Abstract:
A method and system for improved lesion creation. The method generally includes positioning a treatment element of a medical device proximate an area of target tissue and operating a control unit in accordance with a duty cycle, that includes at least one freeze-warm cycle, each freeze-warm cycle including: supplying coolant to the treatment element at a first flow rate that causes the treatment element to reach a first temperature, the first temperature causing ablation of the target tissue and cryoadhesion between the treatment element and target tissue, and supplying coolant to the treatment element at a second flow rate that causes the treatment element to reach a second temperature, the second temperature being higher than the first temperature and the second flow rate being lower than the first flow rate, the second temperature being above a temperature at which ablation occurs and below a temperature at which cryoadhesion is broken.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    n/a 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    n/a 
       FIELD OF THE INVENTION 
       [0003]    The present invention relates to a method and system for cryoablation. Specifically, the present method and system provides effective cryoablation of tissue and enhances patient safety by reducing the need for fluoroscopy. 
       BACKGROUND OF THE INVENTION 
       [0004]    Cryoablation, a therapy that uses that removal of heat from tissue, is often used to treat cardiac conditions such as cardiac arrhythmias. In most cryoablation procedures, a pressurized refrigerant is circulated within the tip of a cryoablation catheter, where the refrigerant expands and absorbs heat from surrounding tissue. As the tissue freezes, blood adjacent the treatment site may also freeze, creating an “ice ball” that temporarily adheres the treatment element (for example, a cryoballoon or thermally conductive area at the tip of the cryoablation device) to the tissue at the treatment site, a phenomenon called cryoadhesion. 
         [0005]    Cryoadhesion is advantageous in that it helps prevent the cryoablation device from moving away from the target treatment site of a beating heart. However, research has shown that a freeze-thaw-freeze cycle more effectively ablates tissue than a single longer freeze-only cycle. Although more efficient lesion creation is desired, the freeze-thaw-freeze cycle may also result in the thawing of the ice ball that keeps the cryoablation device in place. As a result, the device must be repositioned, which may be complicated and time-consuming. Further, some cryoablation procedures, such pulmonary vein isolation (PVI), involve the use of fluoroscopy to visualize the position of the device and to make sure that, for example, the pulmonary vein is completely occluded. Fluoroscopy involves x-ray visualization; consequently, each time the ice ball thaws and the cryoablation device is repositioned, the patient and the user are exposed to an increased amount of radiation. 
         [0006]    Therefore, it is desirable to provide a method and system for more efficient cryoablation, while reducing the need for fluoroscopy. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention advantageously provides a method and system for improved lesion creation in tissue. In one embodiment, the method may generally include positioning a distal end of a medical device proximate an area of target tissue, and operating a control unit according to a treatment cycle. The treatment cycle may include supplying coolant to the distal portion of the medical device at a first flow rate, the first flow rate causing the distal portion of the medical device to reach a first temperature, and supplying coolant to the distal portion of the medical device at a second flow rate, the second flow rate causing the distal portion of the medical device to reach a second temperature, the second temperature being higher than the first temperature and the second flow rate being lower than the first flow rate. The first temperature being sufficiently low so as to ablate the target tissue and cause cryoadhesion between the distal portion of the medical device and the target tissue, the second temperature being above a temperature at which ablation occurs and below a temperature at which cryoadhesion is broken. The treatment cycle may further include supplying coolant to the distal portion of the medical device at a third flow rate, the third flow rate causing the distal portion of the medical device to reach a third temperature, the third temperature being higher than the first temperature but lower than the second temperature, and the third flow rate being lower than the first flow rate but higher than the second flow rate, and supplying coolant to the distal portion of the medical device at a fourth flow rate, the fourth flow rate causing the distal portion of the medical device to reach a fourth temperature, the fourth temperature being higher than the first and third temperatures but lower than the second temperature, and the fourth flow rate being higher than the second flow rate but lower than the first and third flow rates. The third temperature may be sufficiently low so as to ablate the target tissue and cause cryoadhesion between the distal portion of the medical device and the target tissue, the fourth temperature being above a temperature at which ablation occurs and below a temperature at which cryoadhesion is broken. For example, the first temperature may be between approximately −50° C. to approximately −70° C.; the first flow rate may be between approximately 7000 and 7500 standard cubic centimeters per minute (sccm); the second temperature may be between approximately −15° C. and approximately −5° C.; the second flow rate may be between approximately 3500 sccm and approximately 3000 sccm; the third temperature may be between approximately −35° C. to approximately −40° C.; the third flow rate may be between approximately 4500 sccm to approximately 5000 sccm; the fourth temperature may be between approximately −15° C. and approximately −20° C.; and the fourth flow rate may be between approximately 4000 sccm and approximately 4200 sccm. The coolant may be continuously delivered to a distal portion of the medical device. 
