Patent Description:
The present disclosure contemplates that an amount of a fluid being in a mixed phase within a tank may be estimated by subtracting the weight of the empty tank from the weight of the tank and its contents. This approach, however, may not be accurate particularly if the noted weight of the tank is inaccurate and/or if the scale is defective. In order to overcome potential uncertainties about empty tank weight and a weight scale being defective, the present disclosure contemplates determining the amount of liquid within a container, regardless of the container's weight and without using a weight scale. Rather, the instant disclosure provides an apparatus for ascertaining an amount of cryogenic fluid within a tank knowing the fluid contained therein, the pressure of the fluid inside the container, and the temperature of the tank and/or fluid within the tank. In this manner, the instant disclosure provides a novel improvement upon the prior art in determining how much of a liquid is within a tank but avoiding having to know either the weight of the tank or its contents.

<CIT> discloses a method of controlling the depth of cryoablation.

<CIT> discloses a method of protecting the phrenic nerve while ablating cardiac tissue.

<CIT> discloses a refrigerant supply system for cryotherapy including refrigerant recompression and associated devices, systems, and methods.

The invention is defined in appended independent claim <NUM>, and further embodiments are described in the independent claims. Cryosurgical methods described here below are not part of the present invention. It is a first aspect of the present disclosure to provide a cryogenic surgical system, comprising: (a) a cryosurgical probe fluidicly interposing an inlet conduit arranged to supply a cryogenic fluid to the cryosurgical probe and an exhaust conduit arranged to direct the cryogenic fluid away from the cryosurgical probe; and (b) a cryosurgical control unit capable of receiving the cryogenic fluid from a cryogenic fluid source, the cryosurgical control unit comprising an inlet valve fluidicly coupled to the inlet conduit and configured to selectively supply the cryogenic fluid to the cryosurgical probe via the inlet conduit and an exhaust valve fluidicly coupled to the exhaust conduit and configured to selectively permit cryogenic fluid to flow from the exhaust conduit, where the cryosurgical control unit is configured to control cooling of the cryosurgical probe to a temperature lower than a first setpoint temperature by (i) cooling the cryosurgical probe by opening the inlet valve to supply the cryogenic fluid to the cryosurgical probe and having the exhaust valve open to permit the cryogenic fluid to exhaust from the cryosurgical probe, (ii) shutting the inlet valve when a temperature of the cryosurgical probe reaches the first setpoint temperature, and (iii) maintaining the inlet valve shut and maintaining the exhaust valve open while the cryosurgical probe temperature is less than the first setpoint temperature.

In a more detailed embodiment of the first aspect, the cryosurgical control unit is configured to open the inlet valve to supply the cryogenic fluid to the cryosurgical probe when the temperature of the cryosurgical probe reaches a second setpoint temperature, after maintaining the inlet valve shut while the cryosurgical probe temperature is less than the first setpoint temperature. In yet another more detailed embodiment, the second setpoint temperature is higher than the first setpoint temperature. In still a further detailed embodiment, the cryosurgical control unit is configured to receive the cryogenic fluid as a gas, and the cryosurgical probe includes an orifice capable of liquifying at least a portion of the gas. In a more detailed embodiment, the cryogenic fluid comprises at least one of gaseous nitrous oxide and gaseous carbon dioxide.

Examples are described in conjunction with the accompanying drawing figures in which:.

Examples according to the present disclosure are described and illustrated below to encompass devices, methods, and techniques relating to medical procedures. Of course, it will be apparent to those of ordinary skill in the art that the examples discussed below are examples and may be reconfigured without departing from the scope of the present disclosure. It is also to be understood that variations of the examples contemplated by one of ordinary skill in the art shall concurrently comprise part of the instant disclosure. However, for clarity and precision, the examples as discussed below may include optional steps, methods, and features that one of ordinary skill should recognize as not being a requisite to fall within the scope of the present disclosure.

The present disclosure includes, inter alia, medical devices and related methods, and, more specifically, cryogenic surgical systems and related methods. The present disclosure contemplates that some cryogenic surgical systems, such as those utilizing certain cryogenic fluids and/or some cryosurgical probes incorporating small diameter inlet and exhaust conduits, may have a limited ability to cool the cryosurgical probes as much as may be desirable for some surgical procedures. Accordingly, the present disclosure contemplates that it may be beneficial to provide improved techniques and/or apparatus that facilitate enhanced cooling of cryosurgical probes. Additionally, the present disclosure contemplates that it may be beneficial to provide improved techniques and/or apparatus associated with indicating the liquid level in tanks utilized for cryogenic fluids in connection with cryogenic surgical systems.

