Refrigeration source for a cryoablation catheter

Info

Publication number: 20050159735
Type: Application
Filed: Jul 9, 2004
Publication Date: Jul 21, 2005
Inventors: Jay Walton (Carlisle, MA), Lizhong Zheng (Canton, MA), John Ketchum (Harvard, MA), Mark Wallace (Bedford, MA), Steven Howard (Ashland, MA)
Application Number: 10/888,804

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

Description

This application is 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

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

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.

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.

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 N₂O), 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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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 (N₂O). 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.

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.).

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.

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.

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.

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.

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.

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).

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_wto 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.

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.

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).

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

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:

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;

FIG. 2 is a schematic of a second embodiment of the apparatus of the present invention, using a compressor as the primary refrigerant source;

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;

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;

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;

FIG. 6 is a schematic of a parameter display for use with the control equipment of the present invention; and

FIG. 7 is a flow diagram showing one control sequence for use with the control apparatus of the present invention.

FIG. 8 is a perspective view of the system of the present invention;

FIG. 9 is a cross-sectional view of the catheter of the present invention as seen along the line 2-2 in FIG. 8;

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;

FIG. 11 is a schematic view of the interactive components in the console of the present invention;

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

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

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-508b, and a typical refrigerant for the precool circuit would be R-410a. 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.

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.

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.

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.

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.

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.

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 2way, 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.

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.

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.

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.

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.

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.

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 P1, the primary recovery pressure sensor 510 inputs recovery bottle pressure P2, the high pressure primary sensor 210 inputs high side primary pressure P3, the low pressure primary sensor 410 inputs low side primary pressure P4, the catheter tip pressure sensor 310 inputs catheter tip pressure P5, 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.

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 P3, the low pressure primary sensor 610 inputs low side primary pressure P4, the catheter tip pressure sensor 310 inputs catheter tip pressure P5, 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 PL2 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 PL2B to the controller 601 of the high side primary compressor 600, and the control system 700 provides a maximum low side pressure set point PL2A to the controller 619 of the low side primary compressor 618.

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 P3, low side primary pressure P4, 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, P3, P4, and F, as well as the average, maximum, and minimum values of these parameters.

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 PL1=160 psia; PL2=400 psia; PL3=500 psia; PL4=700 psia, PL5=600 psia; PL6=5 psia; PL7=diastolic pressure; PL8=375 psia; and PL9=5 psia. Temperature limits, flow limits, procedure times, and procedure types are set by the operator according to the procedure being performed.

Perform self tests (block 802) of the control system circuitry and connecting circuitry to the sensors and controllers to insure circuit integrity.

Read and store supply cylinder pressure P1, primary low pressure P4, and catheter tip pressure P5 (block 804). At this time, P4 and P5 are at atmospheric pressure. If P1 is less than Pressure Limit PL2 (block 808), display a message to replace the supply cylinder (block 810), and prevent further operation. If P1 is greater than PL2, but less than Pressure Limit PL3, display a message to replace the supply cylinder soon, but allow operation to continue.

Read precool charge pressure PB and recovery cylinder pressure P2 (block 806). If PB is less than Pressure Limit PL1 (block 808), display a message to service the precool loop (block 810), and prevent further operation. If P2 is greater than Pressure Limit PL4 (block 808), display a message to replace the recovery cylinder (block 810), and prevent further operation. If P2 is less than PL4, but greater than Pressure Limit PL5, display a message to replace the recovery cylinder soon, but allow operation to continue.

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.

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 P5. At this time, T is the patient's body temperature and P5 is atmospheric pressure.

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 P3, P4, and P5 (block 824), until all three are less than Pressure Limit PL6 (block 826).

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.

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 P5 exceeds Pressure Limit PL7, start the shutdown sequence (block 840). Pressure Limit PL7 is a pressure above which the low pressure side of the catheter 300 is not considered safe.

If F falls below Flow Limit FL1, and catheter tip temperature T is less than Temperature Limit TL1, start the shutdown sequence (block 840). Flow Limit FL1 is a minimum flow rate below which it is determined that a leak or a plug has occurred in the catheter 300. FL1 can be expressed as a percentage of the flow rate set point SPF. Temperature Limit TL1 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 TL1, a low flow rate will not cause a shutdown.

If P3 exceeds Pressure Limit PL8, and F is less than Flow Limit FL2, start the shutdown sequence (block 840). PL8 is a maximum safe pressure for the high side of the primary system. Flow Limit FL2 is a minimum flow rate below which it is determined that a plug has occurred in the catheter 300, when PL8 is exceeded. FL2 can be expressed as a percentage of the flow rate set point SPF.

