Thermal control and design considerations for a high-performance fluid warmer

The process of warming liquids for intravenous infusion presents several technical challenges for the engineer: Typical liquid inlet temperatures can range from 5 degrees C to 20 degrees C, flow requirements can vary from essentially zero ("Keep Vein Open," or K.V.O.) to 30 L/h, and desired outlet temperature is fixed at a maximum threshold of 41 degrees C to minimize the risk of thermally mediated hemolysis. The primary challenge is developing a control technique that can tailor the energy introduced to the liquid in response to a randomly variable flow in order to achieve, but not exceed, a fixed temperature. The most difficult aspect of this challenge is preventing the transient infusate temperature from exceeding 43 degrees C, even when the power requirement varies by orders of magnitude, such as occurs when the flow suddenly decreases from maximum to zero. Many current-generation fluid warmers are optimized for operation at either low or high flow rates; we believed that it was possible to design an easy-to-use device that could achieve good performance across the entire range of flow rates. This article describes some of the methods that were used successfully to meet these challenges in the design of the Augustine Medical Ranger blood fluid warmer.