Refrigerant pump

Abstract

The present application relates to a cryogen pump. A device includes a probe, which includes an inner cavity and has a distal end configured to contact living tissue. A temperature sensor is located at the distal end, and a pump having a pump motor is connected to deliver a cryogenic fluid to the distal end of the probe through the inner cavity and receive the cryogenic fluid returned from the probe. There is a separator and a flow meter, the separator is connected to separate the returned cryogenic fluid into a returned cryogenic liquid and a returned cryogenic gas, and the flow meter is connected to measure the flow rate of the returned cryogenic gas. The processor is configured to control the pumping rate of the pump motor in response to the temperature measured by the temperature sensor and the flow rate of the returned cryogenic gas.

Description

Field of the Invention

[0001] The present invention relates generally to pumps and, in particular, to pumping cryogenic materials. Background of the Invention

[0002] Pumps used with cryogenic materials must be able to overcome limitations caused by extreme cold (eg, reduced elasticity) in order to operate efficiently. Systems and methods for improving the efficiency of cryogenic pumps are known in the art.

[0003] US Patent 2,888,879 to Gaarder describes the pumping of cryogenically cooled liquefied gases, particularly liquid oxygen. The pump is of the submerged type, wherein the pump is located within a cryogen vessel and is therefore cooled by the cryogen itself.

[0004] US Patent 3,456,595 to Gottzmann et al. describes a pump that serves two purposes: the first purpose is to pump refrigerant, and the second purpose is to meter the volume flow by counting the refrigerant flow through the piston cycle.

[0005] Berrettini, US Patent 3,958,443, describes an apparatus for verifying and calibrating a meter for measuring an amount of cryogenics dispensed from a storage reservoir.

[0006] US Patent 5,616,838 to Preston et al. describes a cryogenic meter mounted in an insulated container having an inlet and an outlet communicating with a liquid natural gas delivery flow path.

[0007] Kottke, US Pat. No. 6,203,288, describes a reciprocating pump comprising a cylinder having a closed internal compartment. The pump can be used to pump a cryogen.

[0008] Gram et al., US Pat. No. 6,659,730, describes an apparatus for supplying cryogenic liquid and vapor from a storage tank to a pump to reduce ventilation requirements. The pump is operable to pump a cryogenic liquid or a mixture of liquid and vapor.

[0009] Baust et al., US Pat. Nos. 7,192,426 and 8,551,081, describe a cryosurgical system for supplying a cryogen to a probe. The system includes a container filled with cryogen, the container having a bellows of a pump immersed in the cryogen.

[0010] Danley et al., U.S. Pat. No. 8,418,480, describes a cooling system that uses a single-acting positive displacement bellows pump to transfer a cryogenic liquid (e.g., liquid nitrogen) from a storage Dewar to a heat exchanger coupled to a measuring chamber of an instrument.

[0011] US Patent 8,671,700 to Duong et al. describes a cryogenic fluid generator that includes at least one pump assembly having an actuator mounted to a container assembly.

[0012] Baust et al., US Patents 8,998,888 and 9,408,654, describe a cryogenic medical device for delivering a supercooled liquid cryogen to a cryoprobe of various configurations. The cryoprobe may be used to treat damaged, diseased, cancerous, or otherwise unwanted tissue.

[0013] US Patent 9,441,997 to Downie et al. describes a method for measuring physical properties of a two-phase fluid using at least one piezoelectric oscillator immersed in the two-phase fluid.

[0014] US Patent Application 2005 / 0274127 to Drube et al. describes a mobile system for dispensing cryogenic liquids to points of use. The system includes a low pressure bulk tank containing a supply of cryogenic liquid and a high pressure storage tank in communication with the bulk tank to receive the cryogenic liquid from the tank.

[0015] US Patent Application 2010 / 0256621 to Babkin et al. describes a cryoablation system that drives a liquid cryogen or refrigerant along a closed fluid path without evaporating the liquid cryogen.