<FIG> is a schematic diagram of an example cryogenic surgical system <NUM>, according to at least some examples of the present disclosure. System <NUM> may include a cryosurgical probe <NUM> and/or a cryosurgical control unit <NUM>, which may be coupled to a cryogenic fluid source <NUM> containing a cryogenic fluid <NUM>.

Generally, the cryosurgical control unit <NUM> may be configured to control cooling and/or heating (e.g., defrosting) of the cryosurgical probe <NUM>, such as by controlling the flow of cryogenic fluid <NUM> to and/or from the cryosurgical probe <NUM>. In some examples, and with the exception of the improvements described in the present disclosure, the illustrative cryosurgical control unit <NUM> of the cryogenic surgical system <NUM> may be generally similar to the "cryoICE BOX" cryogenic surgical units available from AtriCure, Inc. of Mason, Ohio.

The cryogenic surgical system <NUM> may be configured to cool cryosurgical probe <NUM> so that it may have a desired cryosurgical effect (e.g., ablation, cryoanalgesia, etc.) on a target tissue <NUM>. In some example applications, cryosurgical probe <NUM> may form an ice ball <NUM> in, on, and/or around the target tissue <NUM>. Some example cryosurgical probes that may be suitable for use with the illustrative cryosurgical system <NUM> may include the "cryoICE," "cryoFORM," and/or "cryoSPHERE" probes available from AtriCure, Inc. of Mason, Ohio.

The cryosurgical probe <NUM> may be fluidicly coupled to an inlet conduit <NUM>, which may be arranged to supply cryogenic fluid <NUM> from the cryogenic control unit <NUM> to the cryosurgical probe <NUM>. Similarly, the cryosurgical probe <NUM> may be fluidicly coupled to an exhaust conduit <NUM>, which may be arranged to convey cryogenic fluid <NUM> from the cryosurgical probe <NUM> to the cryogenic control unit <NUM>. Accordingly, cryogenic fluid <NUM> may flow through the inlet conduit <NUM>, through the cryosurgical probe <NUM>, and through the exhaust conduit <NUM>, so that the cryosurgical probe fluidicly interposes the inlet conduit <NUM> and the exhaust conduit <NUM>. In some examples, the cryosurgical probe <NUM> may include an orifice <NUM> fluidicly interposing the inlet conduit <NUM> and the exhaust conduit <NUM>. The orifice <NUM> may be configured to liquify at least a portion of a gaseous cryogenic fluid <NUM> flowing through the cryosurgical probe <NUM>, such as by the Joule-Thomson effect. In some examples, the cryosurgical probe <NUM> may include a temperature sensor <NUM>, such as a thermocouple, which may be configured to measure a temperature of the cryosurgical probe <NUM>.

The cryosurgical control unit <NUM> may be configured to receive cryogenic fluid <NUM> from the cryogenic fluid source <NUM>, such as via a cryogenic fluid supply line <NUM>. The cryosurgical control unit <NUM> may include an inlet valve <NUM> fluidicly coupled between the cryogenic fluid supply line <NUM> extending from the cryogenic fluid source <NUM> and the inlet conduit <NUM> extending to the cryosurgical probe <NUM>. The inlet valve <NUM> may be configured to selectively supply cryogenic fluid <NUM> to the cryosurgical probe <NUM> via the inlet conduit <NUM>. The cryosurgical control unit <NUM> may include an exhaust valve <NUM> fluidicly coupled between the exhaust conduit <NUM> extending from the cryosurgical probe <NUM> and an exhaust hose <NUM>, which may be configured to direct exhausted cryogenic fluid <NUM> to an appropriate location. The exhaust valve <NUM> may be configured to selectively permit cryogenic fluid <NUM> to flow from the exhaust conduit <NUM> to the exhaust hose <NUM>. Collectively, the exhaust conduit <NUM>, the exhaust valve <NUM>, and the exhaust hose <NUM> may be referred to as the exhaust flow path <NUM>. As used herein, "back pressure" may refer to the pressure within the cryosurgical probe <NUM> due to the effects of the exhaust flow path <NUM>. For example, increasing the cryogenic fluid <NUM> flow rate through the exhaust flow path <NUM> and/or throttling exhaust valve <NUM> may increase the back pressure. Similarly, lowering the cryogenic fluid <NUM> flow rate through the exhaust path <NUM> and/or fully opening the exhaust valve <NUM> may reduce the back pressure.