If P4 is less than Pressure Limit PL9, and F is less than Flow Limit FL3, start the shutdown sequence (block 840). PL9 is a pressure below which it is determined that a plug has occurred in the catheter 300, when flow is below FL3. FL3 can be expressed as a percentage of the flow rate set point SPF.

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 P3, primary low side pressure P4, and catheter tip pressure P5 (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).

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.

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.

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.

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.

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 (N₂O) 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.

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.

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.

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. N₂O). 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.

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).

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.).

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.

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.

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”.

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.

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.

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_tcan be used to control the mass flow rate of the refrigerant, to thereby control the refrigeration potential of the catheter 912.

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.

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.

Claims

1. A cryogenic medical system comprising:

a medical device;

a refrigeration system for cooling the medical device including a primary cooling loop circulating a coolant through the medical device; and a precooling loop cooling the coolant before the coolant reaches the medical device;

wherein the primary cooling loop includes a recovery system which recovers the coolant after it passes through the medical device and wherein the primary cooling loop is substantially an open loop.

2. The cryogenic medical system of claim 1 wherein the primary cooling loop includes:

a pressure bottle supplying the coolant to the primary cooling loop; and

a compressor interposed between the medical device and a recovery bottle.

3. The cryogenic medical system of claim 1

wherein the precooling loop includes a heat exchanger having an inlet and an outlet, the heat exchanger enveloping a portion of the primary cooling loop; and

wherein the precooling loop is a substantially closed loop further comprising a compressor and an aftercooler.

4. The cryogenic medical system of claim 1

wherein the precooling loop includes a heat exchanger having an inlet and an outlet, the heat exchanger defining a coolant flow path and enveloping a portion of the primary cooling loop; and

further comprising a compressor in fluid communication with an aftercooler outputting coolant to the inlet of the heat exchanger and receiving coolant from the outlet of the heat exchanger.

5. A cryogenic medical system comprising:

a medical device;

a console;

a primary cooling system directing a coolant to the medical device; and,

a precooling system cooling the coolant before the coolant reaches the medical device;

wherein the primary cooling system includes a recovery system which recovers the coolant after it passes through the medical device and wherein the primary cooling system is substantially an open loop.

6. The cryogenic medical system of claim 5 wherein the primary cooling system includes:

a pressure bottle supplying the coolant to the primary cooling system; and

a compressor interposed between the medical device and a recovery bottle.

7. The cryogenic medical system of claim 5

wherein the precooling system includes a heat exchanger having an inlet and an outlet, the heat exchanger enveloping a portion of the primary cooling system; and

wherein the precooling system is a substantially closed loop further comprising a compressor and an aftercooler.

8. The cryogenic medical system of claim 5

wherein the precooling system includes a heat exchanger having an inlet and an outlet, the heat exchanger defining a coolant flow path and enveloping a portion of the primary cooling system; and

further comprising a compressor in fluid communication with an aftercooler outputting coolant to the inlet of the heat exchanger and receiving coolant from the outlet of the heat exchanger.

9. A cryogenic medical system comprising:

a medical device having a connection point;

a console;

a primary cooling system directing a coolant to the medical device; and

a precooling system cooling the coolant before the coolant reaches the connection point;

wherein the primary cooling system includes a recovery system which recovers the coolant after it passes through the medical device and wherein the primary cooling system is substantially an open loop.

10. The cryogenic medical system of claim 9 wherein the primary cooling system includes:

a pressure bottle supplying the coolant to the primary cooling system; and

a compressor interposed between the medical device and a recovery bottle.

11. The cryogenic medical system of claim 9

wherein the precooling system includes a heat exchanger having an inlet and an outlet, the heat exchanger enveloping a portion of the primary cooling system; and

wherein the precooling system is a substantially closed loop further comprising a compressor and an aftercooler.

12. The cryogenic medical system of claim 9

wherein the precooling system includes a heat exchanger having an inlet and an outlet, the heat exchanger defining a coolant flow path and enveloping a portion of the primary cooling system; and

further comprising a compressor in fluid communication with an aftercooler outputting coolant to the inlet of the heat exchanger and receiving coolant from the outlet of the heat exchanger.

13. A cryogenic medical system comprising:

a medical device having a connection point;

a console, the console being used to cool the medical device and being connectable to the medical device at the connection point, the console including a primary cooling system directing a coolant to the medical device along a supply conduit; and a precooling system cooling the coolant within the supply conduit before the coolant reaches the connection point;

wherein the primary cooling system includes a return conduit leading from the medical device to a recovery system which recovers the coolant after it passes through the medical device and wherein the primary cooling system is substantially an open loop.

14. The cryogenic medical system of claim 13 wherein the primary cooling system includes:

a pressure bottle in fluid communication with the supply conduit; and

a compressor interposed between the medical device a recovery bottle.