[0016] US Patent Applications 2014 / 0169993 and 2015 / 0300344 to Berzak et al. describe a cryogen pump that includes a pump portion having a bellows with an outlet opening and an inlet opening at a first end. SUMMARY OF THE INVENTION

[0017] An embodiment of the present invention provides a device comprising:

[0018] a probe comprising an inner lumen and having a distal end configured to contact tissue of a living subject;

[0019] a temperature sensor located at the remote end;

[0020] a pump having a pump motor, the pump being coupled to deliver the cryogenic fluid through the lumen to the distal end of the probe and to receive the cryogenic fluid back from the probe;

[0021] a separator coupled to separate the returning cryogenic fluid into a returning cryogenic liquid and a returning cryogenic gas;

[0022] a flow meter coupled to measure the flow rate of the returning cryogenic gas; and

[0023] A processor is configured to control a pumping rate of the pump motor in response to the temperature measured by the temperature sensor and the flow rate of the returning cryogenic gas.

[0024] In a disclosed embodiment, the pump is a piston pump.

[0025] In another disclosed embodiment, the pump is a bellows pump.

[0026] In yet another disclosed embodiment, as the temperature decreases, the processor sets the pumping rate to a preset high rate without regard to the flow rate of the returning cryogenic gas.

[0027] Typically, when the temperature has dropped to a predetermined steady state value, the processor sets the pumping rate to a preset high rate until the flow rate of the returning cryogenic gas drops from a measured peak high value flow rate to a predetermined lower flow rate.

[0028] In an alternative embodiment, when the flow rate of the returning cryogenic gas comprises a predetermined lower flow rate, the processor reduces the pumping rate to a preset rate that is lower than the preset high rate. Typically, when the pumping rate has been reduced to the lower preset rate, the processor increases the pumping rate when at least one of the temperature and the flow rate changes.

[0029] In another alternative embodiment, the predetermined lower flow rate is a preset percentage of the measured high peak flow rate, the preset percentage being less than 100%. The preset percentage may be 50%.

[0030] In yet another alternative embodiment, the processor sets the pumping rate at a first rate when the temperature is a first steady-state temperature measured by the temperature sensor, and sets the pumping rate at a second rate greater than the first rate when the temperature is a second steady-state temperature less than the first steady-state temperature.

[0031] According to an embodiment of the present invention, there is also provided a method, the method comprising:

[0032] providing a probe including a lumen and having a distal end, the distal end being configured to contact tissue of a living subject;

[0033] positioning a temperature sensor at the distal end;

[0034] coupling a pump including a pump motor to deliver cryogenic fluid through the lumen to the distal end of the probe and to receive cryogenic fluid returned from the probe;

[0035] separating the returning cryogenic fluid into returning cryogenic liquid and returning cryogenic gas;

[0036] measuring the flow rate of the return cryogenic gas; and

[0037] The pumping rate of the pump motor is controlled in response to the temperature measured by the temperature sensor and the flow rate of the returning cryogenic gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The present disclosure will be more fully understood from the following detailed description of embodiments of the present disclosure in conjunction with the accompanying drawings, in which:

[0039] Figure 1 A schematic diagram showing a device being used in surgery according to an embodiment of the present invention;

[0040] Figure 2 is a schematic diagram of a pump according to an embodiment of the present invention;

[0041] Figure 3 is a schematic diagram of a probe according to an embodiment of the present invention;

[0042] Figure 4 According to an embodiment of the present invention Figure 1 A schematic block diagram of a device illustrating how the components of the device are connected together;

[0043] Figure 5A , Figure 5B , Figure 5C a flowchart showing the steps followed by a processor in operating the apparatus according to an embodiment of the present invention; and

[0044] Figure 6 and Figure 7 A graph showing steps of a diagrammatic flow chart according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0045] Overview

[0046] Compared to prior art cryopump systems, embodiments of the present invention measure the temperature obtained by delivering cryogenic fluid to the probe and the flow rate of cryogenic gas returning from the probe. Using these two measurements enables embodiments of the present invention to reduce the volume of cryogenic liquid required to cool the probe to a cryogenic steady state. The reduction in volume occurs both during the transition to a cryogenic steady state and in the steady state itself.