         [0008]    In another embodiment, the method may generally include positioning a treatment element coupled to the distal end of a medical device proximate an area of target tissue, and operating a control unit in accordance with a duty cycle, the control unit being in communication with the treatment element, the duty cycle including at least one freeze-warm cycle, each freeze-warm cycle including: supplying coolant to the treatment element at a first flow rate, the first flow rate causing the treatment element to reach a first temperature, the first temperature causing ablation of the target tissue and causing cryoadhesion between the treatment element and target tissue, and supplying coolant to the treatment element at a second flow rate, the second flow rate causing the treatment element to reach a second temperature, the second temperature being higher than the first temperature and the second flow rate being lower than the first flow rate, the second temperature being above a temperature at which ablation occurs and below a temperature at which cryoadhesion is broken. For example, the duty cycle may include two or more freeze-warm cycles. 
         [0009]    The system may generally include a medical device including a distal portion defining a treatment element, a fluid supply in communication with the treatment element, the fluid supply continuously delivering fluid to the treatment element when the treatment element is activated, and a control unit having a processor, the processor operating to control the flow of fluid according to a duty cycle. The duty cycle may include: a freezing cycle over a first time interval during which the fluid flow rate is increased to lower the temperature of the treatment element to a first ablation temperature; a warming cycle over a second time interval during which the fluid flow rate is decreased to raise the temperature of the treatment element to a first maintenance temperature, the first maintenance temperature being higher than the first ablation temperature; a freezing cycle over a third time interval during which the fluid flow rate is increased to lower the temperature of the treatment element to a second ablation temperature, the second ablation temperature being higher than the first ablation temperature but lower than the first maintenance temperature; and a freezing cycle over a fourth time interval during which the fluid flow rate is decreased to raise the temperature of the treatment element to a second maintenance temperature, the second maintenance temperature being higher than the first ablation temperature but lower than the first maintenance temperature. For example, the first ablation temperature may be between approximately 50° C. to approximately −70° C. and the first flow rate may be between approximately 7000 and 7500 sccm; the first maintenance temperature may be between approximately −15° C. and approximately −5° C. and the first maintenance flow rate may be between approximately 3500 sccm and approximately 3000 sccm; the second ablation temperature may be between approximately −35° C. to approximately −40° C. and the second ablation flow rate may be between approximately 4500 sccm to approximately 5000 sccm; and the second maintenance temperature may be between approximately −15° C. and approximately −20° C. and the second maintenance flow rate may be between approximately 4000 sccm and approximately 4200 sccm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
           [0011]      FIG. 1  shows an exemplary cryoablation system in accordance with the present invention, the system including a first embodiment of a distal end of a cryoablation device; 
           [0012]      FIG. 2  shows cross-sectional view of a second embodiment of a distal end of a cryoablation device in accordance with the present invention; and 
           [0013]      FIG. 3  shows a graphical representation of an exemplary cryoablation system duty cycle in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring now to the drawing figures in which like reference designations refer to like elements, an embodiment of a medical system constructed in accordance with principles of the present invention is shown in  FIG. 1  and generally designated as “ 10 .” The system  10  may generally include a medical device  12  that may be coupled to a control unit  14  or operating console. The medical device  12  may generally include one or more treatment elements for cryotherapy and, optionally, other energy modalities. For example, the distal portion  16  of the device  12  may include one or more treatment elements may be configured to remove heat from tissue, thus being capable of cryotreatment or cryoablation, including of cardiac tissue. The device may further include one or more treatment elements configured to heat tissue to ablation or sub-ablation temperatures (for example, radiofrequency electrodes). 
