A heater cooler apparatus 1 for a perfusion system heat-exchanger 26 comprises a heater cooler device 10 and an antimicrobial fluid. In a method of controlling thermal transfer in the heat-exchanger 26, the heat-exchanger 26 is provided with both a first fluid passage 32, 34 for a heat transfer medium and a second fluid passage 22 for the extracorporeal patient fluid. In such an arrangement the heat transfer medium comprises a component with anti-microbial properties, such as glycol. Additionally, or alternatively, the heater-cooler 10 may be provided with an intermediate heat exchanger 40 between a heater-cooler device 10 and an intermediate passage 30； the heater-cooler 10 supplying heater-cooler fluid 11 to the intermediate heat exchanger 40, and the intermediate passage 30 supplying an intermediate fluid 31 to the heat exchanger 26. The intermediate heat exchanger 40 thus permits heat exchange between the heater-cooler fluid 11 and the intermediate fluid 31. The heater-cooler fluid 11 is thus fluidically isolated from the perfusion system heat exchanger 26, allowing for an antiseptic fluid to be used in the heater-cooler device 10, which in turn reduces the risk of cross-contamination originating from the heater-cooler device 10.
