A catheter system for determining blood flow in a body lumen, the system comprising: a catheter, having a catheter shaft defining a proximal end and a distal end, including: an outer tubular member closed at the distal end of the catheter; an inner tubular member disposed within the outer tubular member, said inner tubular member being opened at the distal end of the catheter; a first, fluid lumen defined between the inner tubular member and the outer tubular member configured to receive a fluid to be infused into the body lumen; a second lumen defined by the inner tubular member being open at the distal end, and an advanceable elongate member, the second lumen having a size adapted to receive the advanceable elongate member; one or more fluid infusion openings located at a distal end region of the catheter, the one or more fluid infusion openings configured to permit fluid to exit the catheter from the first fluid lumen; and one or more fluid holes located at the distal end region of the catheter, the one or more fluid holes configured to permit fluid to pass from the first fluid lumen into the second lumen and the elongate member including a temperature sensor positioned on a distal end portion of the elongate member, the temperature sensor positionable within the second lumen of the catheter to measure a temperature of fluid entering the second lumen of the catheter through the one or more fluid holes. a catheter, having a catheter shaft defining a proximal end and a distal end, including: an outer tubular member closed at the distal end of the catheter; an inner tubular member disposed within the outer tubular member, said inner tubular member being opened at the distal end of the catheter; a first, fluid lumen defined between the inner tubular member and the outer tubular member configured to receive a fluid to be infused into the body lumen; a second lumen defined by the inner tubular member being open at the distal end, and an advanceable elongate member, the second lumen having a size adapted to receive the advanceable elongate member; one or more fluid infusion openings located at a distal end region of the catheter, the one or more fluid infusion openings configured to permit fluid to exit the catheter from the first fluid lumen; and one or more fluid holes located at the distal end region of the catheter, the one or more fluid holes configured to permit fluid to pass from the first fluid lumen into the second lumen and an outer tubular member closed at the distal end of the catheter; an inner tubular member disposed within the outer tubular member, said inner tubular member being opened at the distal end of the catheter; a first, fluid lumen defined between the inner tubular member and the outer tubular member configured to receive a fluid to be infused into the body lumen; a second lumen defined by the inner tubular member being open at the distal end, and an advanceable elongate member, the second lumen having a size adapted to receive the advanceable elongate member; one or more fluid infusion openings located at a distal end region of the catheter, the one or more fluid infusion openings configured to permit fluid to exit the catheter from the first fluid lumen; and one or more fluid holes located at the distal end region of the catheter, the one or more fluid holes configured to permit fluid to pass from the first fluid lumen into the second lumen and the elongate member including a temperature sensor positioned on a distal end portion of the elongate member, the temperature sensor positionable within the second lumen of the catheter to measure a temperature of fluid entering the second lumen of the catheter through the one or more fluid holes.