“Nobody is dead until warm and dead”

Although hypothermia is a rather rare phenomenon, mortality is still relatively high today. Rapid recognition of hypothermia and adequate treatment make the difference between life and death.

Severe hypothermia still has a high mortality today. Although considerable progress has been made in the care of severely hypothermic victims, both prehospital and in the hospital, exact incidence figures are lacking; they are estimated at 1-5 per 100,000 population, depending on the region. In addition to the “classic” risk groups such as mountaineers, adventurers, army or navy personnel, or workers in polar regions, “normal” people who suffer an accident, lie unconscious or helpless in unheated dwellings, or intoxicated persons can also suffer severe hypothermia (Fig. 1).

Pathophysiology of hypothermia

The human body is homoiothermic and tries to keep its core temperature stable around 37°C under all circumstances. To prevent cooling, the body increases its internal heat production primarily by means of increased muscle activity. However, if the ambient temperature is too low and/or the body is no longer able to produce enough heat, the core body temperature (KKT) drops steadily. The resulting clinical symptoms were first described by the Swiss physician and mountain rescuer Bruno Durrer, MD, and Hermann Brugger, MD, the South Tyrolean physician and researcher in mountain emergency medicine, in the so-called “Swiss Staging” [1]. Today, this is the basis for the classification of hypothermia. A second classification is made based on the measured KKT. There, mild hypothermia (KKT 35-32°C) is distinguished from moderate (KKT 32-28°C) and severe hypothermia (KKT <28°C) [2].

Swiss staging

Table 1 illustrates the classification of hypothermia into the four stages according to Durrer and Brugger (“Swiss Staging”). In stage 1, mild hypothermia (typically 35-32°C) is present, characterized by cold shivering with which the body attempts to increase KKT by means of muscle activation. These patients can be treated with isolation measures, blankets, and hot drinks and generally do not require further care.

Stage 2 denotes moderately severe hypothermia (approximately 32-28°C). This stage is characterized by impaired consciousness and possibly already first cardiac symptoms. These patients must be protected from further cooling and should usually be hospitalized until their CTC returns to normal.

In stage 3 (usually <28°C), vital functions are severely impaired. Patients are often unconscious and show unstable circulation and impaired breathing. Further cooling leads to circulatory arrest and thus to stage 4.

Stage 4 is also described as suspended animation. These patients suffer circulatory arrest and also have dilated pupils that are fixed in light. They should be transported as soon as possible to a hospital with ECLS (“Extracorporeal Life Support”) capabilities, where external active rewarming can be performed using ECMO (Extracorporeal Membrane Oxygenation) or conventional heart-lung machine. Only rewarming can reactivate the circulation and improve coagulation, which has been massively disturbed by hypothermia. Severely hypothermic trauma patients have a worse outcome than normothermic trauma victims primarily for this reason [3].

Guidelines and algorithms

In recent years, several algorithms and guidelines for the care of hypothermic patients have been published [2,4]. They are designed to help both rescuers directly at the scene and clinically active teams to recognize hypothermic patients, define the severity of hypothermia, and initiate the correct treatment. Figure 2 presents a simplified algorithm for decision-making and treatment of hypothermic patients in cardiac arrest.

Hospital management

Treatment and rewarming: Mildly hypothermic patients (stage 1) need only warm blankets and drinks and muscle activity and can be managed as outpatients. In moderate hypothermia (stage 2), sometimes even stage 3, cardiovascular and respiratory functions may still be adequate. If this is the case, patients can be warmed up non-invasively with warm cloths or special convective warming blankets (e.g. Bair Hugger®) and warm infusions. However, they must be closely monitored. If hypothermia-related circulatory instability is present, the indication for active invasive rewarming should be generous. Patients with circulatory arrest (stage 4) should be rewarmed using ECLS. Two different systems are used: extracorporeal membrane oxygenation (ECMO) or heart-lung machine (HLM) (Fig.3). A team from Innsbruck was able to show that the two systems do not differ significantly with regard to a ROSC (“return of spontaneous circulation”), but that ECMO, on the other hand, enables better survival; this is probably due to the fact that ECMO enables longer-term cardiopulmonary support [5]. The “Bernese Hypothermia Algorithm” is intended to help correctly determine the indication for these complex techniques [6].

