Abstract:
A cryosurgical apparatus capable of operating both a first cryoprobe with a pressure-resistant return run and a second cryoprobe with an unpressurized return run. The apparatus has a first mode for operating the first cryoprobe and a second mode for operating the second cryoprobe. The apparatus includes a controller, a fluid source for providing a fluid for cooling the cryoprobes by of the Joule-Thomson effect, at least two cryoprobe connections one of which is connected to the first and second cryoprobe in the first and second mode, respectively, and a pressure-setting device with at least one proportional valve. The pressure-setting device can regulate, in the first and second modes, the pressure ratio of the admission run to the return run of the cryoprobe and the pressure in the admission run of the cryoprobe, respectively. The controller controls the cooling power of the cryoprobes by way of setting the proportional valve.

Description:
FIELD OF THE DISCLOSED EMBODIMENTS 
       [0001]    The disclosed embodiments relate to a cryosurgical apparatus for operating a first cryoprobe with a pressure-resistant return run and a second cryoprobe with an unpressurized return run and also to a method of operating such a cryosurgical apparatus. 
       BACKGROUND 
       [0002]    Cryotherapy, including cryosurgery, has a large number of applications. Cryoprobes are used, for example, to destroy diseased tissue, to take tissue samples and/or to remove foreign bodies. In cryotherapy, and in particular in cryosurgery, cold is frequently applied by means of a probe in order to achieve a healing effect. 
         [0003]    There are various methods that may be used to cool the corresponding instrument. The Joule-Thomson effect is frequently utilized for cooling. When cooling by the Joule-Thomson effect, a fluid, in particular a gas, is expanded near the site of application via a nozzle, and as a result of the expansion the gas experiences a change in temperature. The cooling power is based, inter alia, on the difference in pressure present at the site of the expansion. The expansion takes place in an expansion chamber. In order to ensure effective cooling of the instrument, it is necessary to remove the expanded gas from the expansion chamber without causing undesirable congestion. Gas is brought to the site of application via an admission run and removed from via a return run. 
         [0004]    In practice, a distinction is drawn between two types of probes used for cooling: rigid defrostable probes and flexible probes. Rigid probes have a rigid, pressure-resistant admission run and return run. Advantageous regulation of the cooling power may be ensured at high and constant admission run pressure by the setting of the return run pressure. An advantage of rigid probes is that they may also be used for heating the site of application. 
         [0005]    Flexible probes have a pressure-resistant admission run and a non-pressure-resistant return run. The expanded gas is sometimes drawn back inside the probe shank. Probes of this type are frequently much more flexible than a corresponding rigid probe. They may advantageously be used with flexible endoscopes or flexoscopes. However, the composition of the return lines or return guides of the fluid is such that these can only handle a low dynamic pressure. Therefore, the cooling power is regulated by flexible probes via the admission run pressure. After the expansion of the fluid in the expansion chamber, the fluid can flow away with low flow resistance (unpressurized return run). 
         [0006]    It is desirable to be able to operate both types of probes on a single apparatus. A corresponding cryosurgical apparatus conventionally includes a fluid source, for example a gas cylinder containing a working gas (for example CO 2  or N 2 O), a pressure-setting means for setting a suitable pressure on the admission run and/or return run and a controller which controls the pressure-setting device in such a way as to provide a constant and/or reproducible cooling power. 
         [0007]    A corresponding cryosurgical apparatus which can operate both rigid and flexible probes is known from DE 10 2006 003 571 A1. However, in this device, the regulation of the cooling power of the connected probes by the apparatus is insufficient. Using this device, it is not possible to react to different supply pressures and temperature-induced changes. 
       SUMMARY 
       [0008]    Starting from the above described prior art, the object of the disclosed embodiments is to provide a cryosurgical apparatus for efficiently controlling cryoprobes that is embodied in a simple manner while at the same time complying with stringent safety standards. Furthermore, a method of operating such a cryosurgical apparatus is also discussed. 