[0047] Therefore, in an embodiment of the present invention, the device includes a probe including an inner cavity and a distal end configured to contact living tissue. A temperature sensor is located at the distal end, and the device includes a pump with a pump motor, the pump is connected to deliver a cryogenic fluid through the inner cavity to the distal end of the probe, and receives the cryogenic fluid returned from the probe.

[0048] The device also includes: a separator connected to separate the returned cryogenic fluid into a returned cryogenic liquid and a returned cryogenic gas; and a flow meter connected to measure the flow rate of the returned cryogenic gas. The processor is configured to control the pumping rate of the pump motor in response to the temperature measured by the temperature sensor and the flow rate of the returned cryogenic gas.

[0049] Detailed Description

[0050] In the following description, the same elements in the drawings are identified by the same numerals.

[0051] Reference Figure 1 , which is a schematic diagram of a device 20 being used in a surgery according to an embodiment of the present invention. For example, the surgery assumed in the following description is for a breast tumor, but it should be understood that the device 20 can be used for other surgeries, such as treating a prostate tumor or a kidney tumor, and all of these surgeries are considered to be included within the scope of the present invention.

[0052] The procedure is performed by a physician 24 on a patient 28, and the physician is able to view the results of the procedure on a display 32 included in the apparatus 20. (Typically, breast cancer surgery involves performing an ultrasound scan of the breast of the patient 28, and presenting the scan results on the display 32. The scan is typically performed by an ultrasound professional rather than the physician 24. The details of the ultrasound scan are not relevant to the present disclosure, and for simplicity, the ultrasound professional is not described in detail. Figure 1 . )

[0053] The device 20 is controlled by a processor 36, which is coupled to a memory 40 in which software 44 for device operation is stored. The processor 36 and memory 40 are installed in an operating console 42. For example, the software 44 in the memory 40 can be downloaded to the processor in electronic form over a network. Alternatively or additionally, the software can be provided on a non-transitory tangible medium, such as an optical, magnetic or electronic storage medium. The software 44 includes software for a device operating algorithm 48, including steps performed by a processor in the operating device 20. The device operating algorithm 48 is described in more detail below.

[0054] In breast tumor surgery, the device 20 is used to introduce a cryogenic fluid 52 initially maintained in a pump 56 into a probe 60 having a distal end that is inserted near the tumor. The fluid 52 is initially in the form of a liquid, but during surgery, the fluid may change from a liquid to a liquid / gas mixture, or even to a completely gaseous state. Unless otherwise specified, as an example, the cryogenic fluid 52 is assumed herein to include liquid or gaseous nitrogen. However, it should be understood that other cryogenic fluids, such as cryogenic argon, may be used in the device 20, and all of these cryogenic fluids are considered to be included within the scope of the present invention. The pump 56 is connected to the probe 60, and the doctor 24 manipulates the probe 60 so as to correctly position the probe distal end relative to the tumor. (The operation is usually assisted by a doctor observing the ultrasound scan mentioned above.) Both the pump 56 and the probe 60 will be described in detail below.

[0055] Figure 2 is a schematic diagram of a pump 56 according to an embodiment of the present invention. Pump 56 is a piston pump including a piston 64 driven by a motor 68 coupled to the piston. In embodiments of the present invention, cryogenic fluid pumps other than piston pumps such as pump 56, such as bellows pumps, may be used, and the description herein may be modified, mutatis mutandis, to accommodate these different pumps.

[0056] The pump 56 is connected to the Dewar 72 which contains the cryogenic fluid 52 which remains in liquid form in the lower space 80 of the Dewar. Above the liquid in the Dewar, in the upper space 84, there is a gas formed by the evaporation of the cryogenic liquid.

[0057] like Figure 2 As shown, the piston of the pump 56 and the pump element attached to the piston are immersed in the liquid form of the fluid 52. When the motor 68 is running, the cryogenic fluid in liquid form exits the Dewar through the outlet check valve 76 and the outlet tube 94. The fluid exiting from the valve 76 and the tube 94 is typically close to 100% liquid, and the fluid exiting from the pump is delivered to the probe 60 through the outlet tube.

