A guidable catheter for heating or cooling a surrounding fluid in a feeding vessel in a vasculature of a patient. The catheter includes a heat transfer element, the heat transfer element having a plurality of exterior surface irregularities shaped and arranged to create turbulence in a surrounding fluid. The surface irregularities have a depth at least equal to the boundary layer thickness of flow of the surrounding fluid in the feeding vessel. The catheter assembly also includes a supply catheter having a portion disposed within the heat transfer element to deliver a working fluid to an interior of the heat transfer element. The catheter assembly further includes a return catheter to return a working fluid from the interior of the heat transfer element. A guidewire tube is provided adjacent one of the supply catheter or the return catheter and runs substantially parallel to the axis of the guidable catheter to receive a guidewire disposed within the guidewire tube. The guidable catheter may further have a temperature-monitoring device disposed at the distal tip of the guidewire. The temperature-monitoring device may be a thermocouple or a thermistor. Feedback may further be provided to control the temperature of a source of working fluid. A method is also provided for selectively controlling the temperature of a selected volume of blood in a patient. The method includes introducing a guidewire into a blood vessel feeding a selected volume of blood in a patient and introducing a catheter assembly into the blood vessel feeding a selected volume of blood in a patient by inserting the guidewire into a guidewire tube in the catheter assembly. A working fluid is delivered from a source of working fluid through a supply catheter in the catheter assembly and returned through a return catheter in the catheter assembly. Heat is transferred between a heat transfer element forming a distal end of the catheter assembly and the volume of blood in the feeding vessel. The temperature is monitored of the volume of blood in the feeding vessel by measuring the temperature with a temperature-monitoring device disposed at or near the distal tip of the guidewire.