         [0009]    Disclosed embodiments include a cryosurgical apparatus for operating a first cryoprobe with a pressure-resistant return run and a second cryoprobe with an unpressurized or non-pressure-resistant return run, wherein the cryosurgical apparatus has a first mode, namely a back pressure regulating mode for operating the first cryoprobe, and a second mode, namely a front pressure regulating mode for operating the second cryoprobe. The apparatus includes a controller; a fluid source for providing a fluid, in particular a gas, which can be introduced, for cooling the cryoprobes, in particular by means of the Joule-Thomson effect, into the cryoprobes; at least two cryoprobe connections, there being connected to a cryoprobe connection of the cryoprobe connections in the first mode the first cryoprobe and in the second mode the second cryoprobe; and a pressure-setting device with at least one pressure regulating valve which is connected to the cryoprobes in such a way that the cryoprobes can be used to regulate, in both the first and second modes, the pressure ratio of the admission run to the return run of the cryoprobes and/or the pressure in the admission run, the controller controlling the cooling power of the cryoprobes by way of a setting of the pressure regulating valve. 
         [0010]    The disclosed embodiments thus provide a fluid circuit which is as simple as possible and allows both flexible and rigid cryoprobes or cryoprobes with either pressure-resistant or non-pressure-resistant return runs to be operated. Furthermore, according to the disclosed embodiments, at least one cryoprobe connection is used for both types of probe. The complexity when allocating the individual cryoprobe plugs to the corresponding cryoprobe connections may thus be reduced. 
         [0011]    According to the disclosed embodiments, a single pressure-setting device with a pressure regulating valve can be used to regulate both the fluid pressure in the return run of the first cryoprobe and that in the in the admission run of the second cryoprobe. Thus, the construction of the cryosurgical apparatus may be simplified. That affects not only the costs but also the safety of the apparatus. It should also be noted that the term “unpressurized” is to be interpreted herein to mean that an approximately atmospheric pressure is present. The pressure regulating valve may be a proportional valve or a needle valve. 
         [0012]    In the case of the first cryoprobe, defrosting of the probe may be ensured if the pressure ratio of the admission run to the return run is set appropriately. 
         [0013]    The pressure-setting device is connected in the first mode to the first cryoprobe such that the pressure in the admission run and in the return run of the first cryoprobe can be regulated by way of the setting of the pressure regulating valve. The aforementioned advantageous regulation of the cryoprobe power along the boiling-point/dew-point curve of the fluid (e.g., cooling by the Joule-Thomson effect) may in this way be ensured. Further advantages are a low flow speed, low flow resistance, a low loss of pressure on the admission and return run and better heat exchange effects on the probe head. 
         [0014]    The pressure-setting device is connected in the second mode to the second cryoprobe such that, in the case of substantially—usually low—pressure in the return run of the second cryoprobe, the pressure in the admission run can be set. The cooling power of flexible probes may therefore also advantageously be set. 
         [0015]    The pressure-setting device can include a 3/2 proportional valve (a valve with three connections and two main adjusting positions) which is connected by a first proportional valve connection or pressure regulating valve connection to the fluid source, by a second proportional valve connection or pressure regulating connection to the ventilation means and by a third proportional valve connection or pressure regulating valve connection to at least one of the cryoprobe connections. Preferably, this at least one cryoprobe connection is the connection used both by the first cryoprobe and by the second cryoprobe. As a result of the use of a 3/2 proportional valve, the pressure-setting device may be constructed in a very simple manner. The 3/2 proportional valve serves to set the pressure in the return run of the first cryoprobe and the volume of gas supplied into the admission run of the second cryoprobe. 
         [0016]    The cryosurgical apparatus can include a switching device with at least one switching valve, which is connected to the pressure-setting device and at least one cryoprobe connection for changing between the first and the second mode. 
         [0017]    Preferably, this cryoprobe connection is also the cryoprobe connection connected both to the first probe and to the second probe. The switching valve establishes a connection between the cryoprobe connection and the pressure-setting device in such a way that it is possible to set, in the first mode, the pressure in the return run and, in the second mode, the pressure in the admission run by way of the pressure-setting means, in particular by way of the pressure regulating valve therein. 
         [0018]    The controller can activate the switching device. It is thus possible to ensure automatic changing between the control states, i.e. the first mode and the second mode. The doctor or the operating person does not have to take care to set the appropriate mode when connecting a first or second cryoprobe. 
         [0019]    This is particularly advantageous if the plug-in connections to the probes are configured such that a corresponding allocation is carried out in accordance with the type of probe. It is possible to produce the switching device by way of a plurality of, in particular two, 2/2 switching valves (a valve with two connections and two adjusting positions). It is however more advantageous if the switching device comprises a 3/2 switching valve (three connections, two adjusting positions) which is connected by a first switching valve connection to the pressure-setting device, by a second switching valve connection to the ventilation means and by a third switching valve connection to a cryoprobe connection. The construction of the device may thus be further simplified. 