         [0015]    It is noted that the one or more treatment elements and/or the distal portion  16  of the medical device  12  may define or assume a shape that is independent of the coolant flow. Although the shape of the distal portion  16  of a focal catheter (such as shown in  FIG. 2 ) will generally be independent of coolant flow, treatment elements such as cryoballoons are often kept in an inflated or expanded configuration by the pressure and/or volume of the coolant that is circulated within. For example, many cryoablation systems use an initial fixed volume of fluid to inflate a cryoballoon before treatment begins. In such a system, a predetermined volume of fluid that is specific to the device being used may be delivered to an inflation reservoir from the fluid supply reservoir at room temperature or without being cooled. That is, the temperature is above cryoablation or even cryocooling temperatures. For example, the initial volume of fluid may have a temperature that is close to, or slightly above, room temperature. Once the initial volume is received within the inflation reservoir, a valve may be used to close the inflation reservoir from the fluid supply reservoir. Then, another valve may be used to open the fixed initial volume reservoir to the device, wherein the fixed initial volume inflates the cryoballoon to a predetermined inflation level. After the inflation phase is over, the system enters into a transition phase and then an ablation phase that includes a continuous flow of coolant at cryoablation temperatures. If a thawing phase is desired, the injection of coolant from the fluid supply reservoir is stopped altogether. If another ablation cycle is desired, the system is evacuated or flushed by using one or more valves to open the fluid flow path of the system to a vacuum. After evacuation, the inflation reservoir is refilled with fluid. During the refilling phase, the fluid flow path of the system is open to the vacuum. 
         [0016]    Such fixed initial volume systems may only be used for a specific device, as the size of the inflation reservoir is predetermined, and cannot be adapted for use with, for example, a different type or size of device or newer generation of a device. Additionally, these systems are generally “on/off” and do not easily allow for temperature modification during an ablation procedure. 
         [0017]    The system  10  described herein is a “continuous flow” system, rather than the “fixed initial volume” system described above. In this continuous flow system  10 , the details of which are further shown and described in  FIG. 3 , the coolant flow rate may be adjusted upstream and/or downstream of the device  12  to allow the treatment element to reach sub-ablation temperatures without breaking cryoadhesion between the treatment element and target tissue. That is, the flow of coolant from the fluid supply reservoir  36  does not have to be completely shut off to allow for a warm cycle, and the warm cycle does not completely thaw the ice ball between the treatment element and tissue. Further, because the shape of the treatment element and/or the distal portion  16  of the device  12  is independent of coolant flow, the flow rate of the coolant both upstream and downstream of the device may be adjusted to adjust treatment element temperature without the risk of deflation (for example, if a cryoballoon is used). 
         [0018]    Referring now to  FIG. 1  in detail, the medical device  12  may include an elongate body  18  passable through a patient&#39;s vasculature and/or proximate to a tissue region for diagnosis or treatment, such as a catheter, sheath, or intravascular introducer. The elongate body  18  may define a proximal portion  20  and a distal portion  22 , and may further include one or more lumens disposed within the elongate body  18  thereby providing mechanical, electrical, and/or fluid communication between the proximal portion  20  of the elongate body  18  and the distal portion  22  of the elongate body  18  (not shown). 
         [0019]    The medical device  12  may include a shaft  24  at least partially disposed within a portion of the elongate body  18 . The shaft  24  may extend or otherwise protrude from a distal end  22  of the elongate body  18 , and may be movable with respect to the elongate body  18  in longitudinal and rotational directions. That is, the shaft  24  may be slidably and/or rotatably movable with respect to the elongate body  18 . The shaft  24  may further define a lumen  26  therein for the introduction and passage of a guide wire (not shown). The shaft  24  may include or otherwise be coupled to a distal tip  28  that defines an opening and passage therethrough to the lumen  26  through which the guide wire may exit the distal tip  28  of the device  12 . 
         [0020]    The medical device  12  may further include a fluid delivery conduit  30  traversing at least a portion of the elongate body  18  and towards the distal portion  22  of the elongate body  18 . The delivery conduit  30  may be coupled to or otherwise extend from the distal portion  22  of the elongate body  18 , and/or may be coupled to a portion of the shaft  24  and/or distal tip  28  of the medical device  12 . The fluid delivery conduit  30  may define a lumen therein for the passage or delivery of a fluid from the proximal portion of the elongate body  18  and/or the control unit  14  to the one or more treatment elements of the medical device  12 . A distal portion of the fluid delivery conduit  30  may further include one or more apertures or openings therein, to provide for the dispersion or directed ejection of fluid from the lumen to an environment exterior to the fluid delivery conduit  30 . 
