Rebuttal from Nybo and Secher

Abstract

Point:CounterpointHumans do/do not demonstrate selective brain cooling during hyperthermiaRebuttal from Nybo and SecherPublished Online:01 Feb 2011https://doi.org/10.1152/japplphysiol.00992.2010cMoreSectionsPDF (42 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations White et al. mention three mechanisms for selective brain cooling that all represent potential routes for cerebral heat dissipation. However, heat removal through the skull is very limited and both at rest and during exercise with and without hyperthermia the main part of the metabolic heat produced in the brain is released via the venous blood leaving the brain (4, 7, 8). Therefore, the brain temperature is mainly dictated by the cerebral metabolic rate, the cerebral blood flow, and the temperature of the arterial blood. As discussed in the Counterpoint in this debate, ventilation-induced precooling of the carotid blood may increase from rest to exercise, but it should be considered that heat removed via this mechanism is restricted to ∼0.1 J·g−1·min−1 compared with a total cerebral heat production of ∼0.6 J·g−1·min−1. Furthermore, our previous evaluation of the cerebral heat balance reveals that <5% of the heat produced by the brain may be removed through the skull (7). White et al. question this assessment of the cerebral heat balance—specifically the reduction in cerebral blood flow during exercise-induced hyperthermia. Yet it has to be acknowledged that the reduction in the perfusion of the brain during exercise with heat stress has been verified by independent techniques (5, 6) and it is well known that cerebral blood flow declines when hyperventilation reduces the arterial Pco2 as observed during passive and exercise-induced hyperthermia (3, 9).When special cooling devices are applied (1) it is possible to create significant cooling of the brain, but in “natural settings,” i.e., resting or freely moving humans the rate of heat loss through the aforementioned mechanisms is limited. It would require unphysiological flow rates and/or temperature gradients if heat loss through the skull or via the emissary veins should have a significant effect on the brain temperature (2, 4).So, are humans capable of demonstrating selective brain cooling? Yes—but humans can also fly in the sky. However, both situations require specialized technical support.REFERENCES1. Boller M , Lampe JW , Katz JM , Barbut D , Becker LB. Feasibility of intra-arrest hypothermia induction: A novel nasopharyngeal approach achieves preferential brain cooling. Resuscitation 81: 1025–1030, 2010.Crossref | ISI | Google Scholar2. Brengelmann GL. Specialized brain cooling in humans? FASEB 7: 1148–1153, 1993.Crossref | ISI | Google Scholar3. Kety S , Schmidt C. The effects of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men. J Clin Invest 27: 484–492, 1948.Crossref | PubMed | ISI | Google Scholar4. Nelson DA , Nunneley SA. Brain temperature and limits on transcranial cooling in humans: quantitative modeling results. Eur J Appl Physiol Occup Physiol 78: 353–359, 1998.Crossref | ISI | Google Scholar5. Nybo L , Møller K , Volianitis S , Nielsen B , Secher NH. Effects of hyperthermia on cerebral blood flow and metabolism during prolonged exercise in humans. J Appl Physiol 93: 58–64, 2002.Link | ISI | Google Scholar6. Nybo L , Nielsen B. Middle cerebral artery blood flow velocity is reduced with hyperthermia during prolonged exercise in humans. J Physiol 534: 279–286, 2001.Crossref | PubMed | ISI | Google Scholar7. Nybo L , Secher NH , Nielsen B. Inadequate heat release from the human brain during prolonged exercise with hyperthermia. J Physiol 545: 697–704, 2002.Crossref | PubMed | ISI | Google Scholar8. Shevelev IA. Functional imaging of the brain by infrared radiation (thermoencephaloscopy). Prog Neurobiol 56: 267–305, 1998.Crossref | ISI | Google Scholar9. White MD , Cabanac M. Exercise hyperpnea and hyperthermia in humans. J Appl Physiol 81: 1249–1254, 1996.Link | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 110Issue 2February 2011Pages 573-574 Copyright & PermissionsCopyright © 2011 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.00992.2010cHistory Published online 1 February 2011 Published in print 1 February 2011 Metrics