Pathophysiology and epidemiology of accidental hypothermia

Despite strong evidence that unintended hypothermia is detrimental to patient outcome and effective patient warming systems being widely available accidental hypothermia is still a common phenomenon in many patients. A commonly used definition for perioperative hypothermia [1] distinguishes between mild hypothermia (32 to 36 °C), moderate hypothermi a (28 to 32 °C) and deep hypothermia (<28 °C) but there is unfortunately no consistent definition for the different stages of hypothermia. Because humans belong to the homoeothermic species, under normal conditions the core temperature will be kept constant at approximately 37 °C by a closed-loop system – the thermoregulatory system. A typical thermoregulatory response can be characterized by a threshold, a gain, which controls the degree of a response, and the maximum response intensity [2]. The thermoregulatory system is normally able to keep the core temperature constant within 0.2 °C of a targeted value. A typical operating room (OR) temperature will be kept at < 23 °C [3]. This causes a body heat loss so that the typical preoperative patient may have a (still) normal core temperature but the periphery is already cold and vasoconstricted. During surgery exposure of the surgical site to the relatively cold OR environment (evaporation) in addition to other hypothermia promoting effects like infusion of cold fluids contributes further to a significant heat loss. Now autonomic defences alone are available to maintain normothermia. But both, general and regional anaesthesia impair central and peripheral thermoregulatory control. Perioperative hypothermia is an accidental but relatively frequent complication of surgical procedures. Without perioperative warming the majority of patients will become at least slightly hypothermic (core temperature < 36 °C) [4]. Albelha et al. [5] reported an incidence of hypothermia (cut off value < 35 °C) of 60 percent at admission to the intensive care unit (ICU) in non-cardiac surgical patients. [1] Sessler DI. Complications and treatment of mild hypothermia. Anesthesiology 2001; 95: 531-543. [2] Sessler DI. Mild perioperative hypothermia. N Engl J Med 1997; 336: 1730-1737. [3] Morris RH. Operating room temperature and the anesthetized, paralyzed patient. Arch Surg 1971; 102: 95-97. [4] Bennett J, Ramachandra V, Webster J, Carli F. Prevention of hypothermia during hip surgery: effect of passive compared with active skin surface warming. Br J Anaesth 1994; 73: 180-183. [5] Abelha FJ, Castro MA, Neves AM, Landeiro NM, Santos CC. Hypothermia in a surgical intensive care unit. BMC Anesthesiol. 2005; 5: 7 Biomed Tech 2012; 57 (Suppl. 1) © 2012 by Walter de Gruyter · Berlin · Boston. DOI 10.1515/bmt-2012-4228