Survival and rehabilitation: Heart-lung machine (HLM) for rewarming patients with alpine hypothermia-induced circulatory arrest was first performed in the 1980s by Prof. Ueli Althaus, MD, at the Inselspital Bern [7]. This novel treatment, which gave apparent deaths a chance of survival, quickly spread among rescue teams, emergency departments and cardiac surgery teams. A Swiss multicenter study led by Walpoth and Althaus examined mostly young patients five years after HLM treatment [8]. Before treatment, all patients had postoperative complications such as pulmonary edema, neurological deficits, or even long intensive care stays. Many of them needed rehabilitation for their more passive disabilities. Of a total of 32 patients included, who had a mean CST of 21.9°C, 15 survived. After five years, the survivors were healthy, showed no late sequelae, required no medical treatment, and had resumed normal activities. These very promising results were not only central for the patients and the treatment teams, they also showed that the method was usable despite initial complications. Today, active-invasive rewarming of severely hypothermic patients with circulatory arrest is the gold standard worldwide.

Forecast

The outcome of hypothermic patients depends on several factors. Vital signs at initial assessment are the most important positive prognostic factor [9]. This is understood to include preserved, although possibly severely slowed, breathing, functioning circulation, or some cardiac activity. The more disturbed the vital signs, the greater the risk of secondary cooling with subsequent circulatory arrest, e.g. during recovery of a hypothermic victim. This phenomenon is known as “salvage death” and is likely triggered by a shift of cold blood from the periphery to the core, causing KKT to drop acutely (=stage 4). The prognosis is favorable if the patient can be rapidly transported to an ECLS center and actively rewarmed invasively [10]. The primary goal in prehospital care is to prevent a hypothermic patient from cooling further.

Hypothermic patients who experience concomitant asphyxia have the worst prognosis. This often occurs in avalanche accidents or drowning: The victim suffers asphyxia while KKT is still normal and only cools down in the course [11,12]. At normothermia, the brain has an ischemia tolerance of only three minutes, whereas at a KKT of 20°C, the brain can withstand about thirty minutes of hypoxia without damage. This protective effect of hypothermia comes from a slowing of all metabolic processes [13].

Currently, potassium measurement as an expression of cellular damage is the standard to estimate the extent of hypoxia [14]. If the value in a severely hypothermic victim with circulatory arrest is >12 mmol/l or in an avalanche victim >8 mmol/l, it must be assumed that the asphyxia occurred before cooling and the person suffocated [4]. The international guidelines therefore recommend that resuscitation can be stopped at a potassium value >12 or 8 mmol/l [15]. Pasquier et al. have devised the “HOPE score”, which takes into account factors other than potassium (age, sex, asphyxia, etc.) and should thus provide a better indication of the prognosis of severely hypothermic patients with circulatory arrest (stage 4) [16]. A validation of this score is currently underway.

Innovation and future

A Danish publication on a boating accident only a few years ago showed comparatively high  long-term survival rates in adolescents who capsized in freezing water and were all successfully rewarmed using ECLS [17]. The incredible case of a massively hypothermic patient is described in the box; it reinforces the maxim “Nobody is dead until warm and dead”. Despite many positive examples, however, much remains to be done: We need better evidence by collecting and analyzing such individual cases. We are particularly interested in the causes of accidents, the means of rescue and transport used and, of course, the rewarming methods and post-interventional treatment. Acute hypothermia is a “low incidence – high impact” situation: There are few cases per year worldwide, but correct recognition of hypothermia and “state of the art” treatment make the difference between life and death. Therefore, the International Hypothermia Registry (IHR) was developed at the University Hospital of Geneva. Currently, over 50 centers worldwide support the registry, and we hope that by collecting as many cases of accidental hypothermia as possible, more scientific evidence can be obtained and thus improve the outcome of these victims [18].

Take-Home Messages

• Severe hypothermia still has a high mortality today.
• The primary goal in prehospital care is to prevent a hypothermic patient from cooling further.
• Hypothermic patients who experience concomitant asphyxia have the worst prognosis. This often occurs in avalanche or drowning accidents.
• Deeply hypothermic patients in circulatory arrest should be immediately transferred under resuscitation to a center where they can be rewarmed using ECLS. As a general rule, “Nobody is dead until warm and dead.”

Literature:

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CARDIOVASC 2018; 17(6): 27-32