Abstract:
A cryocatheter includes a catheter body defining a coolant flow path, a catheter tip exposed to the coolant flow path, and a heating element associated with the catheter tip. The heating element can be disposed entirely or partially within the catheter tip. Alternatively, the heating element can be exterior to the catheter tip. The heating element can include an electrically resistive element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application Serial No. 60/127,986, filed Apr. 6, 1999. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     FIELD OF THE INVENTION 
     This invention relates to catheters, and more particularly to tip temperature control for cryogenic catheters. 
     BACKGROUND OF THE INVENTION 
     A cryocatheter can generally be described as an elongate, slender, flexible body that is capable of delivering extreme cold to provide a medically therapeutic effect. Exemplary cryocatheters are disclosed in U.S. Pat. Nos. 5,899,898 and 5,899,899 to Arless. 
     Known techniques for creating the extremely low temperatures delivered by a cryocatheter include provision of a cooling chamber where a high pressure gas is allowed to rapidly expand, or where a liquid changes phase to a gas. While both of these techniques can provide extremely cold temperatures (at or above 0° C. to −70° C. or below), it can be very difficult to regulate coolant flow and expansion or phase change with enough precision to ensure that specific temperatures are achieved and maintained. For example, a selected temperature can be therapeutic, but a temperature a few degrees above or below the selected temperature can be either ineffective or injurious. 
     Additionally, many coolants perform differently under certain conditions. For example, coolant performance can be affected if the coolant absorbs moisture, or if subjected to turbulent flow. Coolant performance is also affected by the particular thermal environment in which it is used and the heat load that it is subjected to. 
     Prior art cryogenic devices attempt to control temperature, typically at or near the distal tip of the device, by adjusting the injection pressure and volume of coolant in the tip using combinations of pressure regulators and/or pumps. However, for very small diameter catheters (e.g., 3 Fr to 9 Fr), temperature regulation achieved by precise coolant pressure and/or volume control is difficult, especially if one attempts to correct for coolant sensitivity to ambient humidity, room temperature, and temperature variations of a pumping apparatus and a control console. It would therefore be desirable to provide a cryocatheter with improved temperature control features. 
     SUMMARY OF THE INVENTION 
     The present invention provides a cryocatheter with improved temperature control features. Whereas prior art cryogenic devices adjust device temperature through a reduction or increase in cooling power by control of coolant flow, the present invention provides an optimized coolant flow and adjusts device temperature with a heating element to reach and maintain a desired temperature. 
     In an exemplary embodiment, a cryocatheter includes a catheter body defining a coolant flow path, a catheter tip exposed to the coolant flow path, and a heating element associated with the catheter tip. The heating element can be disposed entirely or partially within the catheter tip. Alternatively, the heating element can be exterior to the catheter tip. The heating element can include an electrically resistive element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 illustrates a cryocatheter in accordance with the invention with the distal tip enlarged to show detail; and 
     FIGS. 2A and 2B are cross-sectional views of exemplary distal tip embodiments. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a cryocatheter in accordance with the invention. The cryocatheter includes a flexible body  10 , as is known in the art, that defines or contains two or more lumens. In the illustrated embodiment, the body  10  defines a first lumen  12  within which a tube  14  (defining a second lumen  16 ) is disposed. The body has a proximal end  18  and a distal end  20 . In FIG. 1, the distal end  20  is enlarged to show additional detail. 
     The distal end  20  includes a tip  22  that seals the body  10  and defines a coolant expansion chamber  24 . The tip  22 , as well as other portions of the body, can be formed from or include a thermally transmisive material, suitable for cooling or heating tissue or for otherwise performing cryotherapy. In the illustrated embodiment, the second lumen  16 , defined by the tube  14 , provides a path for coolant (shown by arrows) to flow from a source (not shown) to the cooling chamber  24 . Coolant exits the cooling chamber  24  through the first lumen  12 , defined by the body  10 . Although the cooling chamber  24  is shown at the distal end  20  of the catheter body  10 , the cooling chamber can also be configured as a “pass-through” structure, such as a continuous or segmented cylinder, disposed at a point between the proximal and the distal end of the catheter body. 
     A steering wire  26 , in communication with a handle unit  28 , is secured to an anchor portion  30 . Applying tension to the steering wire  26  causes the catheter body  10  to deflect. The handle unit  28  provides a connection point for a coolant supply and return umbilical  32 , as well as a connection point for an electrical/sensor umbilical  34 . 
     A heating element  36  is positioned within the distal end  20  at a location where it can heat the tip  22 . A wire  38  connects the heating element  36  to an energy source (not shown). Exemplary heating elements  36  include resistive wires and thin films as are known in the electrical and heating arts. As shown, the heating element  36  is a metal cylinder placed inside the tip  22 . In another embodiment, the heating element  36  is a thin film resistance heater which operates at a power of about 10 to 15 Watts. One or more thermocouples  40  are provided to measure temperature of the tip  22 . 
     FIGS. 2A and 2B illustrate alternative configurations for the heating element  36 . As shown in FIG. 2A, a heating element  36 ′ may be placed on the exterior of the tip  22 , whereas the heating element  36  of FIG. 1 is entirely within the tip  22 . FIG. 2B illustrates a heating element  36 ″ that includes a first portion within the catheter body  10  and a second portion that is external with respect thereto. 
     A complete system includes a control console for controlling coolant flow, monitoring tip temperature, and controlling heater activation. Thus, in operation a coolant injection pressure is set at a fixed value which optimizes the cooling efficiency for the selected catheter dimensions and treatment to be performed, as well as to eliminate turbulent flow, cavitation, and bubble formation to provide a selected tip temperature below a selected therapy temperature. For example, the coolant can be injected to provide a temperature of about −60° C. and the heating element  36  can be activated to raise the temperature of the tip 22 to −55° C. In response to thermal changes in the cryocatheter and the tissue being treated, the heating element  36  is energized and de-energized as required to maintain a consistent, selected temperature. In the illustrated embodiments, the heating element  36  is operative to control the catheter tip&#39;s temperature between minus 80° C. and approximately plus 37° C. 
     A variety of modifications and variations of the present invention are possible in light of the above teachings. Specifically, although the heated tip is shown with respect to a slender and flexible catheter, it is also applicable to other embodiments that are thick and rigid. It is therefore understood that, within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described hereinabove. All references cited herein are expressly incorporated by reference in their entirety.