PROBLEM TO BE SOLVED: To provide a system for increasing the temperature of a fluid being infused into a patient's body while infusion is taking place, and also to provide improved monitoring of air in the infusion system. SOLUTION: A disposable fluid infusion cartridge 100 comprises a heat exchanger 101 having upper and lower aspects and an internal heat exchange zone defined by a first and second plurality of overlapping fins and forming a substantially uniform flow path depth. Each fin has a ratio of height to width of at least 1:2, whereby fluid enters the lower aspect of the heat exchanger via a lower port and fills a lower flow cavity across the width of the heat exchange zone prior to flowing through the heat exchange zone and out an upper port at the upper aspect of the heat exchanger. The cartridge further has an air-trap 110 having upper and lower aspects and an inner surface for receiving the fluid from the heat exchanger, and further comprises an fluid flow disrupter and a purging mechanism for purging air from the air-trap and preventing air from passing beyond the air-trap. COPYRIGHT: (C)2010,JPO&INPIT