Abstract:
A system and method for transferring heat requires a supply tube connected in fluid communication with a capillary tube. A tip member is positioned to surround the distal end of the capillary tube to create a cryo-chamber. In operation, a liquid refrigerant is introduced into the supply tube at a working pressure (e.g. 450 psia). The pressure is then significantly reduced on the liquid refrigerant as it transits through the capillary tube. The refrigerant then exits the distal end of the capillary tube, still in its liquid state. Inside the cryo-chamber, at a pressure of less than about one atmosphere, the refrigerant transitions into its gaseous state. The resultant refrigeration causes heat to transfer into the cryo-chamber.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention pertains generally to interventional medical devices that can be advanced into the vasculature of a patient. More particularly, the present invention pertains to cryo-catheters that are useful for cryoablating tissue in the vasculature. The present invention is particularly, but not exclusively, useful for the construction and deployment of a cryo-catheter wherein a refrigerant fluid transitions from a liquid to a gaseous state, at an operational site in the vasculature, to establish cryoablation temperatures that are below approximately minus eighty four degrees Centigrade.  
       BACKGROUND OF THE INVENTION  
       [0002]     Medical devices that can be advanced into the vasculature of a patient, and properly positioned at a site for an in-situ procedure, have several structural requirements in common with each other. Most importantly, they must be properly dimensioned to allow them to be advanced into the vasculature. This requires they be long and slender. Also, they must be steerable, bio-compatible, flexible and have sufficient structural strength to maintain their integrity while they are employed in the vasculature. With all of this in mind, the interventional device must also be fully capable of performing its intended function.  
         [0003]     Recently, there has been substantial interest in medical procedures involving the cryo-ablation of tissue. In general, such procedures are intended to freeze specifically identified tissue. One procedure for which the cryoablation of tissue is known to be particularly efficacious is in the treatment of atrial fibrillation in the left ventricle of the heart. It happens, however, that cryoablation in general, and this procedure in particular, preferably requires temperatures below about minus eighty four degrees Centigrade (−84° C.). In order to generate such temperatures deep in the vasculature of a patient, several heat transfer principles need to be considered. Specifically, not only must such very low temperatures be generated, these temperatures must be somehow confined to the proximity where tissue is to be cryoablated.  
         [0004]     Fourier&#39;s law of heat conduction states that the rate at which heat is transferred through a body, per unit cross sectional area, is proportional to the temperature gradient existing in the body (dQ/dt=rate of heat transfer). Mathematically, this phenomenon is expressed as: 
 
 dQ/dt=−λAdT/dx  
 
 where λ is the material&#39;s thermal conductivity, “A” is the cross sectional area through which heat is to be transferred, and dT/dx is the local temperature gradient. In the context of a cryo-catheter, “A” will be predetermined and will be necessarily limited by space considerations. Further, because high thermally conductive materials can be used in the manufacture of the cryo-catheter (e.g. copper), the thermal conductivity (λ) for a cryoablation procedure is effectively controlled by the relatively low conductivity of the tissue that is to be ablated. Thus, it can be appreciated that the local temperature gradient “dT/dx” is a control variable of significant importance. In particular, it is desirable that the local temperature gradient between tissue at an operational site, and the refrigerant in a cryo-catheter, be as great as possible. Stated differently, it is desirable to have cryo-catheter temperatures at the operational site that are as low as possible. 
 
         [0006]     In addition to the temperature gradient effect discussed above, it is also to be appreciated that a substantial amount of heat transfer in a substance can result without any change in temperature. Specifically, this phenomenon involves latent heat and occurs wherever a substance, such as a fluid refrigerant, changes state. By definition, “latent heat” is the heat which is required to change the state of a unit mass of a substance from a solid to a liquid, or from a liquid to a gas, without a change of temperature. In the case of a fluid refrigerant, it can be said that prior to such a state change, the liquid refrigerant is “refrigerant in excess”. On the other hand, after the fluid refrigerant begins to boil (i.e. change state from liquid to gas) the gas refrigerant is “refrigerant limited”. Insofar as cryo-catheters are concerned, due to their requirement for low operational temperatures, it is desirable to obtain the additional refrigeration potential that results during the transfer of latent heat. Stated differently, it is preferable for the refrigerant to stay in its liquid state (i.e. remain “refrigerant in excess”) until employed for cryoablation.  