         [0020]    The cryosurgical apparatus can include at least one pressure sensor for determining an input pressure and/or an output pressure of the pressure-setting means. Automatic regulating of the cooling power by way of the controller is thus possible. 
         [0021]    Preferably, the fluid is made up of carbon dioxide and/or nitrous oxide (laughing gas) or a mixture of these gases. These gases have a high Joule-Thomson coefficient and are liquefiable at normal temperature. However, use may also be made of any other gas having a Joule-Thomson inversion temperature above the patient&#39;s body temperature. 
         [0022]    The cryosurgical apparatus can include a detection means which ascertains whether a first cryoprobe or a second cryoprobe is connected to the cryoprobe connections. The detection means is connected to the controller and sends apparatus mark signals which allow the controller to ascertain whether a cryoprobe of the first type (first cryoprobe) or of the second type (second cryoprobe) is connected. This further increases the operator convenience of the apparatus. The controller can thus determine which cryoprobe connections are occupied and set the appropriate mode. The detection means may also serve as a securing means which detects the presence of a pneumatic contact between at least one cryoprobe connection and a cryoprobe. If the pneumatic connection is interrupted, then the securing means interrupts the supply of fluid. The issuing of fluid into the environment may be efficiently avoided in this way. 
         [0023]    The cryosurgical apparatus can have a flow sensor for determining the cooling power of the connected cryoprobe. By determining the amount of fluid which flows through the cryoprobe, it is possible to draw conclusions about the cooling power applied in the cryoprobe. It is also possible to set or to determine the temperature by regulating and measuring the amount of fluid. 
         [0024]    The disclosed embodiments also include a method for operating a cryoprobe with a cryosurgical apparatus, wherein the apparatus provides a fluid for cooling the cryoprobe. The method includes the steps of determining whether a first cryoprobe is connected to a rigid or pressure-resistant return run or a second cryoprobe is connected to a flexible or unpressurized return run; setting a first mode (counterpressure mode or back pressure regulating mode) if the first cryoprobe is connected and setting a second mode (free-running mode or front pressure regulating mode) if the second cryoprobe is connected; and controlling a pressure-setting device with a pressure regulating valve for regulating a pressure ratio between the admission run and return run of the cryoprobes. The step of setting the first or second modes includes a setting of a switching device in such a way that, in the first mode, at least the return run of the first cryoprobe is connected to a pressure regulating valve connection of the proportional valve and, in the second mode, at least the admission run of the second cryoprobe is connected to a pressure regulating valve connection of the proportional valve. 
         [0025]    In this case too, therefore, the pressure ratio in the admission run and return run of the first cryoprobe or the second cryoprobe is advantageously regulated by means of a proportional valve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The disclosed embodiments will be described in greater detail, pointing out further features and advantages, by reference to the example embodiments illustrated in the drawings. 
           [0027]      FIG. 1  shows the components of a cryosurgical apparatus according to the disclosed embodiments. 
           [0028]      FIG. 2  is a fluid circuit diagram for the cryosurgical apparatus according to a first disclosed embodiment. 
           [0029]      FIG. 3  is a fluid flow diagram of the fluid circuit program from  FIG. 2  at maximum cooling power for a flexible cryoprobe. 
           [0030]      FIG. 4  is a fluid flow diagram of the fluid circuit diagram from  FIG. 2  at maximum cooling power for a rigid cryoprobe. 
           [0031]      FIG. 5  is a fluid circuit diagram for the cryosurgical apparatus according to a second disclosed embodiment. 
           [0032]      FIG. 6  is a fluid flow diagram of the fluid circuit diagram from  FIG. 5  at maximum cooling power for a flexible cryoprobe. 
           [0033]      FIG. 7  is a fluid flow diagram of the fluid circuit diagram from  FIG. 5  at maximum cooling power for a rigid cryoprobe. 
           [0034]      FIG. 8  is a fluid circuit diagram for the cryosurgical apparatus according to a third disclosed embodiment 
           [0035]      FIG. 9  is a fluid flow diagram of the fluid circuit diagram from  FIG. 8  at maximum cooling power for a flexible cryoprobe. 
           [0036]      FIG. 10  is a fluid flow diagram of the fluid circuit diagram from  FIG. 8  at maximum cooling power for a rigid cryoprobe. 
           [0037]      FIG. 11  is a fluid circuit diagram for the cryosurgical apparatus according to a fourth disclosed embodiment. 