         [0021]    As shown in  FIG. 1 , the one or more treatment elements may be one or more expandable elements or balloons  32  at the distal portion  22  of the elongate body  18 . For example, the medical device  12  may include a treatment element that includes one or more cryoballoons  32 . As a non-limiting example, the medical device  12  of  FIG. 1  includes an outer cryoballoon  32   a  and an inner cryoballoon  32   b . At least a portion of each cryoballoon  32   a ,  32   b  may be coupled to a portion of the distal portion  22  of the elongate body  18  and at least a portion of each cryoballoon  32   a ,  32   b  may be coupled to a distal portion of the shaft  24  and/or distal tip  28  to define an interior chamber or region  34  within which a portion of the fluid delivery conduit  30  may be contained. For example, the inner cryoballoon  32   b  may define an interior chamber or region  34  that contains coolant or fluid dispersed from the fluid delivery conduit  30 . The coolant may be delivered from a fluid supply reservoir  36  to the interior chamber  34  under pressure, such that the coolant expands within the interior chamber  34  upon exiting the fluid delivery conduit  30 . This expansion causes a decrease in the temperature of the coolant, and therefore of the one or more cryoballoons  32 , to ablation and/or sub-ablation temperatures via the Joule-Thompson effect. The interior chamber  34  may also be in fluid communication with a fluid exhaust conduit  38  defined by or included in the elongate body  18  for the removal of expanded coolant from the interior chamber  34  of the cryoballoon  32  (for example, the inner cryoballoon  32   b  as shown in  FIG. 1 ). The cryoballoon  32  may further include one or more material layers providing for puncture resistance, radiopacity, or the like. Further, each cryoballoon may be either substantially compliant or substantially noncompliant. Although two cryoballoons  32   a ,  32   b  are shown in  FIG. 1 , it will be understood that any number and/or configuration of expandable elements may be used, and that the one or more treatment elements may include expandable elements other than cryoballoons. 
         [0022]    The medical device  12  may include a handle  40  coupled to the proximal portion  20  of the elongate body  18 . The handle  40  can include circuitry for identification and/or use in controlling of the medical device  12  or another component of the system  10 . Additionally, the handle  40  may be provided with a fitting  42  for receiving a guide wire that may be passed into the guide wire lumen  26 . The handle  40  may also include one or more connectors  44  that are matable to the control unit  14  to establish communication between the medical device  12  and one or more components or portions of the control unit  14 . The handle  40  may also include one or more actuators or control mechanisms that allow a user to control, deflect, steer, or otherwise manipulate a distal portion  22  of the medical device  12  from the proximal portion  20  of the medical device  12 . For example, the handle  40  may include one or more components such as steering elements (for example, a lever or knob  46 ) for manipulating the elongate body  18  and/or additional components of the medical device  12 . For example, a pull wire for steering the distal portion  22  of the elongate body  18  may be anchored at its proximal end to a portion of the handle  40  in communication with one or more steering elements  46 . 
         [0023]    Continuing to refer to  FIG. 1 , the medical device  12  may further include one or more conductive segments or mapping electrodes  48  positioned on or about the elongate body  18  for measuring, recording, or otherwise assessing one or more electrical properties or characteristics of surrounding tissue. For example, mapping electrodes  48  may be used for recording electrocardiogram or monophasic action potential (MAP) signals. Additionally, the medical device  12  may include one or more conductive segments or treatment electrodes  50  conveying an electrical signal, current, or voltage to a designated tissue region (for example, as shown in  FIG. 2 ). The electrodes  48 ,  50  may be configured in a myriad of different geometric configurations or controllably deployable shapes, and may also vary in number to suit a particular application, targeted tissue structure or physiological feature. In the non-limiting example shown in  FIG. 1 , the medical device  12  may include a first pair of mapping electrodes  48  proximal to the cryoballoons  32   a ,  32   b  and a second pair of mapping electrodes  48  distal to the cryoballoons  32   a ,  32   b.    