[0058] Pump 56 is connected to receive the returned cryogenic material from probe 60. The returned cryogenic material is typically a liquid-gas mixture, although in some cases the returned material may include nearly 100% gas, or possibly nearly 100% liquid. The returned material is input to liquid / gas separator 88 through receiving tube 92. Separator 88 returns the separated cryogenic liquid to the existing cryogenic liquid in space 80 and allows the separated gas to enter the upper space 84.

[0059] The upper space 84 is connected to a gas flow meter 100 ( Figure 4), the gas flow meter 100 is connected to the atmosphere. Therefore, the separated gas entering the upper space 84 is discharged into the atmosphere through the gas flow meter 100.

[0060] Figure 3 1 is a schematic diagram of a probe 60 according to an embodiment of the present invention. Probe 60 includes a handle 104, which is attached to a shaft 108 of the probe at the proximal end of the shaft. Shaft 108 terminates in a pointed distal end 112, which enables the shaft to pierce tissue, such as a portion of a breast of patient 28. As shown in section 116, shaft 108 includes three concentric tubes, typically made of thin-walled stainless steel. A first inner tube 120 surrounds a central lumen 124, and the inner tube is surrounded by a second tube 128. The first tube and the second tube are separated by an intermediate space 132. A third outer tube 136 surrounds the second tube 128, and the second tube and the third tube are separated by a space 140.

[0061] A temperature sensor 144 (typically a thermocouple or thermistor) is fixedly located within the distal end 112. A cable 148 for the temperature sensor is typically located within the space 140. The cable is connected to the processor 36 and enables the processor to measure the temperature sensed by the sensor 144.

[0062] In operation of the device 20, the space 140 between the second and third tubes of the shaft 108 is maintained in a sealed evacuated state. As explained in more detail below, the central lumen 124 is used to deliver cryogenic fluid from the pump 56 to the distal end 112, and the intermediate space 132 is used to return cryogenic fluid from the distal end to the pump.

[0063] A flexible tube 152 having an internal structure substantially similar to that of shaft 108 is coupled to the shaft via handle 104. The lumen within tube 152 is configured to deliver cryogenic fluid from pump 56 to central lumen 124 and to transfer returning cryogenic fluid from intermediate space 132 to the pump.

[0064] Figure 4 is a block diagram of a device 20 illustrating how the components of the device according to an embodiment of the present invention are connected together. The block diagram illustrates the flow of cryogenic fluids and the communication of signal data between the components.

[0065] Processor 36 controls the operation of device 20 by providing a pump motor control signal to motor 68. When activated, motor operates pump 56 to discharge cryogenic fluid from Dewar 72 through outlet valve 76. The discharged cryogenic fluid flows out of the Dewar, through flexible tube 152 and handle 104, into probe 60 to distal end 112.

[0066] The cryogenic fluid is usually returned as a liquid / gas mixture from the distal end 112 through the probe 60, the handle 104 and the flexible tube 152 to the liquid / gas separator 88 in the Dewar flask 72. The separator separates the returning cryogenic fluid into gas entering the upper space 84 and liquid entering the lower space 80. The gas entering the upper space 84 is discharged into the atmosphere through the gas flow meter 100.

[0067] As shown, a temperature sensor 144 located at the distal end of the probe provides a signal to the processor 36 indicating the temperature of the probe tip. The processor 36 also receives a signal from the gas meter 100 indicating the return gas flow rate. This rate corresponds to the rate of gas flow input from the separator 88 to the space 84. The processor 36 uses these signals to run the algorithm 48, which enables the processor to generate an output signal to control the pump motor 68. The output signal transmitted to the pump motor controls the speed of the motor.

[0068] According to an embodiment of the present invention, Figure 5A , Figure 5B , Figure 5C A flow chart showing the steps followed by processor 36 in operating device 20, and Figure 6 and Figure 7 A graph illustrating the steps of a flowchart is shown. The steps of the flowchart correspond to the steps of the algorithm 48.