           [0038]      FIG. 12  is a fluid flow diagram of the fluid circuit diagram from  FIG. 11  at maximum cooling power for a flexible cryoprobe. 
           [0039]      FIG. 13  is a fluid flow diagram of the fluid circuit diagram from  FIG. 11  at maximum cooling power for a rigid cryoprobe. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    The same reference numerals will be used in the following description for identical and equivalent parts. 
         [0041]    As seen in  FIG. 1 , a cryosurgical apparatus  10  according to the disclosed embodiments includes a fluid source  11  (for example a gas cylinder) for providing fluid at constant pressure, a switching device  15  and a pressure-setting device or means  14 . The cryosurgical apparatus  10  also includes a controller  13  which controls the pressure-setting device  14  and the switching device  15  to operate connected cryoprobes  1 ,  2  with the necessary fluid. Both rigid cryoprobes  1  and flexible cryoprobes  2  can be connected to the cryosurgical apparatus  10  according to the disclosed embodiments, and the controller  13  regulates the pressure-setting device  14  and the switching device  15  accordingly. 
       First Example Embodiment 
       [0042]      FIG. 2  shows a configuration of the pressure-setting device  14  and the switching device  15 , according to a first embodiment. The switching device includes a 3/2 proportional valve  30  and a 3/2 switching valve  50 . A first proportional valve connection  31  of the proportional valve  30  is connected to the fluid source  11  and a second proportional valve connection  32  is connected to the ventilation/fluid removal means  40 . The ventilation/fluid removal means  40  includes a sound absorber  41  and a flow sensor  42 . The ventilation/fluid removal means  40  serves to drain the fluid. The third proportional valve connection  33  of the proportional valve  30  is connected to the first cryoprobe connection  21 . The first cryoprobe connection  21  has a double function. In a first mode the return run of a rigid cryoprobe  1  is connected to the first cryoprobe connection  21  and in a second mode the admission run of a flexible cryoprobe  2  is connected to the first cryoprobe connection  21 . 
         [0043]    Furthermore, a first switching valve connection  51  of the 3/2 switching valve  50  is fluidly connected to the first proportional valve connection  31 . The second switching valve connection  52  leads via a restrictor  17  to the ventilation means  40  and the second proportional valve connection  32 . A third cryoprobe connection  23  is also linked to this line for the return run of the flexible cryoprobe  2 . The third switching valve connection  53  is connected to a second cryoprobe connection  22  for the admission run of the rigid cryoprobe  1 . 
         [0044]    The controller  13  controls the compressed air setting device  14  and the switching device  15  according to  FIG. 2  so that the rigid cryoprobe  1  can be connected in a first mode and a flexible cryoprobe  2  can be connected in a second mode. In order to be able to set the cooling power of the cryoprobes  1 ,  2  in accordance with a user input, the cryosurgical apparatus  10  according to the disclosed embodiments has sensors  19 ,  19 ′,  42  which determine the pressure at the first proportional valve connection  31  and at the third proportional valve connection  33 . The sensors  19 ,  19 ′,  42  issue corresponding sensor signals to the controller  13  which sets the proportional valve  30  and the switching valve  50  using corresponding regulating signals. The proportional valve  30  and the switching valve  50  have appropriate actuators for this purpose. 
         [0045]      FIG. 3  shows the fluid flows in the second mode or in the front pressure regulating mode when the flexible cryoprobe  2  is connected and at maximum cooling power. The flexible cryoprobe  2  is connected to the first cryoprobe connection  21  and the third cryoprobe connection  23 . The proportional valve  30  is activated by the controller  13  in such a way that the fluid from the fluid source  11  flows unrestricted via the first proportional valve connection  31  to the proportional valve  32  and from there into the admission run of the cryoprobe  2 . The return run of the flexible cryoprobe  2  opens directly into the ventilation means  40 . The switching valve  50  is deactivated (i.e. there is no fluid connection between the first switching valve connection  51  and the third switching valve connection  53 ). In order to avoid undesired rapid outflow of the fluid through the second cryoprobe connection  22 , the restrictor  17  is provided there. The cooling power of the flexible cryoprobe  2  can be restricted by setting the proportional valve  30 . As soon as the proportional valve  30  is deactivated (no fluid connection between the first and third proportional valve connection  30 ,  33 ), the flexible cryoprobe  2  is no longer cooled. 
