Prehospital Use of Forced Air Warming

Accidental hypothermia is defined as an involuntary drop in core body temperature below 35°C.

Hypothermia is a risk factor for cardiac arrythmia, pulmonary oedema, coagulopathy and neurological pathology. Isolated accidental hypothermia is potentially lethal on its own, and it is also an independent risk factor for increased mortality in patients with traumatic injury.

When core temperature drops, the body will automatically activate compensatory mechanisms in order to both increase thermic production and minimize heat loss in order to restore normal temperature. The mechanisms for thermal homeostasis are complex, strictly regulated and effective.

Until quite recently, active external rewarming was considered to be harmful and dangerous for patients with accidental hypothermia. A fear of fatal complications such as increased afterdrop and "rearming syndrome" leading to dangerous arrythmias and a drop in blood pressure which in a worstcase scenario could lead to circulatory collapse, shock and potentially death.

Given the potentially detrimental consequences of being cold, any advice to abstain from warming patients should be well-documented, and the adverse effects of warming should outweigh those of being hypothermic. The lack of evidence for this claim that active external rewarming is dangerous along with many reports of successful external rewarming is leading to a change in most guidelines where active external rewarming is now an advisable treatment option. A variety of different technological devices exist for the purpose of rewarming, but there is no established standard which is proven superior to others. Several trials have compared different rewarming methods, but evidence is limited. There is particularly deficient evidence regarding the amount of heat available for transfer from different methods and also the required amount of heat transfer for the rewarming to have a significant clinical impact. There is however reason to be cautious with active external rewarming of hypothermic patients. In a normothermic patient with normal circulatory function, thermal energy will be absorbed by the blood which flows back into central circulation distributing the heat evenly into a large volume. In a hypothermic patient there will be less cutaneous blood flow and less heat will be transferred to systemic circulation. This means that heat will accumulate on the skin surface and increase the risk of cutaneous injury, and there are several reported cases of this. Hypothermia research with volunteer research participants is a complicated matter for many reasons.

One complicating factor is shivering, which is an autonomic defense mechanism in which the muscles contract rapidly producing heat. It is the most effective method for the body to increase the metabolic heat production and re-establish normothermia. The shivering response is complicated in a research setting because there is a large interindividual variation in temperature threshold for initiation of shivering and in shivering intensity. This means that if research participants are cooled down they will start to shiver at different core temperatures and with different intensities.

Standardization of data is therefore impossible, and the researcher will be measuring the interindividual shivering abilities of the different research participants instead of the effect of the intervention. Shivering must therefore be inhibited in order to achieve reliable data.

Inhibition of shivering for research has been done several times before. The most common method to achieve this is through intravenous administration of a drug called Meperidine which is a synthetic opioid with good shivering inhibition attributes. A protocol for inhibition of shivering in patients with simulated accidental hypothermia has been developed by Helland et. al., which we intend to use in this experiment. Given the results of our previous studies (REK 714469 and REK 855256, results not yet published) where we have documented limited effects of traditional treatment options in non-shivering participants, we believe that alternative strategies must be investigated. Forced Air Warming (FAW) is a commonly used method for rewarming from hypothermia in an in-hospital setting, but it has limited evidence for prehospital use. FAW enables heat transfer to a much larger surface area compared to, for example, an electric heating blanket, which may contribute to significantly higher rewarming rates.