Approximately 10 % of metabolic heat production is lost via the respiratory tract, from both the heating and humidification of inspiratory gases. This can be reduced by routine humidification of airway gases, although the overall effect on core temperature is minimal [18].

Administration of 1 l of intravenous (IV) fluid at room temperature (21 °C) or one unit of refrigerated blood decreases the core body temperature by approximately 0.25 °C. Therefore whilst patients cannot be warmed by using fluid warmers (fluid given cannot substantially exceed body temperature), heat loss can be prevented, especially when large amounts of fluid are given. Administration of warmed IV fluid has been shown to decrease the incidence of hypothermia in gynecologic, abdominal, and orthopedic surgery [19]. Their use has been recommended for all intraoperative IV infusions >500 ml in adults [17]. At low flow rates, there are no clinically important differences between any of the available fluid warmers. At higher flow rates the Hotline countercurrent water heat exchanger (Level 1 Technologies Inc, Rockland, MA, USA) consistently delivers the warmest fluid outlet temperatures [19]. During massive transfusion or whenever very high flow rates are needed, high volume systems with powerful heaters are recommended to deliver large amounts of warm fluid quickly (e.g., level 1 infusor, Belmont).

The simplest way to decrease cutaneous heat loss is to apply a cotton blanket, or surgical drape to the skin to trap a layer of still air below the covering and act as a passive insulator. A single layer will reduce heat loss by approximately 30 %, with additional layers only adding marginal benefit [20]. Warming the blanket may increase patient satisfaction but has little benefit, and the benefit is short-lived [20]. Therefore passive warming can help to reduce heat loss but usually insufficient to prevent mild hypothermia.

The most common intraoperative warming systems are forced-air convective warming systems or forced-air warmers, which distribute heated air generated by a power unit through a specially designed blanket. About 90 % of metabolic heat is lost via the skin surface, and forced-air warmers have a dual benefit of almost completely eliminating this loss where they are sited, and transferring heat to the body. Heat transfer per unit area is relatively low, but as long as a large surface area is available for heating forced-air warmers are generally very effective and maintain normothermia even during major procedures [18, 21]. They are superior to passive insulation both in preventing hypothermia and rewarming already hypothermic patients [19]. Forced-air warming is also inexpensive and remarkably safe, and has therefore become the routine method of warming surgical patients. It is preferable to cover most of the exposed area when possible in order to counteract the heat loss coming from large abdominal incisions.