[0069] Algorithm 48 includes three flowcharts: a first flowchart 160, also referred to herein as a temperature flowchart 160; a second flowchart 164, also referred to herein as a gas flow rate flowchart 164; and a third flowchart 220, also referred to herein as a steady-state flowchart 220. When operating device 20, the processor initially starts flowcharts 160 and 164 simultaneously, and does not start steady-state flowchart 220. Typically, the processor starts flowcharts 160 and 164 after probe 60 has been placed in the patient, and physician 24 uses console 42 to send a command to processor 36 to start the flowchart. (As described below, steady-state flowchart 220 will be started later.)

[0070] When starting flowcharts 160 and 164, in the initial operation step 168 of both flowcharts, the physician 24 sets a target steady-state operating temperature T for the device 20. S The steady-state operating temperature of the device is T S Above the boiling point of the cryogenic fluid 52, it is assumed here that the cryogenic fluid 52 comprises liquid nitrogen (boiling point -196°C). The steady-state operating temperature corresponds to the temperature maintained at the probe tip once the pump speed of the motor 68 is set to a constant low speed by the processor 36. Typically, the steady-state operating temperature depends on the constant low speed of the motor, such that the higher the constant low speed, the lower the steady-state operating temperature.

[0071] In the disclosed embodiment, the constant low speed of the motor 68 is approximately 28 rpm (revolutions per minute) for a steady-state operating temperature of approximately -160° C., and the constant low speed of the motor is approximately 8 rpm for a steady-state operating temperature of approximately -80° C. However, it should be understood that these values ​​of constant low speed and steady-state operating temperature are merely examples, and thus, the scope of the present invention includes constant low speeds of the motor 68 other than 8 rpm and 28 rpm, and corresponding steady-state temperatures other than -80° C. and -160° C. It should also be understood that the constant low speed required for a desired steady-state operating temperature can be determined by one of ordinary skill in the art without undue experimentation.

[0072] Additionally, in an initial step 168 , processor 36 increases the speed of motor 68 to a high speed.

[0073] In the gas flow rate flowchart 164 , control passes to a gas flow rate recording step 170 where the processor 36 records the gas flow rate of the gas flow meter 100 .

[0074] At the start, the probe 60 and its connecting flexible tube 152 are typically at approximately room temperature, and the processor accesses the signal from the temperature sensor 144 in the probe tip to determine the probe tip temperature T. In the first decision step 172 of both flowcharts, the processor checks whether the temperature T is greater than the steady-state operating temperature T set in step 168. S .

[0075] In flowchart 160, if decision 172 returns a negative answer, that is, if T>T S , the flow chart proceeds to high pump speed step 176, where the processor continues to operate the motor 68 at high speed. In the disclosed embodiment, high speed is approximately in the range of 40rpm-50rpm. Control in the flow chart then returns to decision 172, causing the processor 36 to re-determine.

[0076] In flowchart 164, if decision 172 returns negative, the flowchart proceeds to temperature flowchart step 180 where the processor sets the speed of motor 68 according to temperature flowchart 160. Control returns from step 180 to decision 172 whereupon processor 36 re-makes the decision as in flowchart 160.

[0077] T>T appears at the beginning of the flowchart S The state at this time corresponds to the initial freezing process of the device 20.

[0078] Figure 6 shows a steady-state temperature T S The curve of the process is about -160℃, and Figure 7 shows a steady-state temperature T SThe curve of the process is about -80℃. Figure 6 In FIG. 3 , graph 300 plots temperature T (° C.) versus time (s), graph 304 plots gas flow rate G (L / min) versus time, and graph 308 plots motor speed S (rpm) versus time. Figure 7 , graph 400 plots temperature T (° C.) versus time, graph 404 plots gas flow rate G (L / min) versus time, and graph 408 plots motor speed S (rpm) versus time.

[0079] As shown in the graph, during the first startup time period 312 and 412, which typically lasts about 30 seconds from the start of operation of the device 20, the temperature T drops sharply to about -160°C ( Figure 6 ) and -80℃( Figure 7 ), while the speed S of the motor 68 rises to 48 rpm ( Figure 6 ) and 30rpm( Figure 7 The time period corresponds to the time when the decision 172 is repeated.