         [0007]     At this point it should also be noted that there is a significant benefit which is obtained by maintaining a fluid refrigerant in its liquid state while it transits through a system. Specifically, this benefit comes from the fact that, any water entrained in the liquid refrigerant is prevented from forming as frost or ice that could clog the system, so long as the refrigerant remains liquid. This is a particularly important consideration whenever a system requires that the refrigerant pass through small or narrow orifices.  
         [0008]     As discussed above, for the operation of a cryoablation system, it is necessary to select a fluid refrigerant that is capable of generating very low temperatures (i.e. &lt;−84° C.). Prior to its use in the system, however, the fluid refrigerant is typically stored in vessels under very high pressure (i.e. around 700 psia). On the other hand, when it is to be used in a cryo-catheter, the pressure on the refrigerant needs to be reduced in stages to about one atmosphere. In addition to the refrigeration effect, an important consideration here is that the pressure be reduced to a level below normal blood pressure for safety reasons.  
         [0009]     Although there are several well known ways in the pertinent art for reducing the pressure on a fluid, a convenient way for accomplishing this pressure reduction in a cryo-catheter is by passing the fluid refrigerant through a capillary tube. For capillary tubes that can be considered as being long, straight, uniform pipes, the “Darcy equation” is applicable. According to the Darcy equation a pressure drop along the length of the pipe (tube) (i.e. head loss “h l ”) can be mathematically expressed as: 
 
 h   l   =f ( l/d )( V   2 /2 g ) 
 
         [0010]     In the above expression: “f” is a friction factor, “l” is the length of the tube, “d” is the diameter of the tube, “V” is the velocity of the fluid through the tube, and “g” is the acceleration due to gravity.  
         [0011]     From the Darcy equation it is to be noted that the head loss (h l ) is proportional to the ratio “l/d”. This is the same as saying that the head loss is inversely proportional to the aspect ratio (“d/l”) of the pipe (tube). Regardless how viewed, the pressure drop along the entire length of a pipe will increase by reducing the inside diameter of the pipe “d” or by increasing the length “l” of the pipe. In any event, the dimensions of a tube that is to be used in a cryo-catheter for the purpose of reducing pressure on a fluid refrigerant should be selected so that the fluid is “refrigerant in excess” (i.e. in a liquid state) as it transits through the tube. Empirical results can be helpful when determining the most effective dimensions for such a tube.  
         [0012]     In light of the above, it is an object of the present invention to provide a heat transfer system that will maintain a fluid refrigerant in a liquid state during a pressure drop on the fluid that is greater than four hundred psia, when the final pressure on the fluid is to be less than approximately one atmosphere. Another object of the present invention is to provide a heat transfer system that effectively avoids frost or ice build-up in the system as refrigerant passes through a relatively small orifice. Still another object of the present invention is to provide a heat transfer system that can be safely introduced into the vasculature of a patient where it will create temperatures as low as about minus eighty four degrees Centigrade. Another object of the present invention is to provide a heat transfer system that is relatively easy to manufacture, is simple to use and is comparatively cost effective.  
       SUMMARY OF THE INVENTION  
       [0013]     A cryo-catheter (i.e. heat transfer system) in accordance with the present invention includes a hollow supply tube having a distal end that is connected in fluid communication with the proximal end of a capillary tube. Additionally, a tip member is positioned to surround the distal end of the capillary tube to thereby create a cryo-chamber that is located at the distal end of the cryo-catheter.  