The cryosurgical control unit <NUM> may be configured to control cooling and/or heating of the cryosurgical probe <NUM>, such as by selectively supplying cryogenic fluid <NUM> to the cryosurgical probe <NUM> via the inlet conduit <NUM>. To supply the cryogenic fluid <NUM> to the cryogenic probe <NUM> via the inlet conduit <NUM>, the cryogenic control unit <NUM> may at least partially open the inlet valve <NUM>, which may allow cryogenic fluid <NUM> to flow to from the cryogenic fluid source <NUM>, through the cryogenic fluid supply line <NUM>, through the inlet valve <NUM>, through the inlet conduit <NUM>, and to cryogenic probe <NUM>.

The cryosurgical control unit <NUM> may be configured to control cooling and/or heating of the cryosurgical probe <NUM>, such as by selectively permitting cryogenic fluid <NUM> to flow from the cryosurgical probe <NUM> via the exhaust conduit <NUM>. To permit cryogenic fluid <NUM> to flow from the cryogenic probe <NUM> via the exhaust conduit <NUM>, cryogenic control unit <NUM> may at least partially open the exhaust valve <NUM>, which may allow cryogenic fluid <NUM> to flow from the cryogenic probe <NUM>, through the exhaust conduit <NUM>, through the exhaust valve <NUM>, and through the exhaust hose <NUM>.

In some examples, the cryogenic fluid source <NUM> may include a pressurized tank <NUM> containing liquid <NUM> and gas <NUM> phases of the cryogenic fluid <NUM>. In some examples, the cryogenic control unit <NUM> may be configured to receive the cryogenic fluid <NUM> as a gas. In some examples, the cryogenic fluid <NUM> may include nitrous oxide and/or carbon dioxide.

In some examples, the cryogenic surgical system <NUM> may include a heater in thermal communication with the cryogenic fluid <NUM> in the tank <NUM>. For example, a heater band <NUM> may be removably disposed at least partially around and/or may be thermally coupled to an exterior wall <NUM> of the tank <NUM>. In some examples, the heater band <NUM> may be disposed on a lower portion <NUM> of the exterior wall <NUM> of the tank <NUM>. The cryogenic surgical system <NUM> may include a pressure sensor <NUM> arranged to measure the pressure of the fluid <NUM> coming from or within the tank. For example, the pressure sensor <NUM> may be disposed within the cryogenic control unit <NUM> and/or may be fluidicly coupled to the cryogenic fluid supply line <NUM>. During operation of the heater band <NUM>, the heat output of the heater band <NUM> may be varied to maintain the pressure of the cryogenic fluid <NUM> in the tank <NUM> within a pressure setpoint range, such as by cycling electrical power to the heater band on and off.

The cryogenic surgical system <NUM> may include a temperature sensor <NUM> arranged to measure a temperature associated with the tank <NUM>, such as the temperature of the exterior wall <NUM> of the tank <NUM>. In some examples, the temperature sensor <NUM> may be disposed on the heater band <NUM> proximate an upper end <NUM> of the heater band <NUM>.

In some examples, the cryogenic surgical system <NUM> may be configured to display one or more indicia associated with the liquid level <NUM> in the tank <NUM>. As used herein, "displaying an indicium" may refer to providing any audible indication (e.g., beep and/or tone), visual indication (e.g., numerical and/or graphical indication), and/or tactile indication (e.g., shaking and/or vibration), which may be understood to convey information, such as information about the liquid level <NUM> in the tank <NUM>. For example, the cryogenic control unit <NUM> may include a graphical indicator <NUM> configured to visually display one or more indicia associated with the liquid level <NUM> in the tank <NUM>. In some other examples, indicators may include sound-emitting devices (e.g., speakers, buzzers, beepers, etc.) and/or movement-causing devices (e.g., electric motors with unbalanced masses on the driveshafts).

