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Description / Table of Contents: Abstract Heat loses during surgery occur mainly to the environment and due to infusions and irrigations. Infusions given at room temperature account for a great deal of the total heat deficit during major operations, e.g., the infusion of 53 ml/kg 20° C fluid leads to a loss of 1° C in mean body temperature. Hence, heating i.v. fluids will add to the effect of other measures aimed at reducing heat loss to the environment. We investigated the efficacy of different warming methods for i.v. fluids in an experimental model by measuring the temperature at the end of the delivery line. Methods. The following in-line warmers were studied: Hotline HL-90 and System H-250/heat exchanger D-50 (Level 1 Technologies, Marshfield, USA), Astotherm IFT 260 (Stihler Elektronic GmbH, Stuttgart, Germany), RSLB 30 H Gamida (Productions Hospitalieres Francaises, Eaubonne, France), Bair Hugger 241/Modell 500 Prototype (Augustine Medical, Eden Prairie, USA). They were compared with prewarming infusions (39° C) only using the Clinitherm S (Labor Technik Barkey GmbH, Bielefeld, Germany) and prewarming with “active insulation” of the delivery line using the Autotherm/Autoline system (Labor Technik Barkey GmbH, Bielefeld, Germany). We investigated the influence of four variables on the efficacy of warming: (1) flow rate (50–15,000 ml/h); (2) ambient temperature (20° C and 25° C); (3) infusion bag temperature (6° C, 20° C, and 39° C); and (4) length of infusion system downstream from the heat exchanger. Fluid temperatures were measured using thermistors of 1 mm diameter (Modell YSI 520, Yellow Springs Instruments Co., Yellow Springs, USA) incorporated into 3-way stopcocks. Temperatures were recorded using Hellige temperature monitors (Hellige GmbH, Freiburg im Breisgau, Germany) and the signals were collected at 10 Hz through an AD converter and averaged over 1 min. Flows were calculated by timed collection into calibrated cylinders; 10 to 12 different flow rates were taken to define one temperature/flow plot. Effective warming was defined as a temperature 〉33° C at the end of the infusion line. Results. At high flow rates (〉2,500 ml/h) using 20° C fluids at 20° C ambient temperature, the H-250/D-50 system gave the highest temperatures throughout the range and showed effective warming from 1,300 ml/h on over the entire range tested (35° C at 17,000 ml/h) compared to the RSLB 30 H Gamida system (3,000–18,000 ml/h) (Fig. 2). This difference in performance was almost abolished with fluids at 6° C (Fig. 4). Similar efficacy could be reached by using prewarmed infusions that gave effective warming at 〉2,000 ml/h and reached 39° C at 13,000 ml/h. Prewarmed infusions could be used effectively down to 〉80 ml/h applying “active insulation” (Autotherm/Autoline) to the whole infusion system. The Hotline HL-90 (50–4,700 ml/h) appeared to be the most effective in-line warmer in the low (〈250 ml/h) and middle (250–2,500 ml/h) flow range, followed by the Astotherm IFT 260 (400–4,000 ml/h), but only if used with a length of 40 cm down-stream from the heat exchanger (Fig. 1). Increasing this distance to 145 cm markedly reduced its efficacy below the range of 2,000 ml/min (1,200–3,000 ml/h) (Fig. 5). The Bair Hugger 241 Prototype showed a narrow effective range (700–1,300 ml/h) that could be extended beyond 1,300 ml/h by the use of prewarmed infusions (Figs. 1 and 3). The performance for 6° C solutions and ambient temperatures of 25° C are given in Fig. 4 and Table 1. Conclusions. The importance of infusion warming increases with the amount of fluid given. In general, the infusion bag temperature only influenced the efficacy of in-line warmers within the high-flow range, challenging the performance of the heat exchanger. The length of uninsulated i.v. line downstream from the heat exchanger influenced the efficacy within the low- and middle-flow range, as did the room temperature. Prewarmed solutions can be infused very effectively within the high-flow range. This efficiency can be preserved down to the low-flow range by using “active insulation” of the infusion system. In-line warming is essential for emergency and rapid massive transfusions.