A catheter-based system for performing a cryoablation procedure uses a
precooler to lower the temperature of a fluid refrigerant to a sub-cool
temperature (-40.degree. C.) at a working pressure (400 psi). The
sub-cooled fluid is then introduced into a supply line of the catheter.
Upon outflow of the primary fluid from the supply line, and into a tip
section of the catheter, the fluid refrigerant boils at an outflow
pressure of approximately one atmosphere, at a temperature of about
-88.degree. C. In operation, the working pressure is computer controlled
to obtain an appropriate outflow pressure for the coldest possible
temperature in the tip section.