         [0014]     The source of refrigerant fluid mentioned above is connected in fluid communication with the proximal end of the supply tube. Preferably, the refrigerant fluid is nitrous oxide (N 2 O), and it is introduced into the supply tube at a working pressure “p w ” that will typically be in a range between three hundred and fifty psia and five hundred psia (350-500 psia). The refrigerant fluid then sequentially transits through the supply tube and through the capillary tube. Importantly, as the refrigerant fluid exits from the distal end of the capillary tube, it is substantially still in a liquid state. The dimensions of both the supply tube and capillary tube, as well as the working pressure “p w ” for the refrigerant fluid are specifically chosen for this purpose.  
         [0015]     For the construction of the present invention, the supply tube is formed with a lumen having a length “l s ” and a diameter “d s ”. Further, the capillary tube is formed with a lumen having a length “L” and a diameter “d”. More specifically, the diameter “d” of the capillary tube lumen is less than the diameter “d s ” of the supply tube lumen. As specifically intended for the present invention, the refrigerant fluid experiences much more resistance and a much greater pressure drop as it passes through the capillary tube than it did while passing through the supply tube. In detail, while the supply tube may have an aspect ratio “d s /l s ” of around 0.1 or 0.05, the capillary tube will preferably have an aspect ratio “d/l” that is in a range of 0.0008 to 0.0017. When calculating the aspect ratio for the capillary tube, the length “l” will preferably be in a range between approximately four and one half inches and ten inches (4.5 in-10 in.), and the diameter “d” of the capillary tube will be between about 0.008 inches and 0.010 inches.  
         [0016]     As indicated above, for the operation of the present invention, the working pressure “p w ” on the refrigerant fluid at the proximal end of the supply tube, will preferably be in a range between three hundred and fifty psia and five hundred psia (350-500 psia). On the other hand, the tip pressure “p t ” on the refrigerant fluid as it leaves the distal end of the capillary tube and enters the cryo-chamber is preferably less than about one atmosphere. Within this environment, after the refrigerant fluid has transitioned into its gaseous state in the cryo-chamber, it will create a tip temperature “p t ” that is less than about minus eighty four degrees Centigrade (p t &lt;−84° C.). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0018]      FIG. 1  is a schematic view of a system incorporating the present invention;  
         [0019]      FIG. 2  is a cross-sectional view of the distal portion of a cryo-catheter as seen along the line  2 - 2  in  FIG. 1 ;  
         [0020]      FIG. 3  is a graphical plot of pressure vs. temperature for a duty cycle in the operation of the present invention; and  
         [0021]      FIG. 4  is an exemplary graphical representation of changes in the tip temperature (T t ) of a cryo-catheter as a function of the working pressure (p w ) on the fluid refrigerant. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     Referring initially to  FIG. 1 , a system in accordance with the present invention is shown and is generally designated  10 . In detail, the system  10  is shown to include a cryo-catheter  12  that is connected to a console  14  and in fluid communication with a pair of fluid refrigerant sources  16   a  and  16   b  that are mounted inside the console  14 . The sources  16   a  and  16   b  shown in  FIG. 1  are, however, only exemplary. As envisioned for the present invention, the refrigerant sources  16   a  and  16   b  may be of any type pressure vessel known in the pertinent art that is suitable for holding a fluid under relatively high pressures (e.g. 700 psia). For the purposes of the present invention, the fluid refrigerant that is held in sources  16   a  and  16   b  will preferably be nitrous oxide (N 2 O). Still referring to  FIG. 1  it is seen that the fluid refrigerant sources  16   a  and  16   b  are connected in fluid communication with a pre-cooler  18 . The pre-cooler  18 , in turn, is connected in fluid communication with the cryo-catheter  12 . Further, for purposes to be subsequently disclosed, the cryo-catheter  12  is connected in fluid communication to a vacuum source  20 .  
         [0023]     At the extreme distal end of the cryo-catheter  12 , is a tip  22 . Importantly, the tip  22  should be made of a material having very high thermal conductivity, such as copper or steel which respectively have thermal conductivities of 385 and 46 Watts/° K.*m. To put this in perspective, water has a thermal conductivity of only 0.627 Watts/° K.*m.  