15. The cryogenic medical system of claim 13

wherein the precooling system includes a heat exchanger having an inlet and an outlet, the heat exchanger enveloping a portion of the supply conduit; and

wherein the precooling system is a substantially closed loop further comprising a compressor and a condenser.

16. The cryogenic medical system of claim 13

wherein the precooling system includes a heat exchanger having an inlet and an outlet, the heat exchanger defining a coolant flow path and enveloping a portion of the supply conduit; and

further comprising a compressor in fluid communication with an aftercooler outputting coolant to the inlet of the heat exchanger and receiving coolant from the outlet of the heat exchanger.

17. A cryogenic medical system comprising:

a medical device;

a console, the console being connectable to the medical device at a connection point, the console controlling temperature of the medical device, and the console including a first cooling system directing coolant to the medical device at a first temperature along a coolant supply line; and a second cooling system chilling the coolant within the coolant supply line to a temperature below the first temperature before the coolant reaches the connection point;

wherein the first cooling system includes a coolant return line leading from the catheter to a coolant scavenging system, and wherein the first cooling system and the catheter comprise a substantially open-loop.

18. The cryogenic medical system of claim 17 wherein the first cooling system includes:

a coolant reservoir in fluid communication with the fluid supply line; and

a vacuum pump interposed between the catheter and the coolant collection tank.

19. The cryogenic medical system of claim 17

wherein the second cooling system includes an enclosure having an inlet and an outlet; the enclosure defining a fluid path from the inlet to the outlet, and the enclosure enveloping a portion of the coolant supply line; and

further comprising a compressor in fluid communication with a condenser outputting coolant to the inlet of the enclosure and receiving coolant from the outlet of the enclosure.

20. A cryogenic medical system comprising:

a medical device, wherein the medical device includes a catheter;

a refrigeration system for cooling the medical device including a primary cooling loop circulating a coolant through the medical device; and a precooling loop cooling the coolant before the coolant reaches the catheter;

wherein the primary cooling loop includes a recovery system which recovers the coolant after it passes through the catheter and wherein the primary cooling loop is substantially an open loop; and

wherein the precooling loop includes a heat exchanger having an inlet and an outlet, the heat exchanger enveloping a portion of the primary cooling loop.

21. The cryogenic medical system of claim 20

wherein the coolant is supplied to the primary cooling loop in a substantially gaseous state; and

wherein the precooling loop cools the coolant within the primary cooling loop from a substantially gaseous state to a substantially liquid state.

22. A cryogenic medical system comprising:

a medical device, wherein the medical device includes a catheter;

a console;

a primary cooling system directing a coolant to the catheter;

a precooling system cooling the coolant before the coolant reaches the catheter;

wherein the primary cooling system includes a recovery system which recovers the coolant after it passes through the catheter and wherein the primary cooling system is substantially an open loop; and

wherein the precooling system includes a heat exchanger having an inlet and an outlet, the heat exchanger enveloping a portion of the primary cooling system.

23. The cryogenic medical system of claim 22

wherein the coolant is supplied to the primary cooling system in a substantially gaseous state; and

wherein the precooling system cools the coolant within the primary cooling system from a substantially gaseous state to a substantially liquid state.

24. A cryogenic medical system comprising:

a medical device having a connection point, wherein the medical device includes a catheter;

a console;

a primary cooling system directing a coolant to the catheter;

a precooling system cooling the coolant before the coolant reaches the connection point;

wherein the primary cooling system includes a recovery system which recovers the coolant after it passes through the catheter and wherein the primary cooling system is substantially an open loop; and

wherein the precooling system includes a heat exchanger having an inlet and an outlet, the heat exchanger enveloping a portion of the primary cooling system.

25. The cryogenic medical system of claim 24

wherein the coolant is supplied to the primary cooling system in a substantially gaseous state; and

wherein the precooling system cools the coolant within the primary cooling system from a substantially gaseous state to a substantially liquid state.

26. A cryogenic medical system comprising:

a medical device having a connection point, wherein the medical device includes a catheter;

a console, the console being used to cool the catheter and being connectable to the medical device at the connection point, the console including a primary cooling system directing a coolant to the catheter along a supply conduit; and a precooling system cooling the coolant within the supply conduit before the coolant reaches the connection point;

wherein the primary cooling system includes a return conduit leading from the catheter to a recovery system which recovers the coolant after it passes through the catheter and wherein the primary cooling system is substantially an open loop; and

wherein the precooling system includes a heat exchanger having an inlet and an outlet, the heat exchanger enveloping a portion of the supply conduit.

27. The cryogenic medical system of claim 26

wherein the coolant is supplied to the primary cooling system in a substantially gaseous state; and

wherein the precooling system cools the coolant within the primary cooling system from a substantially gaseous state to a substantially liquid state.

