IN homeothermic species, a thermoregulatory system coordinates defenses against cold and heat to maintain internal body temperature within a narrow range, thus optimizing normal physiologic and metabolic function. The combination of anesthetic-induced thermoregulatory impairment and exposure to a cool environment makes most unwarmed surgical patients hypothermic. Although shivering is but one consequence of perioperative hypothermia, and rarely the most serious, it occurs frequently (i.e., 40–60% after volatile anesthetics), and it remains poorly understood. While coldinduced thermoregulatory shivering remains an obvious etiology, the phenomenon has also been attributed to numerous other causes. Our first goal is to review the organization of the thermoregulatory system, and particularly the physiology of postanesthetic shivering. We then discuss the pharmacology of thermoregulation and review the putative mechanisms and sites of action of various antishivering drugs.

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Perioperative shivering: physiology and pharmacology.

The Reticuloendothelial System

The neurobiological mechanisms of both sleep and circadian regulation have been unraveled partly in the last decades. A network of brain structures, rather than a single locus, is involved in arousal state regulation, whereas the suprachiasmatic nucleus (SCN) has been recognized as a key structure for the regulation of circadian rhythms. Although most models of sleep regulation include a circadian component, the actual mechanism by which the circadian timing system promotes—in addition to homeostatic pressure—transitions between sleep and wakefulness remains to be elucidated. Little more can be stated presently than a probable involvement of neuronal projections and neurohumoral factors originating in the SCN. This paper reviews the relation among body temperature, arousal state, and the circadian timing system and proposes that the circadian temperature rhythm provides an additional signaling pathway for the circadian modulation of sleep and wakefulness. A review of the literature shows that increased brain temperature is associated with a type of neuronal activation typical of sleep in some structures (hypothalamus, basal forebrain), but typical of wakefulness in others (midbrain reticular formation, thalamus). Not only local temperature, but also skin temperature are related to the activation type in these structures. Warming of the skin is associated with an activation type typical of sleep in the midbrain reticular formation, hypothalamus, and cerebral cortex (CC). The decreasing part of the circadian rhythm in core temperature is mainly determined by heat loss from the skin of the extremities, which is associated with strongly increased skin temperature. As such, alterations in core and skin

More than a marker: interaction between the circadian regulation of temperature and sleep, age-related changes, and treatment possibilities

The maintenance of normothermia is an important function Physiology of the autonomic nervous system in homeothermic mammals Thermoregulation is achieved by a physiological control such as man, as cellular and tissue dysfunction become system consisting of peripheral and central thermoreceptors, evident at even minor deviations from normal core body an integrating control centre and efferent response systems temperature. 48 53 113 In man, core temperature is normally which take compensatory action. 113 Afferent thermal input maintained within narrow limits of 36.5–37.5°C, 48 even in comes from anatomically distinct cold and warmth recepthe presence of an adverse environmental temperature, by tors, which may be peripheral or central. 53 The central a combination of behavioural and physiological responses. control mechanism, situated in the hypothalamus, deterAnaesthesia abolishes behavioural mechanisms and has mines mean body temperature by integrating thermal signals the potential to disrupt the physiological mechanisms of from peripheral and core structures, and comparing mean thermoregulation. Adverse postoperative outcomes, includ- body temperature with a pre-determined ‘set-point’ ing wound infection, increased surgical bleeding and morbid temperature. 48 In man, the efferent response to effect change in body heat content as required is by behavioural and cardiac events are being associated with mild perioperative autonomic means. The latter involves control of cutaneous hypothermia (33.0–36.4°C). 21 22 26 42–44 52 74 80 84 85 90 109

J. D. Hardy

Papers

Unit responses in the rabbit's brain stem to changes in brain and cutaneous temperature