Catheter systems and methods for determining blood flow rates based on temperature measurements by thermodilution. The catheter may include a fluid lumen defined between inner and outer tubular members for delivering an indicator fluid, and a guidewire lumen defined by the inner tubular member. The catheter may include fluid infusion openings at the distal end region of the outer tubular member configured to permit the indicator fluid to exit the catheter from the fluid lumen, and a fluid hole located at the distal end region of the inner tubular member configured to permit the indicator fluid to pass from the fluid lumen into the guidewire lumen. A temperature sensor positioned on a guidewire may be positioned within the guidewire lumen to measure the temperature of the indicator fluid entering the guidewire lumen through the fluid hole. A blood flow rate may be calculated based on the measured temperature.