         [0024]    Continuing to refer to  FIG. 1 , the system  10  may include one or more fluid reservoirs, such as a fluid supply reservoir  36  and a fluid return reservoir  52 . For example, the fluid supply reservoir  36  and/or fluid return reservoir  52  may be located within the control unit or console  14 . The fluid supply reservoir  36  may be in fluid communication with the fluid delivery conduit  30 , and the fluid return reservoir  52  may be in fluid communication with the fluid exhaust conduit  38 , and may be in communication with an exhaust or scavenging system (not shown) for recovering or venting expended fluid for re-use or disposal, as well as various control mechanisms and a vacuum pump  54 . The vacuum pump  54  may create a low-pressure environment in one or more conduits or lumens within the medical device  12  (such as the fluid exhaust conduit  38 ) so that expanded fluid is drawn into the lumen of the elongate body  18  from, for example, the interior chamber  34 , towards the proximal portion  20  of the elongate body  18 . In addition to providing an exhaust function for the fluid or coolant supply reservoir  36 , the control unit  14  may also include pumps, valves, controllers or the like to recover and/or re-circulate fluid delivered to the handle  40 , the elongate body  18 , and/or the fluid pathways of the medical device  12 . 
         [0025]    The system  10  may further include one or more sensors to monitor the operating parameters throughout the system, including for example, pressure, temperature, flow rates, volume, power delivery, impedance, or the like in the control unit  14  and/or the medical device  12 , in addition to monitoring, recording or otherwise conveying measurements or conditions within the medical device  12  or the ambient environment at the distal portion of the medical device  12 . The sensor(s) may be in communication with the control unit  14  for initiating or triggering one or more alerts or therapeutic delivery modifications during operation of the medical device  12 . One or more valves, controllers, or the like may be in communication with the sensor(s) to provide for the controlled dispersion or circulation of fluid through the lumens/fluid paths of the medical device  12 . Such valves, controllers, or the like may be located in a portion of the medical device  12  and/or in the control unit  14 . For example, each electrode  48 ,  50  may include a sensor, such as a thermocouple, an electrical conductivity sensor, a spectrometer, a pressure sensor, a fluid flow sensor, a pH sensor, and/or a thermal sensor (not shown) coupled to or in communication with the electrodes  48 ,  50 . The sensors may also be in communication with a feedback portion of the control unit  14  to trigger or actuate changes in operation when predetermined sequences, properties, or measurements are attained or exceeded. 
         [0026]    Continuing to refer to  FIG. 1 , the control unit  14  may additionally include an energy generator or power source  56  as a treatment or diagnostic mechanism in communication with the electrodes  48 ,  50  of the medical device  12 . For example, the control unit  14  may include a radiofrequency (RF) generator  56  for providing RF energy delivery in addition to cryotreatment functionality. Additionally or alternatively, energy may be used for pacing myocardial cells in a mapping procedure. The radiofrequency generator  56  may have a plurality of output channels, with each channel coupled to an individual electrode  48 ,  50 . For example, if RF treatment is used in addition to cryotreatment, the radiofrequency generator  56  may be operable in one or more modes of operation, including for example bipolar energy delivery between at least two treatment electrodes  50  on the medical device  12  within a patient&#39;s body, monopolar or unipolar energy delivery to one or more of the electrodes  50  on the medical device  12  within a patient&#39;s body and through a patient return or ground electrode (not shown) spaced apart from the electrodes  50  of the medical device  12  (for example, on a patient&#39;s skin), and combinations thereof. 
         [0027]    The control unit  14  may include one or more controllers, processors, and/or software modules containing instructions or algorithms (collectively referred to as “computers  58 ”) to provide for the automated operation and performance of the features, sequences, calculations, or procedures described herein. For example, the control unit  14  may include a signal processing unit or computer  58  to measure one or more electrical characteristics between the electrodes  48 ,  50  of the medical device  12 . An excitation current may be applied between one or more of the electrodes  48 ,  50  on the medical device  12  and/or a patient return electrode, and the resulting voltage, impedance, or other electrical properties of the target tissue region may be measured, for example, in an electrogram. Unipolar electrograms may be recorded with the mapping electrode  48  as the positive electrode, and another electrode on the body surface or remote from the field or cardiac excitation as the negative electrode. The control unit  14  may further include one or more displays or screens  60  to display the various recorded signals and measurement, for example, an electrogram. 
         [0028]    As is shown and described in greater detail in  FIG. 3 , the one or more computers  58 , controllers, processors, and/or software modules containing instructions or algorithms of the control unit  14  may be used for the automatic or manual control of coolant flow within the system according to a preferred duty cycle. Although the control unit  14  may be programmed to follow a predefined duty cycle, feedback signals from one or more system  10  components may allow for the duty cycle to be adjusted in real time in order to effectively ablate target tissue. 
