Do standard monitoring sites affect true brain temperature when hyperthermia is rapidly induced and reversed

Background: Accurate measurements of brain and core temperatures during warming and cooling of the whole organism, accidentally or therapeutically, are important for studies of thermoregulation and cerebral insults and resuscitation. Hypothesis: During steady states and normal circulation, temperatures in the brain, nasopharynx, esophagus and rectum (the latter are core temperatures) equilibrate quickly; and that during rapid cooling or warming, slight temperature gradients occur, with esophageal core temperature reflecting brain temperature better than rectal temperature. Methods: We evaluated 5 mongrel dogs and 12 pigtail monkeys. The animals were exposed to total body hyperthermia by immersion into water at 45°C to achieve cerebral temperature 42°C which was maintained until cardiac arrest. In monkeys, at cardiac arrest, surface cooling and cardiopulmonary resuscitation were attempted for up to 30 min to determine resuscitability at 38.5°C. Continuously monitored were brain (epidural) (Tep), esophageal (Tes), rectal (Tre) and nasopharyngeal temperatures (Tnp). Also monitored were mean arterial pressure and intracranial pressure. Results: At normothermia, in dogs and monkeys, Tep, Tre, Tes and Tnp correlated well. In the dogs, during heating, Tes, Tnp and Tre at first correlated well. Vigorous panting started as Tep reached 41°C, which immediately lowered Tnp and Tep to increase less steeply than Tes and Tre. After about 40 min of panting, with cerebral perfusion pressure still normal, Tep decreased sharply and reached the levels of Tnp, while Tre remained high. In the monkeys during heating, Tep, Tes and Tre correlated well. When cerebral perfusion pressure decreased below 50 mmHg, Tep declined significantly as compared with Tre, which continued to be high in severe arterial hypotension. Tes at that time achieved levels between Tep and Tre. During cooling in monkeys, the decline in Tre was slower as compared with the decline in Tes and Tep. Conclusions: In normal dogs and monkeys, rectal, esophageal and nasopharyngeal temperatures are almost identical with brain temperatures; but during rapid external warming or cooling, brain temperature is reflected in nasopharyngeal temperature, somewhat in higher esophageal temperature, but not in even higher rectal temperature. For clinical monitoring during temperature changes, one should use primarily esophageal temperature and, if feasible, brain (epidural) temperature as well.