Letter to the EditorReply to KenneyTravis D. GibbonsTravis D. GibbonsCentre for Heart, Lung and Vascular Health, School of Health and Exercise Science, University of British Columbia-Okanagan, Kelowna, British Columbia, CanadaPublished Online:02 Jan 2023https://doi.org/10.1152/japplphysiol.00699.2022MoreSectionsPDF (187 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat to the editor: We thank Professor Kenney (1) for his support of our findings and also acknowledge his important point about the teleological relevance of heat-induced hyperventilation in humans. Prof. Kenney poses the argument that heat-induced hyperventilation serves a cardiovascular benefit that outweighs its physiological costs, and therefore, still exists in humans.The primary benefit that Prof. Kenney raises is the enhancement of venous return in a setting in which central venous pressure is challenged. In the upright heated, healthy human, this is a unique and considerable challenge. Given that exercising stroke volume is immediately reduced when normal breathing-mediated intrathoracic pressures are blunted (via positive pressure ventilation; 2), hyperventilation may well increase venous return and stroke volume in passive and active heat stress. However, heat-induced hyperventilation can also incur considerable cardiovascular costs independently of its hypocapnic effects. First, during heavy exercise the added ventilation will increase the work of breathing and this will: 1) come with additional cardiac output and oxygen consumption requirements (2, 3), and 2) further stimulate respiratory muscle afferents, leading to locomotor muscle vasoconstriction and accelerated locomotor muscle fatigue (4, 5). Second, the added ventilation will increase the probability of approaching expiratory flow limitation, higher abdominal pressures, and higher expiratory intrathoracic pressures, which will constrain any increases in cardiac filling and stroke volume.It should also be noted that during exercise in the heat, which is presumably the environmental setting that forced our evolution, the typical breathing response is tachypneic hyperventilation (6). This pattern of breathing is less effective in driving intrathoracic pressure excursions and has a higher energetic cost for a given ventilation, making it an inefficient and physiologically costly way to promote venous return. This pattern of breathing mimics what is observed in truly adapted panting species that have the specialized vascular anatomy that justifies the cost of such extreme tachypneic hyperventilation (7).Perhaps the costliest consequence of heat-induced hyperventilation is hypocapnic-mediated cerebral vasoconstriction. Rowell’s idea that the respiratory muscle pump counteracts peripheral volume displacement is valid, but simply turning up the pump to promote venous return comes at the expense of the brain since hypocapnia is a potent cerebral vasoconstrictor. The maintenance of venous return and arterial blood pressure is futile if the most vulnerable organ does not receive any of the perfusion (8).Last, we suggest that human heat-induced hyperventilation has not been selected for, just not selected out. For example, tachypneic hyperventilation in humans resembles [though extremely underdeveloped (9)] the panting response observed in many mammals and birds that absolutely require respiratory evaporative heat loss for species survival. Thus, it is plausible that human heat-induced hyperventilation is a vestigial response akin to excessive hypoxic pulmonary vasoconstriction, i.e., a common maladaptive response in humans that has been largely selected out in native high-altitude species such as the llama (10).DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the author.AUTHOR CONTRIBUTIONST.D.G. conceived and designed research; performed experiments; interpreted results of experiments; drafted manuscript; edited and revised manuscript; approved final version of manuscript.ACKNOWLEDGMENTSI acknowledge Prof. Jerome Dempsey, Prof. James Cotter, and Prof. Philip Ainslie.REFERENCES1. Kenney WL. Importance of hyperthermic hyperventilation. J Appl Physiol (1985). doi:10.1152/japplphysiol.00672.2022.Link | Google Scholar2. Harms CA, Wetter TJ, McClaran SR, Pegelow DF, Nickele GA, Nelson WB, Hanson P, Dempsey JA. 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Harris P, Heath D, Smith P, Williams DR, Ramirez A, Krüger H, Jones DM. Pulmonary circulation of the llama at high and low altitudes. Thorax 37: 38–45, 1982. doi:10.1136/thx.37.1.38. Crossref | PubMed | ISI | Google ScholarAUTHOR NOTESCorrespondence: T. D. Gibbons (travis.[email protected]ca). Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation Collections Related ArticlesImportance of hyperthermic hyperventilation 02 Jan 2023Journal of Applied Physiology More from this issue > Volume 134Issue 1January 2023Pages 131-132 Crossmark Copyright & PermissionsCopyright © 2023 the American Physiological Society.https://doi.org/10.1152/japplphysiol.00699.2022PubMed36592407History Received 18 November 2022 Accepted 23 November 2022 Published online 2 January 2023 Published in print 1 January 2023 Keywordsheathyperventilationhypocapniastroke volume Metrics