James C. Eisenach, M.D., Editor
Poikilothermia in Man: Pathophysiological. Aspects and Clinical Implications. By M. A. MacKenzie. Uitgeverij KU Nijmegen, Nifmegen University Press, 1996. Pages: 192. Price:$39.00.
Poikilothermia in Man is a doctoral thesis on poikilothermia syndrome and summarizes Dr. MacKenzie's decade-long interest in the topic.
The first section was written in conjunction with Professor Schonbaum, a respected authority on thermoregulation. Not surprisingly, it is an excellent review of thermoregulatory physiology with an emphasis on inadvertent thermal perturbations. Included are several comprehensive tables listing the etiologies of various disturbances. The section generally is well referenced, although there are occasional citations to a discredited treatment for accidental hypothermia: inhalation of heated oxygen.
I was struck, however, by the author's definition of hypothermia as being less than 35 [degree sign] Celsius. A core temperature near 35 [degree sign] Celsius is never normal; for that matter, core temperature does not normally reach even 36 [degree sign] Celsius without triggering behavioral and autonomic cold defenses. Further, increasing evidence indicates that core temperatures only [nearly =] 1 [degree sign] Celsius below normal (i.e., near 35 [degree sign] Celsius) provide significant benefits including protection against cerebral ischemia. These same temperatures trigger complications, including morbid myocardial events. One might, thus, make the case that a more physiologic and useful definition of hypothermia would be 36 [degree sign] Celsius.
The authors' definition of hyperthermia, in contrast, is perhaps excessively constrained:“one standard deviation above the mean core temperature under resting conditions in a thermoneutral environment.” Not only does this definition fail to account for the normal 1 [degree sign] Celsius circadian variation, but it implies that 16% of the population is routinely hyperthermic.
These definitions of hypothermia and hyperthermia, one too generous and the other excessively restrictive, simply reflect long-standing confusion in the thermoregulatory literature. This problem is compounded by numerous papers propagating different-but equally arbitrary-definitions of “mild,”“moderate,” and “severe” hypothermia. I would propose that the time has come to define thermal perturbations either physiologically (temperatures that trigger various defenses) or in terms of the adverse or beneficial consequences. Sufficient information now is available to apply either type of definition in most situations.
Despite this book's title, the second section consists of a detailed evaluation of four women with acquired poikilothermia syndrome. So why should anesthesiologists be interested in a syndrome so rare that only a dozen or so cases are reported? Because acquired poikilothermia resembles the thermoregulatory impairment produced by general anesthesia.
Acquired poikilothermia and general anesthesia increase the sweating-to-vasoconstriction interthreshold range from its normal value near 0.2 [degree sign] Celsius, to 4 [degree sign] Celsius or even more. As with general anesthesia, the sweating threshold is increased [nearly =] 1 [degree sign] Celsius, whereas the vasoconstriction threshold is reduced considerably (and not necessary evident in Dr. Mackenzie's patients). It seems likely that both disturbances result from inhibition of normal hypothalamic function, leaving body temperature control to less precise lower centers.
Poikilothermic patients present a unique occasion to evaluate the effects of core-temperature perturbations on behavioral and physiologic functions. Dr. MacKenzie has made the most of this opportunity-and some apparently extraordinarily cooperative patients. Thus, he is able to present the cognitive and autonomic consequences of hypothermia, without the confounding effects of thermoregulatory defenses (as would occur in normal volunteers) or anesthesia (as would occur during surgery).
In summary, the minor interpretive issues mentioned previously in no way detract from Dr. MacKenzie's tour de force. His book clearly presents a comprehensive review of poikilothermia syndrome. This is a topic he is well able to discuss, being undoubtedly the world's authority on the disease. Although the syndrome is rare, it is analogous to the poikilothermia anesthesiologists observe daily in the operating room; thus, there is much for us to learn from Dr. MacKenzie's analysis.
Daniel I. Sessler, M.D.
Outcomes Research[trademark symbol] Laboratory; Department of Anesthesia; University of California, San Francisco; 374 Parnassus Ave, 3rd Floor; San Francisco, California 94143–0648