         [0024]     Referring now to  FIG. 2 , it will be seen that inside the distal portion  24  of cryo-catheter  12 , a capillary tube  26  is connected to a supply tube  28 . Preferably, both the capillary tube  26  and the supply tube  28  will be made of a polymer material that has a relatively low thermal conductivity. Further, the capillary tube  26  preferably has a length “l” that is equal to, or preferably, shorter than the length “l s ” of supply tube  28  (l≦l s ). While the supply tube  28  is dimensioned so as to cause a minimal pressure drop on the fluid refrigerant as it passes through the supply tube  28 , this is not so insofar as the capillary tube  26  is concerned. More specifically, in its structural aspects, the capillary tube  26  is formed with a lumen  30  that extends the entire length of the capillary tube  26  from its proximal end  32  to its distal end  34 . As indicated, the lumen  30  of capillary tube  26  has a diameter “d” and a length “l”. In their relationship to each other, the diameter and length of lumen  30  in capillary tube  26  define an aspect ratio “d/l” that effectively determines the fluid flow characteristics of the capillary tube  26 . For the present invention, the aspect ratio “d/l” is preferably in a range of 0.0008 to 0.0017, with the diameter “d” being selected in the range of about 0.008 inches to about 0.010 inches, and the length “l” being selected in the range of approximately four and one half inches to approximately ten inches. Recall, for selected embodiments, the supply tube  28  may also be of length “l”. Preferably, however, the capillary tube  26  will be shorter than the supply tube  28 .  
         [0025]     Still referring to  FIG. 2  it is to be appreciated that the tip  22  is attached to the distal end  36  of the cryo-catheter  12 . Specifically, the tip  22  is attached to the cryo-catheter  12  to create a cryo-chamber  38  around the distal end  34  of the capillary tube  26 . The structural consequence here is that a fluid refrigerant in the lumen  40  of the supply tube  28  can flow from lumen  40 , through the lumen  30  of the capillary tube  26 , and into the cryo-chamber  38 . Once the fluid is in the cryo-chamber  38 , it can then be exhausted from the cryo-catheter  12  through the return path  42  by vacuum source  20 . As shown, this return path  42  is established between the wall  44  of the cryo-catheter  12  and the respective outside surfaces of the capillary tube  26  and the supply tube  28 . The thermodynamics of fluid flow along this pathway through the cryo-catheter  12  will be best appreciated with reference to  FIG. 3 .  
         [0026]      FIG. 3  shows a pressure-temperature graph for a fluid refrigerant, such as nitrous oxide (N 2 O), and a typical plot of the relationship between these variables as the refrigerant transits through the system  10  of the present invention. In particular, the curve  46  shown in  FIG. 3  is indicative of a phase change for the refrigerant between a gaseous state  48  and a liquid state  50 .  
         [0027]     When cross-referencing  FIG. 3  with  FIG. 1  it is to be appreciated that the pressure and temperature conditions for the fluid refrigerant, as stored in the fluid refrigerant sources  16   a  and  16   b , is indicated by the point A in  FIG. 3 . Specifically, it is expected that the fluid refrigerant will be stored in sources  16   a  and  16   b  at ambient temperature (i.e. room temperature) under a pressure of about 700 psig. When in use, a pressure regulator (not shown) then reduces the pressure on the fluid refrigerant to a working pressure (“p w ”) that will be about 400 to 450 psia (see point B in  FIG. 3 ). The pre-cooler  18  then reduces the temperature of the fluid refrigerant to a temperature of about minus forty five degrees Centigrade while maintaining the fluid refrigerant at the working pressure “p w ” (see point C in  FIG. 3 ). Note that with this cooling, the fluid refrigerant is transformed into its liquid state  50 . Also, it is to be appreciated that the fluid refrigerant is introduced into the supply tube  28  under the conditions indicated at point C.  