         [0029]    Referring now to  FIG. 2 , a second embodiment of a medical device  12  is shown. The medical device  12  may be configured for use with a system  10  such as that shown and described in  FIG. 1 . The device of  FIG. 2  may have a fixed diameter (that is, may not include an expandable element or balloon  32 , as in  FIG. 1 ), such as a focal catheter. The one or more treatment elements may include one or more electrodes  48 ,  50  coupled directly to the distal portion  22  of the elongate body  18 . In the non-limiting example of  FIG. 2 , the medical device  12  may include a treatment electrode  50  at the distal tip  62  and one or more mapping electrodes  48  in the distal portion  22  of the elongate body  18 . Additionally, the treatment electrode  50  may also have mapping functionality. The electrodes  48 ,  50  may have any of a myriad of shapes, sizes, and configurations. As shown in  FIG. 2 , for example, the mapping electrodes  48  may be band electrodes and the treatment electrode  50  may be configured to fit about the distal tip  62  of the device  12 . 
         [0030]    The medical device  12  of  FIG. 2  may include a fluid delivery conduit  30  in fluid communication with an interior chamber  34  in which coolant may expand. The interior chamber  34  may be in thermal communication with a treatment electrode  50  at the distal tip  62  that is composed of a thermally conductive material (for example, a metal), such that expansion of the coolant cools the treatment electrode  50  to a temperature sufficient to cool tissue to ablation or sub-ablation temperatures. As with the device of  FIG. 1 , coolant flow into the interior chamber  34  may be controlled, either automatically or manually, by the control unit  14 . For example, the coolant flow may be according to a duty cycle algorithm. 
         [0031]    Referring now to  FIG. 3 , a graphical representation of an exemplary cryoablation system duty cycle  64  is shown. In general, the duty cycle  64  (or “duty-controlled cycle”) presents a freeze-warm-freeze cycle that more efficiently creates tissue lesions than a single, longer freeze cycle. The duty cycle  64  may include any number of consecutive freeze cycles  68  and warm cycles  70  necessary to create a desired lesion in the target tissue (for example, myocardial tissue). The duty cycle  64  may be controlled either manually by the user with one or more user input devices (which may include a computer  58 ) or automatically by the control unit  14  or component thereof. For example, the duty cycle  64  may be controlled by programming a computer  58  or processor including one or more algorithms to automatically operate the system  10  according to a predetermined duty cycle  64 , or by a computer  58  or processor including one or more algorithms to automatically control the system  10  generally based on a predetermined duty cycle  64  but adjusted in response to one or more feedback signals. Further, one or more parameters of the duty cycle  64  may be shown on one or more displays or screens  60  for monitoring, controlling and/or modifying the duty cycle  64  as necessary. 
         [0032]    As described above, the system  10  may be a continuous flow system instead of a fixed initial volume system. In order to adjust the temperature of the one or more treatment elements (for example, to transition between a freeze cycle  68  and a warm cycle  70 ), the flow rate of the fluid within the fluid flow path of the system  10  may be adjusted upstream and/or downstream of the device  12 . This adjustment in flow rate will consequently adjust the temperature of a treatment element of the device  12 . For example, flow rates may be adjusted using one or more pressure transducers controlling fluid injection pressure in the fluid delivery conduit  30  and controlling vacuum pressure in the fluid exhaust conduit  38  (not shown). 
         [0033]    The duty cycle  64  may include one or more freeze or ablation cycles  68  and one or more thaw or maintenance cycles  70 . Several freeze and warm cycles  68 ,  70  are shown in  FIG. 3 . In general, an exemplary duty cycle  66  is graphically expressed in  FIG. 3  in terms of both temperature (line  72 ) and flow rate (line  74 ). The y-axis of the graph represents temperature (T), and y-axis of the graph represents time (t). Although specific flow rates are not expressly shown on the y-axis of the graph, the approximate flow rate (F) is shown adjacent the flow rate line  74  at each time interval (t 1 , t 2 , t 3 , t 4 , t 5 , etc.). As is shown in  FIG. 3 , the duty cycle  64  may include pulsatile coolant flow, such that flow rate and therefore temperature may fluctuate many times over the course of a duty cycle  64 . Further, temperature and flow rate may be inversely related, such that temperature increases as flow rate decreases and vice versa. In general, the greater the flow rate of the coolant within an interior chamber  34  proximate a treatment element, the greater the amount of heat that may be removed from tissue adjacent the treatment element. 