28. A cryogenic medical system comprising:

a medical device, wherein the medical device includes a catheter;

a system component, the system component being connectable to the medical device at a connection point, the system component controlling temperature of the medical device, and the system component including a first cooling system directing coolant to the medical device at a first temperature along a coolant supply line; and a second cooling system chilling the coolant within the coolant supply line to a temperature below the first temperature before the coolant reaches the connection point,

wherein the first cooling system includes a coolant return line leading from the catheter to a coolant scavenging system, and wherein the first cooling system and the catheter comprise a substantially open-loop, and

wherein the second cooling system includes an enclosure having a fluid inlet and a fluid outlet; the enclosure defining a fluid path from the inlet to the outlet, and the enclosure enveloping a portion of the coolant supply line, and

wherein the inlet is provided inside the enclosure.

29. The cryogenic medical system of claim 28

wherein coolant is supplied to the first cooling system in a substantially gaseous state; and,

wherein the second cooling system chills the coolant within the first cooling system and coolant supply line from a substantially gaseous state to a substantially liquid state.

30. Apparatus for performing cryosurgery, comprising:

a refrigerant supply source connectable to a high pressure duct;

a cryosurgery catheter having an inlet connectable to said high pressure duct, said catheter having a tip;

a Joule-Thomson refrigerant expansion element in said catheter;

a temperature sensor on said catheter;

a pressure sensor adapted to sense pressure inside said catheter tip;

a low pressure duct connectable to an outlet of said catheter;

a flow sensor in said low pressure duct downstream of said catheter; and

a control system connected and programmed to maintain a selected catheter temperature, in response to signals from said temperature sensor, said pressure sensor, and said flow sensor.

31. An apparatus as recited in claim 30, further comprising:

a precool heat exchanger in said high pressure duct;

a precool compressor for compressing a secondary refrigerant; and

a precool expansion element connected to said precool compressor for expanding said secondary refrigerant to cool said precool heat exchanger.

32. An apparatus as recited in claim 31, further comprising:

a bypass valve connected between an outlet of said precool compressor and an inlet of said precool compressor.

33. An apparatus as recited in claim 30, wherein:

said refrigerant supply source comprises a pressure bottle; and

a fluid controller in said high pressure duct; and

further comprising a recovery bottle connected to said low pressure duct.

34. An apparatus as recited in claim 33, wherein said fluid controller comprises a pressure controller.

35. An apparatus as recited in claim 33, wherein said fluid controller comprises a flow controller.

36. An apparatus as recited in claim 33, further comprising:

a vacuum pump having an inlet connected to said low pressure duct;

a recovery pump having an inlet connected to an outlet of said vacuum pump, said recovery pump having an outlet connected to said recovery bottle;

a bypass valve in a bypass duct connected between said high pressure duct and said low pressure duct; and

a vent valve connected to said bypass duct between said bypass valve and said high pressure duct.

37. An apparatus as recited in claim 30, wherein:

said refrigerant supply source comprises a compressor;

said high pressure duct is connected to an outlet of said compressor;

a compressor controller; and

said control system operates said compressor controller to maintain refrigerant pressure above a selected level in said high pressure duct.

38. An apparatus as recited in claim 37, further comprising a second compressor with a second compressor controller;

wherein:

said low pressure duct is connected to an inlet of said second compressor;

an outlet of said second compressor is connected to an inlet of said first compressor;

said control system operates said first compressor controller to maintain refrigerant pressure above a selected level in said high pressure duct; and

said control system operates said second compressor controller to maintain refrigerant pressure below a selected level in said low pressure duct.

39. Apparatus for performing-cryosurgery, comprising:

a primary refrigerant pressure bottle connectable to a high pressure duct;

a fluid pressure controller in said high pressure duct;

a precool heat exchanger in said high pressure duct;

a precool compressor for compressing a secondary refrigerant;

a secondary expansion element connected to expand said secondary refrigerant to cool said precool heat exchanger;

a cryosurgery catheter having an inlet connectable to said high pressure duct;

a primary Joule-Thomson expansion element in said catheter connected to expand said primary refrigerant to cool a portion of said catheter;

a temperature sensor on said catheter;

a low pressure duct connectable to an outlet of said catheter;

a pressure sensor in said low pressure duct;

a flow sensor in said low pressure duct;

a vacuum pump having an inlet connected to said low pressure duct;

a recovery pump having an inlet connected to an outlet of said vacuum pump;

a recovery bottle connected to an outlet of said recovery pump;

a bypass valve in a bypass duct connected between said high pressure duct and said low pressure duct; and

a control system connected and programmed to operate said pressure controller to maintain a selected primary refrigerant flow rate, in response to signals from said temperature sensor, said pressure sensor, and said flow sensor.