The cryosurgical control unit <NUM> may include a processor <NUM> and/or a data storage device <NUM>, which may be operatively coupled to each other and/or to other components of the cryogenic surgical system <NUM> and/or which may be configured to control various functions and operations of the cryogenic surgical system <NUM>, such as those described in the present disclosure. For example, the processor <NUM> and/or the data storage device <NUM> may be operatively coupled to receive information from, send information to, and/or direct operation of the temperature sensor <NUM> of the cryosurgical probe <NUM>, inlet valve <NUM>, exhaust valve <NUM>, heater band <NUM>, pressure sensor <NUM>, temperature sensor <NUM>, and/or graphical indicator <NUM>. The cryosurgical control unit <NUM> may also include an actuator <NUM> communicatively coupled to the processor <NUM>. The actuator may be configured to be manually actuated when a tank <NUM> replacement occurs. By way of example, when the actuator <NUM> is actuated, the processor <NUM> receives a signal indicative of first discrete liquid level <NUM> (see <FIG>) and causes the processor <NUM> to store and indication of the first discrete liquid level in the data storage device <NUM>.

<FIG> are detailed views of the example graphical indicator <NUM> of the cryogenic control unit <NUM>, according to at least some examples of the present disclosure. The graphical indicator <NUM> may be configured to visually display an indicium associated with a first discrete liquid level <NUM> (<FIG>), an indicium associated with a second discrete liquid level <NUM> (<FIG>), and/or an indicium associated with a third discrete liquid level <NUM> (<FIG>). Referring to <FIG>, the indicium associated with the first discrete liquid level <NUM> may correspond to a relatively higher liquid level <NUM>, the indicium associated with the second discrete liquid level <NUM> may correspond to an intermediate liquid level <NUM>, and/or the indicium associated with the third liquid level <NUM> may correspond to a relatively lower liquid level <NUM> in the tank.

In general, any indicia associated with any liquid levels may include part or all of at least some other indicia. For example, in examples including bar-graph type displays, one or more of the indicia corresponding to an emptier level may be included in one or more of the indicia corresponding to a fuller level. For example, in graphical indicator <NUM>, the indicium associated with the second discrete level <NUM> may include a single bar extending approximately half way up, or it may include two or more bars, such that the bar shown as the indicium associated with the third liquid level <NUM> on the bottom and a second bar above and extending to approximately half way up. Similarly, the indicium associated with the first discrete liquid level <NUM> may include the indicium associated with the third liquid level <NUM>, a second bar extending to approximately half way up, and/or a third bar extending substantially to the top. As another example, if the indicia include audible beeps, the indicium associated with the third discrete liquid level (e.g., three beeps) may include the indicium associated with the second liquid level (e.g., two beeps), and the indicium associated with the first liquid level (e.g., one beep). More generally, the indicium associated with the first discrete liquid level, the indicium associated with a second discrete liquid level, and/or the indicium associated with a third discrete liquid level are to be considered in their entireties as presented to a user, and the scope of the present disclosure is not limited by the particular format or inclusion of some indicia as elements of another indicium.

<FIG> is a plot of an example cryogenic fluid flow rate (on the axis labeled "F") and an example cryosurgical probe temperature (on the axis labeled "T") versus time (on the axes labeled "t"), according to at least some examples of the present disclosure. Generally, <FIG> illustrates an example temperature response of the cryosurgical probe <NUM> when cryosurgical control unit <NUM> controls cooling of the cryosurgical probe <NUM> by selectively supplying cryogenic fluid <NUM> to the cryosurgical probe <NUM> by controlling inlet valve <NUM>.

In this example, the cryogenic fluid <NUM> may include nitrous oxide, which may be supplied to the cryosurgical control unit <NUM> and to the cryosurgical probe <NUM> as gas. At least some of the gaseous nitrous oxide flowing through the orifice <NUM> may liquify due the Joule-Thomson effect. Accordingly, at least some of the cooling effect in the cryosurgical probe <NUM> may be due to boiling of the liquified nitrous oxide in the cryosurgical probe <NUM>.

Further, in this example, the exhaust valve <NUM> remains fully open throughout the time shown on the plots. Accordingly, the exhaust flow path <NUM> for the cryogenic fluid <NUM> leaving the cryosurgical probe <NUM> remains substantially the same throughout the time shown on the plots.

At time t0, the cryogenic fluid <NUM> is flowing to the cryosurgical probe <NUM> at flow rate F1. For example, flow rate F1 may be approximately the steady state flow rate when the inlet valve <NUM> is fully open. Temperature T1 may be a first setpoint temperature at which the cryosurgical control unit <NUM> is configured to shut the inlet valve <NUM>.

