 ′. Preferably, point A corresponds to ambient temperature (i.e. room temperature) and a pressure greater than around 700 psig. After leaving the fluid source  914 , the pressure on the refrigerant is lowered to a working pressure “p w ” that is around 400 psig. This change is controlled by the regulator valve  964 , is monitored by the sensor  962 , and is represented in  FIG. 12  as the change from point A to point B. The condition at point B corresponds to the condition of the primary refrigerant as it enters the precooler  922 . 
         [0083]    In the precooler  922 , the primary refrigerant is cooled to a sub-cool temperature “T sc ” (e.g. −40° C.) that is determined by the boiling point of the secondary refrigerant in the precooler  922 . In  FIG. 12  this cooling is represented by the transition from point B to point C. Note that in this transition, as the primary fluid refrigerant passes through the precooler  922 , it changes from a gaseous state (area  978 ) into a liquid state (area  976 ). Point C in  FIG. 12  represents the condition of the primary fluid refrigerant as it enters the supply line  946  of cryocatheter  12  at the proximal end  938  of supply tube  934 . Specifically, the pressure on the primary fluid refrigerant at this point C is the working pressure “p w ”, and the temperature is the sub-cool temperature “T sc ”. 
         [0084]    As the primary fluid refrigerant passes through the supply line  946  of catheter  12 , its condition changes from the indications of point C, to those of point D. Specifically, for the present invention, point D is identified by a temperature of around minus eighty eight degrees Centigrade (−88° C.) and an outlet pressure “p o ” that is close to 15 psia. Further, as indicated in  FIG. 11 , point D identifies the conditions of the primary fluid refrigerant after it has boiled in the tip section  956  as it is leaving the supply line  946  and entering the return line  958  of the catheter  12 . 
         [0085]    The exhaust unit  924  of the catheter  912  is used to evacuate the primary fluid refrigerant from the expansion chamber  954  of tip section  956  after the primary refrigerant has boiled. During this evacuation, the conditions of the primary refrigerant change from point D to point E. Specifically, the conditions at point E are such that the temperature of the refrigerant is an ambient temperature (i.e. room temperature) and it has a return pressure “p r ”, measured by the sensor  968 , that is slightly less than “p o ”. For the transition from point D to point E, the main purpose of the exhaust unit  924  is to help maintain the outlet pressure “p o ” in the tip section  956  as near to one atmosphere pressure as possible. 
         [0086]    Earlier it was mentioned that the mass flow rate of the primary fluid refrigerant as it passes through the catheter  912  has an effect on the operation of the catheter  912 . Essentially this effect is shown in  FIG. 13 . There it will be seen that for relatively low mass flow rates (e.g. below point F on curve  980  shown in  FIG. 13 ), increases in the mass flow rate of the refrigerant will cause lower temperatures. Refrigerant flow in this range is said to be “refrigeration limited.” On the other hand, for relatively high mass flow rates (i.e. above point F), increases in the mass flow rate actually cause the temperature of the refrigerant to rise. Flow in this range is said to be “surface area limited.” Because the system  910  is most efficient at the lowest temperature for the refrigerant, operation at point F is preferred. Accordingly, by monitoring the temperature of the refrigerant in the tip section  956 , “T t ”, variations of T t  can be used to control the mass flow rate of the refrigerant, to thereby control the refrigeration potential of the catheter  912 . 
         [0087]    In operation, the variables mentioned above (p w , p o , p r , and T t ) can be determined as needed. System  910  then manipulates the regulator valve  964 , in response to whatever variables are being used, to vary the working pressure “p w ” of the primary fluid refrigerant as it enters the supply line  946 . In this way, variations in “p w ” can be used to control “p o ” and, consequently, the refrigeration potential of the catheter  912 . 
         [0088]    While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.