referring initially to fig1 , 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 fig1 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 fig1 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 . 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 . referring now to fig2 , 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 . still referring to fig2 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 fig3 . fig3 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 fig3 is indicative of a phase change for the refrigerant between a gaseous state 48 and a liquid state 50 . when cross - referencing fig3 with fig1 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 fig3 . 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 fig3 ). 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 fig3 ). 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 . in overview , conditions on the fluid refrigerant change from the values at point c to those at point d on the graph shown in fig3 , 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 . as shown in fig3 , 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 fig3 ). 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 fig3 ) to its distal end 34 ( point d in fig3 ) in its liquid state 50 . 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 fig3 , 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 ). fig3 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 . 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 cryo - chamber 38 . fig4 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 fig4 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 fig4 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 . 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 .
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 . 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 .