When the cryosurgical probe <NUM> temperature reaches first setpoint temperature T1 at time t1, the cryosurgical control unit <NUM> may fully shut the inlet valve <NUM>. Accordingly, the cryogenic fluid <NUM> flow rate drops to flow rate F0, which may be substantially zero flow. With no inlet flow to the cryosurgical probe <NUM>, the exhaust flow rate may drop, which may result in a lower back pressure in the exhaust flow path <NUM>. The lower back pressure may result in a lower pressure in the cryosurgical probe <NUM>, which may allow the cryogenic fluid <NUM> therein to boil at a lower temperature. Accordingly, a downward temperature transient may occur as shown between time t1 and time t2. The lowest cryosurgical probe <NUM> temperature reached is temperature T2, which may be lower than the first setpoint temperature T1. This downward temperature transient and/or temperature T2 lower than the first setpoint temperature T1 may facilitate formation of a larger ice ball <NUM> and/or further cooling of the ice ball <NUM> and/or target tissue <NUM>.

Eventually, sufficient cryogenic fluid <NUM> in the cryosurgical probe <NUM> may boil and exit via the exhaust path <NUM> to allow the cryosurgical probe <NUM> to warm, such as by heating from its surroundings. During this transient, while the cryosurgical probe <NUM> is cooled to a temperature T2 that is colder than the first setpoint temperature T1, the inlet valve <NUM> may remain fully shut.

At time t2, the cryosurgical probe <NUM> has reached a second setpoint temperature T3. Second setpoint temperature T3 may be the temperature at which cryosurgical control unit <NUM> is configured to open the inlet valve <NUM>. When the cryosurgical control unit <NUM> fully opens the inlet valve <NUM>, the cryogenic fluid <NUM> flow rate may rise to and maintain flow rate F1. Restoring the flow of the cryogenic fluid <NUM> to the cryosurgical probe <NUM> may cool the cryosurgical probe <NUM>, reducing its temperature.

In some examples, the second setpoint temperature T3 may be warmer than the first setpoint temperature T1. Generally, the back pressure between time t0 and time t1 may be greater than the back pressure between time t1 and time t2.

The present disclosure contemplates that the amount of cryogenic fluid <NUM> in the tank <NUM> could be estimated by weighing the tank <NUM> and subtracting the weight of the empty tank <NUM>. The present disclosure contemplates that this approach, however, would typically require the cryogenic surgical system <NUM> to include additional instrumentation, such as a load cell configured to weigh the tank <NUM>. Moreover, attempting to ascertain the amount of cryogenic fluid <NUM> in the tank <NUM> by weight relies on knowing the weight of the empty tank <NUM> and that the scale measuring the weight is accurate - both of which may be unknown or inaccurate.

Some example cryogenic surgical systems <NUM> may be configured to indicate the liquid level <NUM> in the tank <NUM> based on the pressure of the cryogenic fluid <NUM> in the tank <NUM> and the temperature of the exterior wall <NUM> of the tank <NUM>, such as when the heater band <NUM> is operating to maintain the pressure of the cryogenic fluid <NUM> in the tank <NUM> within the pressure setpoint range. <FIG> is a flow diagram of an example method <NUM> of indicating the liquid level <NUM> in the tank <NUM>, according to at least some examples of the present disclosure. In some examples, the method <NUM> may be utilized in connection with heating the tank <NUM> using the heater band <NUM>.

The method <NUM> may begin with operation <NUM>, which may include measuring the pressure of the fluid <NUM> in the tank <NUM>. It should be noted that, by way of example, the method <NUM> may be reinitialized upon actuating the actuator <NUM> indicating a tank <NUM> replacement, and/or may be reinitialized periodically based upon a predetermined timing such as, without limitation, every thirty seconds, every minute, every two minutes, and every five minutes. Operation <NUM> may be followed by operation <NUM>, which may include measuring the temperature of the exterior wall <NUM> of the tank <NUM>. Operation <NUM> may be followed by operation <NUM>, which may include determining whether an immediately previously displayed indicium was the indicium associated with the first discrete liquid level <NUM>, the indicium associated with the second discrete liquid level <NUM>, or the indicium associated with the third discrete liquid level <NUM>. Operation <NUM> may be followed by operations <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>, which may include determining, in sequence, which sets of criteria are satisfied and, upon determining that the particular set of criteria is satisfied, displaying the indicium associated with the liquid level <NUM> in the tank <NUM> that is associated with that particular set of criteria.

