A Cryogenic Catheter for Treating Heart Arrhythmia

Progress in the development of a cryogenic catheter to treat heart arrhythmia is discussed. This system uses a mixed-gas Joule-Thomson (J-T) refrigerator to cool the tip of a catheter that can be inserted into the body through the large veins leading into the heart. The cryogenic catheter is intended to treat heart arrhythmia characterized by an abnormally rapid heart rate, although the system has a wide variety of other medical applications. Approximately 2 million people in the U.S. suffer from rapid-rate heart arrhythmia. Catheter therapy has proven to be a more effective and less expensive method of treatment than alternatives such as drugs or surgery. A cryogenic catheter has significant advantages over existing catheters used in this form of therapy. The catheter has coaxial tubes for the high and low pressure streams with a miniature heat exchanger and J-T orifice at the catheter tip. The high pressure is maintained at 2.5 MPa. The largest diameter is 3 mm, the length is 90 cm, and all but the last 10–20 mm is flexible. The gas mixture has been optimized for the required operating conditions using nonflammable and low ozone depletion gases. Low cost techniques have been incorporated into the fabrication of the cold tip so that each catheter can be disposable. Several prototype catheters have been built. No-load temperatures down to 85 K were achieved with the cold tip exposed to ambient air. Using room temperature gelatin to simulate tissue heat loads, catheter tip temperatures of 160 to 175 K have been achieved, and ice balls about 26 mm in diameter weighing 11 g were created. We estimate that ice balls about 10 mm in diameter weighing 1.5 to 2.0 g are required to treat ventricular arrhythmia. Although the heat load in our experiments was less than the in vivo load, we think the current refrigeration power is sufficient to meet the clinical requirement.