[0080] In flowchart 160, when decision 172 returns affirmative, that is, T ≤ T S When , control moves to gas flow rate flowchart step 184, where the processor sets the speed of motor 68 according to flowchart 164. Even if the processor accesses flowchart 164, the processor continues to iterate decision 172 to check the inequality T≤T S It is certain that it will continue to return.

[0081] In the flowchart 164, when the decision 172 returns affirmative, control moves to the second decision 188, where the processor accesses a signal indicating the flow rate of the gas flow meter 100. In the second decision 188, the processor checks whether the flow rate of the gas through the meter 100 has dropped from a high value to a predetermined lower gas flow rate. The predetermined lower gas flow rate is a gas flow rate that, when reached, indicates that the speed of the motor 68 can be reduced without adversely affecting the steady-state temperature that has been reached. The predetermined lower gas flow rate can be found by one of ordinary skill in the art without undue experimentation.

[0082] As shown in the graph, the gas flow rate generally initially increases from zero during the first startup time period 312, 412. Even when the first startup time period ends, the graph illustrates that the gas flow rate continues to increase, generally increasing during the second startup time period 316 ( Figure 6 ) reaches a high peak value of about 100 L / min and in the second start-up period 416 ( Figure 7) reaches a high peak of about 80 L / min. In one embodiment, after the first startup time period ends, the second startup time period 316, 416 has a duration of about 30 seconds. In the third startup time period 320 ( Figure 6 ) and the third start time period 420 ( Figure 7 ), starting from the end of the second start-up time period, the gas flow rate then begins to decrease while still remaining relatively high until a predetermined lower gas flow rate is reached.

[0083] The predetermined lower gas flow rate is less than 100% of the high peak rate and is generally set as a fraction of the high peak rate, i.e., a percentage. In one embodiment, the percentage is set to about 50%, so that for a high peak of about 100 L / min, the predetermined lower gas flow rate is about 50 L / min ( Figure 6 ), and for a high peak of about 80 L / min, the predetermined lower gas flow rate is about 40 L / min ( Figure 7 ). It will be appreciated that the 50% value is approximate and exemplary, and thus the scope of the present invention includes values ​​of the predetermined lower gas flow rate that are lower or higher than 50% of the high peak rate.

[0084] The high gas flow rates during the first three initial time periods are due to the cryogenic fluid cooling components of the device 20, including the flexible tube 152 and the probe 60. As the components approach a near steady state, the gas flow rates approach a near steady state value.

[0085] As described above, in the second decision step 188, the processor 36 checks whether the gas flow rate through the meter 100 has dropped from a high value to a predetermined lower value gas flow rate. The processor performs this check by accessing the gas flow rate stored in step 170.

[0086] If the second decision step 188 returns negative, that is, the gas flow rate has not reached the predetermined lower value of the gas flow rate, then control proceeds to the high speed pump step 192, wherein the processor continues to maintain the speed S of the motor 68 at a high rate. Control returns from step 192 to the second decision step so that the processor repeats the step.

[0087] If the second decision step 188 returns positive, that is, the gas flow rate has reached the predetermined lower value of the gas flow rate, then control is transferred to the reduce pump speed step 196, wherein the processor iteratively reduces the speed of the motor 68 by a predetermined fraction or percentage, where the predetermined fraction or percentage is assumed to be 10%, however, the predetermined percentage may be lower or higher than this value. As described below, the iteration of step 196 depends on the third decision 200 returning negative.

[0088] The second decision step 188 that returns an affirmative result corresponds to the third start-up time period 320 ( Figure 6 ) and the third start time period 420 ( Figure 7 ) terminates and transition time periods 324 and 424 begin.

[0089] Control passes from step 196 to a third decision step 200, where the processor 36 checks whether the gas flow rate has decreased to an approximately constant value. S = -160℃, the approximate constant value is about 35L / min. S = -80 °C, the approximate constant value is about 18 L / min, but other embodiments may have a specific T S Values ​​above or below these values ​​are approximately constant.

[0090] If the third determination returns negative, and therefore the gas flow rate is not approximately constant, control returns to the reduce pump speed step 196 so that a further reduction in motor speed is performed and the third determination is repeated.