         [0046]    In the first mode or in the back pressure regulating mode (cf.  FIG. 4 ), when the rigid cryoprobe  1  is connected and at maximum cooling power, the switching valve  50  is activated. There is thus a fluid connection between the second cryoprobe connection  22  and the fluid source  11 . The fluid can flow unrestricted into the admission run of the rigid cryoprobe  1 . The return run of the rigid cryoprobe  1  may be regulated by setting the proportional valve  30 . As shown in  FIG. 4 , the maximum cooling power is achieved by deactivation of the proportional valve  30 . In this valve position, the fluid flows unrestricted from the return run of the rigid cryoprobe  1  into the ventilation/fluid removal means  40 . The outflow is measured by means of the flow sensor  42 . Both the cooling power and the fluid flow may be restricted as a result of the setting of the proportional valve  30 . In one position of the proportional valve  30  (intensive restriction), although the fluid continues to circulate, only an insignificant difference in pressure remains in the expansion chamber, so that cooling power is no longer produced here. If the temperature of the cryoprobe  1  is lower than that of the fluid, the fluid absorbs and discharges cold. The circulation of fluid therefore leads to defrosting of the rigid cryoprobe  1 . 
       Second Example Embodiment 
       [0047]      FIG. 5  shows a second fluid circuit which has a similar function to the described first fluid circuit. The 3/2 proportional valve  30  and the 3/2 switching valve  50  are however in this case replaced by two 2/2 proportional valves  30 ,  30 ′ and a 2/2 switching valve respectively. The first 2/2 proportional valve  30  has a first proportional valve connection  31  and a second proportional valve connection  32 . The first proportional valve connection  31  is connected to the fluid source  11  and the second proportional valve connection  32  is connected to the first cryoprobe connection  21 . The second proportional valve  30 ′ is also suspended by its first proportional valve connection  31 ′ from the first cryoprobe connection  21 , while the second proportional valve connection  32 ′ is fluidly connected to the ventilation/fluid removal means  40 . The third cryoprobe connection  23  and the second switching valve connection  52  are also connected to the ventilation/fluid removal means  40 . The first switching valve connection  51  is connected to the fluid source  11  and the third switching valve connection  53  is connected to the second cryoprobe connection  22 . Sensors  19 ,  19 ′,  19 ″ determined measured values at the fluid source  11  and the first cryoprobe connection  21  and the ventilation/fluid removal means  40  respectively. 
         [0048]    In the second mode (cf.  FIG. 6 ), for maximum cooling power of the flexible or second cryoprobe  2 , the first proportional valve  30  is activated (open), the second proportional valve  30 ′ deactivated (closed) and the first switching valve  50  deactivated (fluid connection between the second and third switching valve connection  52 ,  53 ). The fluid flows from the fluid source  11  via the first proportional valve  30  to the first cryoprobe connection  21 . The return run of the flexible cryoprobe  2  opens into the third cryoprobe connection  23  which is directly connected to the ventilation means  40 . The power of the flexible cryoprobe  2  can be regulated by gradually deactivating the first proportional valve  30  which restricts the inflow of fluid depending on the position. 
         [0049]    The maximum power in the first mode is, as shown in  FIG. 7 , achieved in that the first proportional valve  30  is deactivated, the second proportional valve  30 ′ activated and the switching valve  50  activated (fluid connection between the first and third switching valve connection  51 ,  53 ). The fluid therefore flows from the fluid source  11  unrestricted via the switching valve  50  into the second cryoprobe connection  22  and is removed from the first cryoprobe connection  21  via the second proportional valve  30 ′ to the ventilation means  40 . The pressure at the first cryoprobe connection  21 , and thus the cooling power of the first or rigid cryoprobe  1 , may be set by way of the proportional valve  30 ′. The fluid circuit according to  FIG. 5  also allows the flow of fluid in the first mode to be reversed in such a way that fluid is introduced into the rigid cryoprobe  1  via the return run or cryoprobe connection  21  and drained via the admission run or cryoprobe connection  22 . The rigid cryoprobe  1  can thus be defrosted. 
       Third Example Embodiment 
       [0050]      FIG. 8  shows a functionally equivalent fluid circuit with two 2/2 switching valves  50 ,  50 ′ and two 2/2 proportional valves  30 ,  30 ′. The first proportional valve  30  and the first switching valve  50  are arranged parallel to each other, the first proportional valve connection  31  and the first switching valve connection  51  being fluidly connected to the fluid source  11 . The second proportional valve connection  32  of the first proportional valve  30  is connected to the first proportional valve connection  31 ′ of the second proportional valve  30 ′ and the first cryoprobe connection  21 . The second switching valve connection  52  of the first switching valve  50  is connected to the second cryoprobe connection  22  and, via a restrictor  17 , to the first switching valve connection  51 ′ of the second switching valve  50 ′. The second proportional valve connection  32 ′ of the second proportional valve  30 ′, the second switching valve connection  52 ′ of the second switching valve  50 ′ and the third cryoprobe connection  23  are fluidly connected to the ventilation/fluid removal means  40 . Sensors  19  to  19 ″ are accordingly provided. 
