What’s New in Hypothermia?

Abstract & Commentary

Source: Dixon RG, et al. Transcutaneous pacing in a hypothermic-dog model. Ann Emerg Med 1997;29:602-606.

Hypothermia continues to be a significant problem, affecting countless numbers of individuals annually. A recent report suggests that between 1979 and 1992, a total of 10,550 persons died of hypothermia in the United States, for an average of about 754 deaths per year.1 The majority of current therapeutic approaches to hypothermic patients are based largely on anecdotal evidence and limited experimental and clinical studies. Poorly supported dogma about the dos and don’ts of hypothermia can routinely be found in most general textbooks. Only recently have we learned how to properly interpret the arterial blood gas when determining the adequacy of ventilation,2 and that intubation is both safe and effective.3 A recent article by Dixon and colleagues evaluated the use of external pacemakers in experimental hypothermia. Twenty dogs were cooled to core temperatures of 27° C (79° F). Animals were then assigned to one of two groups: warming blanket plus external pacing or warming blanket plus sham pacing. The paced animals had higher cardiac indices, higher heart rates, and lower diastolic blood pressures than the nonpaced animals. Additionally, the paced group of dogs rewarmed at a significantly faster rate. No animal had an adverse effect associated with pacing.

COMMENT BY ROBERT S. HOFFMAN, MD

Most authorities agree that the hypothermic myocardium is irritable, and techniques such as the placement of pacemakers and long central lines are contraindicated. Furthermore, while most authors agree that rewarming is advantageous, many experts still debate the value of techniques that produce very rapid rewarming rates, such as bypass. Opponents to the routine use of aggressive therapy state that although therapeutic success is seen with patients who rewarm more rapidly, techniques that rewarm faster are not necessarily better.

This article poses an interesting question: How do we rewarm? Clearly, rewarming results from a combination of reduction in heat loss, endogenous heat generation, and absorption of exogenously applied heat. For the last two mechanisms, blood flow is required to move warmed blood throughout the body and bring cold blood to the location of warming. The bradycardia commonly associated with hypothermia is often felt to be somewhat physiologic, resembling the hibernation effects seen in animals. Yet, if we wish to rewarm more rapidly, an increase in cardiac output would seem to be a logical way to increase rewarming rates. What has yet to be determined is the safety of this technique and whether rapid rewarming improves prognosis. I plan to try it in the next patient who is resistant to more conventional rewarming techniques.

References

1. Danzl DF, Pozos RS. Multicenter hypothermia survey. Ann Emerg Med 1987;16:1042-1055.

2. Delaney KA, et al. The assessment of acid-base disturbances in hypothermia and their physiological consequences. Ann Emerg Med 1989;18:72-82.

3. Siciliano L, et al. Hypothermia-related deaths—Vermont, October 1994-February 1996. Morb Mortal Wkly Rep MMWR 1996;45:1093-1095.