Induction of Mild Therapeutic Hypothermia

Rapid core cooling may be required when the target core temperature is low (e.g., 32°C or 33°C) or for stroke or acute myocardial infarction when cooling must be induced expeditiously. In such cases, passive cooling is not practical. Immersion in cold water is by far the quickest noninvasive method of actively cooling patients. However, immersion is difficult under clinical conditions and poses a substantial electrical safety risk. Administration of refrigerated intravenous fluids is also effective and reduces mean body temperature 0.5°C/L.^[183] However, this method is usually impractical in neurosurgical patients, in whom fluids must be restricted.

Forced-air cooling is easy to implement, but relatively slow, with approximately 2.5 hours needed to cool neurosurgical patients to 33°C.^[152] Conventional circulating-water mattresses are unlikely to be efficient because relatively little skin surface contacts the mattress and the patient's own body weight reduces blood-borne convection of heat to the back. That is, circulating water probably does not cool well for the same reason that it heats poorly.^[139] Newer circulating-water systems include garment-like covers that cover far more skin surface and transfer large amounts of heat, and they are fairly effective.^{[177] [184]}

The best way to rapidly induce therapeutic hypothermia is probably endovascular cooling. These systems consist of a heat-exchanging catheter, usually inserted

Figure 40-22 Relative effects of warming methods on mean body temperature (ΔMBT) as a function of time (upper portion) or administered fluid (lower portion). Mean body temperature is the average temperature of body tissues and is usually somewhat less than core temperature. The calculations assume an undressed 70-kg patient with a metabolic rate of 80 kcal/hr, in thermal steady state, and with a typical 21°C operating room environment. Only the changes resulting from specific treatments are shown; changes from combined interventions will be additive. a to d, Changes in MBT per liter of administered blood or crystalloid at various fluid temperatures; e, inspiring warmed, humidified gas; f and g, warmed or unwarmed blankets, with all skin below the neck covered; savings are similar with a single layer of other passive insulators; h, full-length circulating water mattress; i, full-length forced-air warmer. (Redrawn from Sessler AM, Sessler DI: Consequences and treatment of perioperative hypothermia. Anesthiol Clin North Am 12:425–456, 1994. See the original figure legend for equations and citations justifying the assumptions.)

Inducing therapeutic hypothermia during surgery is relatively easy because anesthetics profoundly impair thermoregulatory responses. In contrast, unanesthetized patients—even those who have suffered a stroke—vigorously defend core temperature by vasoconstricting and shivering.^[186] It is thus necessary to pharmacologically induce tolerance to hypothermia. The best method thus far identified is the combination of buspirone and meperidine, drugs that synergistically reduce the shivering threshold to approximately 34°C without provoking excessive sedation or respiratory toxicity.^[187] The combination of dexmedetomidine and meperidine may also be helpful, although the interaction is simply additive in this case.

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Figure 40-23 Average (± SD) intraoperative esophageal temperatures (T_esoph ) during the cooling, temperature maintenance, and rewarming periods in eight neurosurgical patients who were cooled with endovascular heat-exchanging catheters in the vena cava. Time zero identified the beginning of each thermal management period; the duration of these periods differed in individual patients, depending on the length of surgery and other factors. The regression lines (gray lines) are shown for the cooling and rewarming periods. (Redrawn from Doufas AG, Akça O, Barry A, et al: Initial experience with a novel heat-exchanging catheter in neurosurgical patients. Anesth Analg 95:1752–1756, 2002.)