         [0051]    In the second mode the first proportional valve  30  is, as shown in  FIG. 9 , activated (fully opened) and the second proportional valve  30 ′ is deactivated (closed) at full power. The first and second switching valves  50 ,  50 ′ are deactivated in this mode. The fluid flows from the fluid source  11  via the first proportional valve  30  to the first cryoprobe connection  21  and is removed from the third cryoprobe connection  23  to the ventilation/fluid removal means  40 . In the second mode the fluid pressure applied to the first cryoprobe connection  21 , and thus the cooling power, may be set by regulating the first proportional valve  30 . 
         [0052]    In the first mode, at maximum power, the first proportional valve  30  is deactivated, the second proportional valve  30 ′ activated, the first switching valve  50  activated and the second switching valve  50 ′ deactivated (cf.  FIG. 10 ). The fluid flows unrestricted from the fluid source  11  to the second cryoprobe connection  22  and from the first cryoprobe connection  21  via the second proportional valve  30 ′ to the ventilation means  40 . The power of the first cryoprobe  1  may be set by restricting the outflowing flow of fluid by means of the second proportional valve  30 ′. 
       Fourth Example Embodiment 
       [0053]    The fourth example embodiment according to  FIG. 11  has a 3/2 proportional valve  30  and two 2/2 switching valves  50 ,  50 ′. The first proportional valve connection  31  is connected to the fluid source  11 , the second proportional valve connection  32  to the ventilation/fluid removal means  40  and the third proportional valve connection  33  of the proportional valve  30  to the first cryoprobe connection  21 . There are further connections between the second cryoprobe connection  22  and the first switching valve connection  51  of the first switching valve  50  and also the second switching valve connection  52 ′ of the second switching valve  50 ′. The second switching valve connection  52  of the first switching valve  50  is connected to the ventilation/fluid removal means  40  and the first switching valve connection  51  of the second switching valve  50 ′ is connected to the fluid source  11 . There is a fluid connection between the third cryoprobe connection  23  and the ventilation/fluid removal means  40 . Sensors  19 ,  19 ′,  19 ″ are provided at the fluid source  11 , the first cryoprobe connection  21  and the ventilation/fluid removal means  40 . A restrictor  17  is connected downstream of the second switching valve connection  52  of the first switching valve  50 . 
         [0054]    In the second mode, at maximum power, the first proportional valve  30  is activated (unrestricted connection between the first and third proportional valve connections  31 ,  33 ) and the two switching valves  50 ,  50 ′ are deactivated (cf.  FIG. 12 ). The fluid flows unrestricted from the fluid source  11  via the proportional valve  30  to the first cryoprobe connection  21  and from the second cryoprobe connection  22  into the ventilation/fluid removal means  40 . The flow of fluid can be regulated via a restriction in the proportional valve  30 . 
         [0055]    In the first mode (cf.  FIG. 13 ), at maximum power, the proportional valve  30  is deactivated (unrestricted connection between the first and third proportional valve connections  32 ,  33 ), the first switching valve  50  is deactivated and the second switching valve  50 ′ is activated. The fluid flows from the fluid source  11  unrestricted into the second cryoprobe connection  22  (via the second switching valve  50 ′) and from the first cryoprobe connection  21  via the proportional valve  30  into the ventilation/fluid removal means  40 . The return run from the first cryoprobe  1  can be regulated by setting the proportional valve  30 . The proportional valve  30  then serves as a restrictor and increases the pressure in the return run line. The power of the rigid cryoprobe  1  thus decreases. 
         [0056]    The example embodiments described hereinbefore serve merely to illustrate different circuit arrangements which achieve the claimed effect. For marketable implementation, it may be necessary to provide further restrictors  17  or check valves. Furthermore, it is possible to combine the second and third cryoprobe connections  22 ,  23  by means of a further 3/2 switching valve into one connection. 
         [0057]    The person skilled in the art will be familiar with numerous further embodiments of the circuits that achieve the same effect.