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Hypothermia: A Critical Problem in the Intensive Care Unit

Hypothermia (Le., core temperature < 35°C) occurs commonly. Although the reported mortality rate for hypothermia has been conservatively estimated at 17 deaths per million per year in the United States, most authorities agree that hypothermia is frequently missed and underreported [11. Numerous publications now strongly support that hypothermia is a yearround, common occurrence in Emergency Departments (EDs) and Post-Anesthesia Care Units (PACUs) (2). In the. ED, hypothermia frequently results from exposure, drug or alcohol use, hypoglycemia, or trauma. Even healthy, young individuals exposed to mild, cool weather on a windy, rainy day may easily become hypothermic. Although everyone is at risk for hypothermia, in some series up to 91%of hypothermia in the ED setting is due to drugs or alcohol (3), Up to 42% of patients with moderate trauma experience hypothermia on the first day of hospital admission (4). Many of these patients seen in the ED setting require urgent surgery, and they remain hypothermic on admission to the PACU. Hypothermia frequently develops intraoperatively and postoperatively in patients who are normothermic prior to surgery due to the decreased heat production associated with paralysis and anesthesia, coupled with the increased heat loss from exposed body cavities in cool operating rooms. Temperatures below 35°C develop perioperatively in 13% of patients undergoing elective surgery [5). Although hypothermia in intensive care units (lCU) is less well studied, virtually all the patients seen in EDs or PACUs are eventually cared for in the ICU. Hypothermia may also develop in other ICU patients as a result of sedation, paralysis, hypoglycemia, myxedema, stroke, burns, or other skin disorders [2]. Review of an APACHE II database from my institution indicates that 3.2% of patients in medical and surgical ICUs experience hypothermia within 48 hours of admission. It can be easily argued that busy intensive care physicians in any type ofIeD see at least 1 hypothermic patient per week. The mortality from hypothermia is high. Although most agree that moderate hypothermia of almost any cause if left untreated will be almost universally fatal, the mortality rate for treated hypothermia ranges from 12 to 73% [6,7). Mortality varies with the severity of the underlying disease and the initial body temperature. In mountain climbers suffering exposure, mortality is 25% for temperatures above 32°C and 66% for temperatures below 27°C (8). The overall mortality rate of hypothermic ED patients is only 12%, but it climbs to 50% if serious underlying disease is present (7). The independent contribution of hypothermia to ICU mortality has been recognized in all the APACHE scoring systems. Despite recognition of the prevalence and morbidity of hypothermia in the medical literature, hypothermia remains under-recognized, and treatment regimens for hypothermia remain both understudied and controversial. In a retrospective review of ICU care for hypothermic patients 25% of hypothermia went untreated by the ICU staff, and, when treated, treatment was not initiated for an average of 4 hours after hypothermia was identified [9]. Treatment for hypothermia usually consists of treating the underlying cause of the hypothermia and rewarming. Identifying the underlying cause of hypothermia can usually be achieved by history, physical examination, and a small number of laboratory tests (2). However, there is little consensus as to the best way to rewarm patients because (1) the desired speed of rewarming may vary with the severity of the hypothermia and the underlying cause, (2) most studies that have evaluated rewarming protocols have done so on small groups of poorly characterized patients in a nonrandomized, uncontrolled fashion, and (3) the best technique may vary depending on the point of care-what is best on the mountainside may not be so in a PACU. In this issue, Gentilello and Moujaes provide an elegant thermodynamic description of rewarming. By using fundamental scientific principles regarding heat transfer and by relying on estimates of the heat transfer characteristics of body tissues, they developed a mathematical model of heat flux that permits formation of some generalizations about rewarming. They apply classic thermodynamic approaches traditionally used to discuss temperature regulation in normal individuals [10,11) in such a way as to model rewarming in ill patients. They provide sufficient comparisons to real patient data to support their hypothesis that the model actually characterizes rewarming well. Although the thermodynamic properties of individual patients may vary due to percentage of body fat, size, and cardiac output, the model provides an excellent method for making generalizations and comparisons. The average rates of rewarming for different methods of rewarming are shown to depend on the fundamental differences in the thermodynamics involved with each method.