Operation <NUM> may include determining whether the immediately previously displayed indicium was the indicium associated with the first discrete liquid level <NUM>. This operation <NUM> may include accessing the data storage device <NUM> to determine what liquid level is currently saved in memory. If the liquid level saved in the data storage device <NUM> matches the liquid level displayed on the graphical indicator <NUM>, the method <NUM> may proceed to operation <NUM>. If the liquid level saved in the data storage device <NUM> does not match the liquid level displayed on the graphical indicator <NUM>, the method <NUM> may proceed to operation <NUM>.

Operation <NUM> may include determining whether the pressure of the fluid <NUM> in the tank <NUM> is greater than a first threshold pressure and whether the temperature of the exterior wall <NUM> is less than a first threshold temperature. If the pressure of the fluid <NUM> in the tank <NUM> is greater than a first threshold pressure and the temperature of the exterior wall <NUM> is less than a first threshold temperature, the method <NUM> may proceed to operation <NUM>. If at least one of (<NUM>) the pressure of the fluid <NUM> in the tank is less than a first threshold pressure and (<NUM>) the temperature of the exterior wall <NUM> is greater than a first threshold temperature, the method <NUM> may proceed to operation <NUM>.

Operation <NUM> may include displaying the indicium associated with the first discrete liquid level <NUM> and updating the data storage device <NUM> to indicate the first discrete liquid level <NUM> is being displayed. Following operation <NUM>, the method may proceed to operation <NUM>.

Operation <NUM> may include determining whether the immediately previously displayed indicium was the indicium associated with the second discrete liquid level <NUM> by comparing the data storage device <NUM> stored liquid level with the liquid level displayed on the graphical indicator <NUM>. If the liquid level saved in the data storage device <NUM> matches the liquid level displayed on the graphical indicator <NUM> (i.e., the second discrete liquid level <NUM>), then the method <NUM> may proceed to operation <NUM>. If the liquid level saved in the data storage device <NUM> does not match the liquid level displayed on the graphical indicator <NUM>, then the method <NUM> may proceed to operation <NUM>.

Operation <NUM> may include determining whether the pressure of the fluid <NUM> in the tank <NUM> is greater than the second threshold pressure and whether the temperature of the exterior wall <NUM> is less than the second threshold temperature. If the pressure of the fluid <NUM> in the tank <NUM> is greater than the second threshold pressure and the temperature of the exterior wall <NUM> is less than the second threshold temperature, the method <NUM> may proceed to operation <NUM>. If at least one of (<NUM>) the pressure of the fluid <NUM> in the tank <NUM> is less than the second threshold pressure and (<NUM>) the temperature of the exterior wall <NUM> is greater than the second threshold temperature, the method <NUM> may proceed to operation <NUM>.

Operation <NUM> may include displaying the indicium associated with the second discrete liquid level <NUM> and updating the data storage device <NUM> to indicate the second discrete liquid level <NUM> is being displayed. Following operation <NUM>, the method may proceed to operation <NUM>.

Operation <NUM> may include determining whether the immediately previously displayed indicium was the indicium associated with the third discrete liquid level <NUM> (or some other predetermined liquid level besides the first and second discrete liquid levels <NUM>, <NUM>). If the liquid level was not one of the first discrete liquid level <NUM> or the second discrete liquid level <NUM>, by reference to the liquid level saved in the data storage device <NUM>, the method may proceed to operation <NUM>.

Operation <NUM> may include determining whether the pressure of the fluid <NUM> in the tank <NUM> is less than the third threshold pressure and the temperature of the exterior wall <NUM> is greater than or equal to the third threshold temperature. If the pressure of the fluid <NUM> in the tank <NUM> is less than the third threshold pressure and the temperature of the exterior wall is greater than the third threshold temperature, the method <NUM> may proceed to operation <NUM>. If at least one of (<NUM>) the pressure of the fluid <NUM> in the tank <NUM> is greater than the third threshold pressure and (<NUM>) the temperature of the exterior wall <NUM> is less than the third threshold temperature, the method <NUM> may proceed to operation <NUM> that includes maintaining the instant/current indicium displayed via operation <NUM>, <NUM>, or <NUM> and proceed to operation <NUM>.

Operation <NUM> may include displaying the indicium associated with the third discrete liquid level <NUM>. Following operation <NUM>, the method may proceed to operation <NUM>.