[0091] If the third determination returns positive, and therefore the gas flow rate is approximately constant, control transfers to step 204 of setting the pump speed at a lower rate, where the processor gradually reduces the speed of the motor 68 to a preset fixed lower rate. S = -160℃, fixed lower speed is about 28rpm, at T S = -80°C, the fixed lower speed is about 8 rpm.

[0092] The third decision 200 that returns an affirmative response corresponds to the device 20 achieving a steady state, so step 204 also includes starting a steady state flowchart 220. The achievement of a steady state corresponds to the end of a transition period 324 and the beginning of a steady state period 328 ( Figure 6 ), and corresponds to the end of the transition period 424 and the beginning of the steady-state period 428 ( Figure 7 ).

[0093] At the beginning of the steady-state time periods 328 and 428 , ie, when performing step 204 , the processor stops the execution of the temperature flow chart 160 and the gas flow rate flow chart 164 and starts the steady-state flow chart 220 .

[0094] In the steady state flow chart 220, in decision step 224, the processor checks whether both the temperature T and the flow rate G are constant. If the determination returns in the affirmative, then in a maintain pump speed constant step 228, the processor maintains the motor 68 at its low speed. If the determination returns in the negative, then in an increase pump speed step 232, the processor increases the speed of the motor 68 by a preset amount, typically about 2-6 rpm. After steps 228 and 232, control returns to decision step 224, and the processor 36 repeats this step.

[0095] It should be appreciated that by measuring the temperature at the distal end of the probe and the flow rate of the returning cryogenic gas, embodiments of the present invention significantly improve the operating efficiency of the cryogenic pumping system. Using both temperature and flow rate enables steady-state cryogenic temperatures to be achieved and maintained while reducing the volume of cryogenic fluid used compared to prior art systems.

[0096] It will therefore be appreciated that the embodiments described above are cited as examples, and that the present invention is not limited to what has been particularly shown and described hereinabove. On the contrary, the scope of the present invention includes combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to a person skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Claims

1. A device for pumping cryogenic fluid, comprising: a probe comprising an inner lumen and having a distal end configured to contact tissue of a living subject; a temperature sensor located at the distal end; a pump including a pump motor, the pump being coupled to deliver cryogenic fluid through the lumen to the distal end of the probe and to receive cryogenic fluid returned from the probe; a separator coupled to separate the returning cryogenic fluid into a returning cryogenic liquid and a returning cryogenic gas; a flow meter coupled to measure a flow rate of the returning cryogenic gas; and A processor is configured to control a pumping rate of the pump motor in response to the temperature measured by the temperature sensor and the flow rate of the returning cryogenic gas.

2. The device according to claim 1, wherein: The pump comprises a piston pump.

3. The device according to claim 1, wherein: The pump comprises a bellows pump.

4. The device according to claim 1, wherein: When the temperature decreases, the processor sets the pumping rate to increase the pumping rate to a first preset rate regardless of the flow rate of the returning cryogenic gas.

5. The device according to any one of claims 1 to 4, wherein: When the temperature has decreased to a predetermined steady state value, the processor sets the pumping rate to increase the pumping rate to a first preset rate until the flow rate of the returning cryogenic gas decreases from a measured peak flow rate to a predetermined gas flow rate.

6. The device according to claim 5, wherein: When the flow rate of the returning cryogenic gas comprises the predetermined gas flow rate, the processor reduces the pumping rate to a second preset rate that is lower than the first preset rate.

7. The device according to claim 6, wherein: When the pumping rate has decreased to the second preset rate, the processor causes the pumping rate to increase when at least one of the temperature and the flow rate changes.

8. The device according to claim 5, wherein: The predetermined gas flow rate is a preset percentage of the measured peak flow rate, the preset percentage being less than 100%.

9. The device according to claim 8, wherein: The preset percentage is 50%.

10. The device according to any one of claims 1 to 4, wherein: The processor sets the pumping rate at a first rate when the temperature comprises a first steady-state temperature measured by the temperature sensor, and sets the pumping rate at a second rate greater than the first rate when the temperature comprises a second steady-state temperature less than the first steady-state temperature.

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