         [0034]    The freeze cycle  68  of the exemplary duty cycle  64  shown in  FIG. 3  may be a freeze cycle  68  of a cryoablation treatment (such as shown in time interval t 1 ). As such, the initial temperature of the first freeze cycle  68  may be between approximately 37° C. and room temperature. However, in freeze cycles  68  occurring after warm cycles  70 , this initial temperature may be much lower. As shown in  FIG. 3 , a freeze cycle  68  may occur, for example, in each of time intervals t 1 , t 3 , and t 5  Likewise, a warm cycle  70  may occur, for example, in each of time intervals t 2  and t 4 . Additionally, although not shown, a final warm cycle in which the ice ball is allowed to completely thaw may occur in a time interval after the cryotreatment procedure is completed. 
         [0035]    Coolant flow within the freeze cycle  68  may be referred to as “ablation flow” (which appears as a “hill” in the graph), and the temperature of the treatment element within the freeze cycle  68  may be referred to as “ablation temperature.” Likewise, coolant flow within the warm cycle  70  may be referred to as “maintenance flow” (which appears as a “valley” in the graph), and the temperature of the treatment element within the warm cycle  70  may be referred to as “maintenance temperature.” Flow rate and temperature may be adjusted many times within a single freeze or warm cycle  68 ,  70  in order to optimize lesion creation. Additionally or alternatively, the time interval of a freeze or warm cycle  68 ,  70  may be adjusted for the same reason. As a non-limiting example, such adjustments may be based on changes in tissue biophysical properties detected by one or more system sensors. Further, although not shown, the coolant temperature may be monitored using, for example, surface thermocouples or by measurement of the coolant temperature after it has expanded (for example, proximal the interior chamber  34  of a cryoballoon  32 , within the fluid return lumen). In general, however, the average ablation flow may be sufficient to produce treatment element temperatures within a range that cause disruption or destruction of tissue (i.e. tissue ablation), which may be a permanent effect. Further, the average maintenance flow may be sufficient to produce treatment element temperatures within a range that allows the treatment element and adjacent tissue to warm above ablation temperatures, but within a range that does not break cryoadhesion between the treatment element and tissue (that is, does not allow the ice ball to completely thaw). 
         [0036]    Continuing to refer to the exemplary duty cycle  64  of  FIG. 3 , during time interval t 1 , a freeze cycle  68 , the flow rate of the coolant (for example, as it is injected into an interior chamber  34  of a cryoballoon) may increase to between approximately 7000 and 7500 standard cubic centimeters per minute (sccm) (e.g., approximately 7200 sccm). Consequently, the temperature of the treatment element may decrease to between approximately −50° C. to approximately −70° C. (e.g., approximately −70° C.). During time interval t 2 , a warm cycle  70 , the flow rate of the coolant may decrease to between approximately 3500 sccm and approximately 3000 sccm (e.g., approximately 3000 sccm), and the temperature of the treatment element may increase to between approximately −15° C. and approximately −5° C. (e.g., approximately −5° C.). During time interval t 3 , a freeze cycle  68 , the flow rate of the coolant may increase to between approximately 4500 sccm to approximately 5000 sccm (e.g., approximately 5000 sccm), and the temperature of the treatment element may decrease to between approximately −35° C. to approximately −40° C. (e.g., approximately −40° C.). During time interval t 4 , a warm cycle  70 , the flow rate may decrease to between approximately 4000 sccm and approximately 4200 sccm (e.g., approximately 4000 sccm), and the temperature of the treatment element may increase to between approximately −15° C. and approximately −20° C. (e.g., approximately −20° C.). During time interval t 5 , a freeze cycle  68 , the flow rate of the coolant may increase to approximately 4500 sccm, and the temperature of the treatment element may decrease to approximately −30° C. 
         [0037]    Even though flow rates are shown in  FIG. 3 , the flow rate during any warm cycle  70  may be reduced to zero temporarily, as long as the temperature of the treatment element is not allowed to reach temperatures at which cryoadhesion would be broken. 
         [0038]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.