         [0028]     In overview, conditions on the fluid refrigerant change from the values at point C to those at point D on the graph shown in  FIG. 3 , as the fluid refrigerant transits through the supply tube  28  and the capillary tube  26 . The vast majority of this change, however, occurs in the capillary tube  26 . Specifically, as the fluid refrigerant enters the lumen  30  at the proximal end  32  of capillary tube  26 , it will be at a temperature of about minus forty five degrees Centigrade. Also, it will be under a working pressure “p w ” of about four hundred to four hundred and fifty psia (point C). As the fluid refrigerant transits capillary tube  26 , the pressure on the fluid refrigerant in lumen  30  is reduced from “p w ” in the supply tube  28  to a tip pressure “p t ” in the cryo-chamber  38 . For the present invention, the tip pressure “p t ” will preferably be less than approximately one atmosphere of pressure. Accordingly, as intended for the present invention, there will be a pressure drop (i.e. head loss “h l ”) that will be around 450 psia.  
         [0029]     As shown in  FIG. 3 , along with the pressure reduction from “p w ” to “p t ” (i.e. head loss “h l ”), the temperature of the fluid refrigerant will be reduced to a tip temperature “t t ” at the distal end  34  of the capillary tube  26  (point D in  FIG. 3 ). For the present invention, the tip temperature “t t ” in the cryo-chamber  38  will be less than about minus eighty four degrees Centigrade. Importantly, as this temperature is achieved, the fluid refrigerant transits the capillary tube  26  from its proximal end  32  (point C in  FIG. 3 ) to its distal end  34  (point D in  FIG. 3 ) in its liquid state  50 .  
         [0030]     As the fluid refrigerant exits into the cryo-chamber  38  from the distal end  34  of capillary tube  26  it evaporates. After boiling has occurred, the consequent rapid rise in temperature of the fluid refrigerant in the cryo-chamber  38  is due, in large part, to heat transfer from the tissue being cryoablated in the patient (not shown). In  FIG. 3 , this heat transfer is represented by the change in conditions on the fluid refrigerant (now in its gaseous state  48 ) indicated by the transition from the tip temperature “t t ” (point D) to a generally ambient temperature (point E).  FIG. 3  also indicates that the heat transfer to the fluid refrigerant in the cryo-chamber  38  is accomplished at a substantially constant tip pressure “p t ”. As mentioned above, the establishment and maintenance of this tip pressure “p t ” is facilitated by the action of the vacuum source  20  that operates to evacuate the fluid refrigerant from the system  10 .  
         [0031]     In the operation of the present invention, the vacuum source  20  is activated to establish a tip pressure “p t ” in the cryo-chamber  38  that is less than about one atmosphere. The exact value of this tip pressure “p t ” may, however, vary to some extent. Importantly, “p t ” is established to evacuate fluid refrigerant from the system  10  and reduce back pressure on the capillary tube  26 .  
         [0032]      FIG. 4  is a plot of the variations in the tip temperature (“t t ”) at the distal end  34  of capillary tube  26 , as a function of the working pressure (“p w ”) at the proximal end  32  of the capillary tube  26 . In particular, the specific measurements shown in  FIG. 4  were obtained using a capillary tube  26  having a length “l” equal to 7.35 inches and a diameter “d” equal to 0.008 inches (aspect ratio “d/l”=0.00109). Although the plot shown in  FIG. 4  is specific for a capillary tube  26  having the given dimensions, this plot can be taken as being generally representative of similarly dimensioned capillary tubes  26 . In any event, it will be noted that when the working pressure “p w ” (e.g. 450 psia) maintains the fluid refrigerant in its liquid state  50  (i.e. “refrigerant in excess”) as it transits the lumen  30  of capillary tube  26 , the tip temperature “t t ” in cryo-chamber  38  will be minimized. On the other hand, if the fluid refrigerant is allowed to boil and become gaseous (i.e. “refrigerant limited”) inside the lumen  30 , the tip temperature “t t ” rises sharply.  
         [0033]     While the particular Improved Distal End for Cryoablation Catheters as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.