The benefit of additional active warming in trauma patients deserves further research. Peter Lundgren, Umea University 9 March 2012 Authors: Peter Lundgren, Otto Henriksson, Peter Naredi and Ulf Bjornstig Affiliation: Division of Surgery, Department of Surgery and Perioperative Sciences, Umea University, Sweden. Admission hypothermia is an independent predictor of increased mortality and morbidity in trauma patients and early application of adequate insulation to reduce heat loss and prevent body core cooling is an important part of prehospital trauma care (1). In addition, most guidelines on protection against cold also suggest the application of active external warming, to aid in protection of further cooling during evacuation and transportation to definitive care (2). Previous laboratory studies on mildly hypothermic shivering subjects have found that exogenous skin heating attenuates shivering heat production by an amount equivalent to the heat donated (3-6). Thus, in a mildly hypothermic shivering victim, external warming is not likely to decrease afterdrop or increase rewarming rate, however it might provide other advantages including increased comfort, decreased cardiac work and preserved substrate availability. Accordingly, in this study on lucid trauma patients with a mean initial body core temperature of 35.4 plus or minus 1.0 degree Celsius and preserved shivering capacity, additional active warming using a large chemical heat pad applied to the upper torso provided no advantage over passive warming alone on body core temperature rewarming during prehospital transportation. However, in accordance with the argumentation above, this does not mean that active warming is of no additional benefit to mildly hypothermic conscious trauma patients, as active warming is likely to reduce shivering metabolism, relieving both cardiac and respiratory demands and preserving oxygen availability for vital organs. The statistically significant reduction in heart rate and respiratory rate with additional active warming seen in this study might be an indicator of reduced cold stress, although we do agree on the fact that the clinical relevance of this small reduction might be limited. Another possible beneficial effect from additional active warming is increased thermal comfort. Although body core temperature was increased in both groups, only 2/3 of the patients assigned to passive warming presented a decrease in cold discomfort whereas all patients assigned to additional active warming presented a decrease in cold discomfort during transportation. This beneficial effect on thermal comfort by application of a chemical heat pad to the upper torso is probably explained by a combination of reduction in shivering thermogenesis and increased skin temperature. There is, opposite to what is suggested in the letter, no reliable relation between cold discomfort and body core temperature (7). We agree that there might be a favourably biased effect on comfort from having a non-blinded additional active warming intervention, but since thermal comfort is a subjective measure, a possible favourably biased effect is desirable and a valuable part of the beneficial effect of active warming. The objective of this study was to evaluate the effect of active warming intervention, and we choose the chemical heat pad as one of many possible warming devices. Effective heat transfer capacity is mainly dependent on surface area, heat content and duration (8). As pointed out the risk of burn injuries should be carefully considered and thus the surface temperature in contact with wet and cold skin should not be allowed to rise above 45 degrees Celsius (9). In optimal conditions, the chemical heat pad surface temperature reaches about 50 degrees Celsius within 2 minutes after activation and then gradually declines (8). Keeping a thin layer of clothing between the patient and the heat pad or as in this study placing the heat pad in an ordinary pillow-case effectively prevents the risk of a too high initial heat transfer and no adverse effects or events were reported from the use of the chemical heat pad during the study. Another important factor affecting heat transfer is the thermal conductivity and pressure applied on the skin from the heating device (9). The weight of the chemical heat pad (1.400 g) thus possess an advantage over more lighter chemically or electrically heated thermal blankets or forced-air warming devices. If, in rare cases, this weight restricts breathing of a respiratory compromised patient or prevents assessment of the anterior chest wall, the heat pad can easily be replaced at other areas of high heat transfer capacity such as in the groins, axillae or under the torso. To assure a high protocol consistency we decided to use only one chemical heat pad in this study. However, in cases where an increased thermal transfer is desired, the results from our previous study using a human model for severe hypothermia support the use of an additional heat pad, possibly placed under the upper back (8). We conclude that in mildly hypothermic trauma patients, with preserved shivering capacity, adequate passive warming (blankets alone) is an effective treatment to establish a slow rewarming rate and to reduce cold discomfort during prehospital transportation. However, the addition of active warming using a chemical heat pad applied to the torso will significantly improve thermal comfort even further and might also reduce the cold induced stress response. Although recommended in most guidelines, active warming in prehospital trauma care has only been evaluated in a few previous clinical trials and the results are diverging (10,11). All studies are also relatively small and included patients suffer from not more than mild hypothermia. Thus, active warming in prehospital trauma care deserves further research, especially including more severely injured patients suffering from moderate or severe hypothermia. References 1. Ireland S, Endacott R, Cameron P, Fitzgerald M, Paul E. The incidence and significance of accidental hypothermia in major trauma ¿ A prospective observational study. Resuscitation 2011 March; 82(3): 300-306. 2. Tisherman SA: Hypothermia, cold injury and drowning. In: Peitzman AB (editor). The Trauma Manual. 2. ed. Philadelphia: Lippincott Williams and Wilkins, 2002, pp 404-410. 3. Giesbrecht GG, Bristow GK, Uin A, Ready AE, Jones RA: Effectiveness of three field treatments for induced mild (33.0 degrees Celsius) hypothermia. J Appl Physiol 1987, 63:2375-79. 4. Sterba JA: Efficacy and safety of prehospital rewarming techniques to treat accidental hypothermia. Ann Emerg Med 1991, 20:896-901. 5. Giesbrecht GG, Sessler DI, Mekjavic IB, Schroeder M, Bristow GK: Treatment of mild immersion hypothermia by direct body-to-body contact. J Appl Physiol 1994, 76:2373-9. 6. Vanggaard L, Eyolfson D, Xu X, Weseen G, Giesbrecht GG: Immersion of distal arms and legs in warm water (AVA rewarming) effectively rewarms mildly hypothermic humans. Aviat Space Environ Med 1999, 70:1081-8. 7. Parsons KC. Human thermal physiology and thermoregulation. In: Parsons KC. Human thermal environments: the effects of hot, moderate, and cold environments on human health, comfort and performance. 2. ed. London, UK: Taylor and Francis; 2003, pp 31-70. 8. Lundgren JP, Henriksson O, Pretorius T, Cahill F, Bristow G, Chochinov A, Pretorius A, Bjornstig U, Giesbrecht GG: Field Torso Warming Modalities: A Comparative Study Using a Human Model. Prehosp Emerg Care 2009, 3:371-378 9. Parsons KC. Human skin contact with hot, moderate and cold surfaces. In: Parsons KC. Human thermal environments: the effects of hot, moderate, and cold environments on human health, comfort and performance. 2. ed. London, UK: Taylor and Francis; 2003, pp 350-387. 10. Kober A: Effectiveness of resistive heating compared with passive warming in treating hypothermia associated with minor trauma: a randomized trial. Mayo Clin Proc 2001, 76:369 375. 11. Watts DD: The utility of traditional prehospital interventions in maintaining thermostasis. Prehosp Emerg Care 1999, 3:115-122. Competing interests The authors declare that they have no competing interests.