What's new in emergencies, trauma, and shock: Intentional or accidental hypothermia in Intensive Care Unit patients: Time to strike the colors?

For centuries, scientists have been intrigued by the concept of hibernation as a means to preserve bodily functions. Over the years, intentional lowering of core temperature has evolved as the strongest neuroprotective intervention in clinical medicine. Hypothermia basically acts by blunting the excess oxidative stress, glutamate release, calcium influx, and cytotoxic edema that accompany neuronal injury.[1] As such, targeting temperatures as low as 32°C has been promoted as a part of standard medical care in traumatic brain injury, neonatal hypoxic-ischemic encephalopathy, and in patients remaining comatose after resuscitation from cardiac arrest.[2] However, unconditional belief in the benefit of hypothermia started to falter as the results of large randomized trials comparing induced hypothermia with “standard” temperature or fever control (i.e. targeting temperatures of 36–37°C) failed to show improved outcome in all of the aforementioned populations.[345] Moreover, some of these studies were prematurely suspended owing to safety reasons.[45]

In this context, the study of Balvers et al. published in this Journal’s issue is interesting. The authors describe that unintentional hypothermia, defined as a core temperature ≤ 35°C, is common in trauma patients upon admission in the Intensive Care Unit (ICU). Hypothermia was identified as the most important independent physiological predictor of both early and late mortality. A late worse outcome was also determined by the evidence of coagulopathy or presence of brain injury.[6] In contrast with intentional hypothermia, its accidental counterpart is inherently more complex but also to some extent “induced”! In the prehospital setting, the body may quickly lose heat after prolonged exposure to a cooler ambient temperature. Alcohol and certain medications can impair the body’s ability to control heat loss. Weeping skin or burn wounds may provoke cooling of the body as well. If the injured patient is in shock, hypoperfusion will add to failing temperature regulation. In the hospital, resuscitation manoeuvres enhance the problem by infusing large amounts of cold isotonic solutions or insufficiently warmed up blood. Finally, prompt “injury-to-incision” decisions result in rapid transport of the patient to a “chilly” operation room.

The study of Balvers et al. once more underpins that trauma-associated hypothermia is far from being a physiological adaptation mechanism to protect the body from ongoing harmful stress but rather a life-threatening exponent of unanticipated exposure in an inadequately prepared person. Hypothermia, acidosis, and coagulopathy indeed have been repeatedly recognized as a “lethal triad” propelling trauma patients into a deadly vicious cycle.[7] Based on the findings of Balvers et al., one may wonder whether the outcome of trauma patients with hypothermia on ICU admission could be beneficially influenced by active rewarming. Unfortunately, clinical studies looking specifically at the effect of aggressive rewarming in accidental hypothermia on ICU mortality are scarce and of low methodological quality. One relevant recent study found no difference in the neurological outcome and mortality in patients with severe traumatic brain injury randomized to either prolonged therapeutic hypothermia with slow rewarming or to temperatures kept between 35.5°C and 37°C.[8] Moreover, attempting to rewarm a hypothermic trauma is potentially deleterious as vasodilation and mobilization of pooled peripheral blood may flood the tissues with waste and acidosis.

Taken together, we must humbly adapt our thinking that a better outcome is guaranteed when arousing humans from intentional hypothermia. In addition, accidental hypothermia definitely is a “bad sign” in trauma victims and probably requires early but gentle correction and at the least, close monitoring within the “golden hour (s)” of acute trauma care. Future research in this population should focus on prehospital temperature preservation, optimal time to initiate rewarming (immediately after arrival in the emergency ward or upon ICU arrival), rewarming speed (slow or aggressive), and most adequate rewarming techniques (intravenous or external).