Operations <NUM>, <NUM>, and <NUM> may function as a timer, which may be configured to periodically reinitiate method <NUM> at operation <NUM>. Operation <NUM> may increment a timing counter. Operation <NUM> may determine whether the timing counter has reached a timing threshold (e.g., <NUM> seconds). If no, the method <NUM> may return to operation <NUM>, which may again increment the timing counter. Once the timing counter has reached the timing threshold, operation <NUM> may follow operation <NUM>. Operation <NUM> may reset the timing counter incremented in operation <NUM> (e.g., to zero). The method <NUM> may return to operation <NUM> following operation <NUM>. In the next cycle, the indicium indicated in the previous cycle in operation <NUM>, operation <NUM>, or operation <NUM> may be used as the immediately previously displayed indicium in operation <NUM>, operation <NUM>, and/or operation <NUM>.

In some examples, method <NUM> may take into account whether the tank <NUM> is a full replacement tank <NUM>. For example, in the first cycle through method <NUM> with a full replacement tank, method <NUM> may consider the immediately previously displayed indicium to be the indicium associated with the first discrete liquid level <NUM>.

In some examples, the first threshold pressure and the second threshold pressure may be substantially the same. In some examples the third threshold pressure may be higher than at least one of the first threshold pressure and the second threshold pressure. In some examples, the third threshold temperature may be higher than the first threshold temperature and the second threshold temperature. In some examples, the liquid level may be displayed based on only the parameters explicitly described above in connection with the method <NUM> (e.g., the pressure of the fluid in the tank and the temperature of the exterior wall of the tank), and not utilizing additional parameters such as the differential pressure between two heights in the tank, the tank weight, the temperatures at multiple locations on the tank wall, etc. In some examples, the method <NUM> may include consideration of any of these additional parameters and/or other additional parameters.

By way of example, the first threshold pressure may be greater than or equal to <NUM> Bar (<NUM> psi), while the first threshold temperature may be between 20C to 30C and, optionally less than 27C. By way of further example, the second threshold pressure may be greater than or equal to <NUM> Bar (<NUM> psi), while the second threshold temperature may be between 21C to 40C and, optionally less than 31C, and even less than 29C. By way of still further example, the third threshold pressure may be less than or equal to <NUM> Bar (<NUM> psi), optionally between <NUM> Bar (<NUM> psi) and <NUM> Bar (<NUM> psi), while the third threshold temperature may be greater than the second threshold temperature, and optionally greater than or equal to 31C.

Claim 1:
A cryogenic surgical system (<NUM>), comprising:
a cryosurgical probe (<NUM>) fluidicly interposing an inlet conduit (<NUM>) arranged to supply a cryogenic fluid (<NUM>) to the cryosurgical probe (<NUM>) and an exhaust conduit (<NUM>) arranged to direct the cryogenic fluid (<NUM>) away from the cryosurgical probe (<NUM>); and
a cryosurgical control unit (<NUM>) capable of receiving the cryogenic fluid (<NUM>) from a cryogenic fluid source (<NUM>), the cryosurgical control unit (<NUM>) comprising an inlet valve (<NUM>) fluidicly coupled to the inlet conduit (<NUM>) and configured to selectively supply the cryogenic fluid (<NUM>) to the cryosurgical probe (<NUM>) via the inlet conduit (<NUM>) and an exhaust valve (<NUM>) fluidicly coupled to the exhaust conduit (<NUM>) and configured to selectively permit cryogenic fluid (<NUM>) to flow from the exhaust conduit (<NUM>).
characterized in that the cryosurgical control unit (<NUM>) is configured to control cooling of the cryosurgical probe (<NUM>) to a temperature (T2) lower than a first setpoint temperature (T1) by:
i. cooling the cryosurgical probe (<NUM>) by opening the inlet valve (<NUM>) to supply the cryogenic fluid (<NUM>) to the cryosurgical probe (<NUM>) and having the exhaust valve (<NUM>) open to permit the cryogenic fluid (<NUM>) to exhaust from the cryosurgical probe (<NUM>),
ii. shutting the inlet valve (<NUM>) and maintaining the exhaust valve (<NUM>) open when a temperature of the cryosurgical probe (<NUM>) reaches the first setpoint temperature (T1), and
iii. maintaining the inlet valve (<NUM>) shut and maintaining the exhaust valve (<NUM>) open while the cryosurgical probe (<NUM>) temperature is less than the first setpoint temperature (T1).