PHYSIOLOGY OF EXTREME BREATH-HOLDING By Anthony R Bain MSc, University of Ottawa, 2011 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE COLLEGE OF GRADUATE STUDIES (Health and Exercise Sciences) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) July 2016 © Anthony R Bain, 2016 The undersigned certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis entitled: Physiology of Extreme Breath-holding submitted by Anthony Bain in partial fulfilment of the requirements of the degree of Doctor of Philosophy . Philip Ainslie. Health and Social Development. UBCO Supervisor, Professor (please print name and faculty/school above the line) Supervisory Committee Member, Professor (please print name and faculty/school in the line above) Supervisory Committee Member, Professor (please print name and faculty/school in the line above) Bill Milsom. Science / Zoology. UBC University Examiner, Professor (please print name and faculty/school in the line above) Kevin Shoemaker. Heath Sciences. University of Western Ontario External Examiner, Professor (please print name and university in the line above) July 5th/ 2016 (Date Submitted to Grad Studies) Additional Committee Members include: Neil Eves. Health and Social Development. UBCO (please print name and faculty/school in the line above) (please print name and faculty/school in the line above) ii Abstract The practice of competitive breath-hold (apnea) diving has provided a gateway for studying the physiologic limits of severe hypoxemia and hypercapnia beyond otherwise possible in healthy humans. In elite apnea competitors, the broad objectives of this Thesis were to, a) quantify the impact of peripheral and central chemoreception, and lung volume on the elite dry-land apnea breakpoint, and b) examine the consequences of prolonged apnea on the cerebral metabolic functioning. These objectives were achieved in four experimental studies. Study 1 explored the impact of peripheral chemoreflex silencing with low-dose dopamine. Here, compared to placebo, dopamine blunted the ventilatory response to hypercapnic-hypoxia by ~27%; however, maximal apnea duration was only increased by ~5%. At the breakpoint, arterial hypoxemia was identical with dopamine compared to placebo, indicating that the apnea termination may largely be determined by a threshold level of hypoxemia to maintain consciousness. To eliminate the influence of hypoxia, Study 2 assessed the main determinants of an apnea breakpoint following hyperoxic pre-breathing. Here, the apnea duration was related to the individual forced vital capacity, and unrelated to the central chemoreflex. Respiratory muscle fatigue and pending atelectasis likely determined the capacity of a maximal hyperoxic apnea. Study 3 quantified the cerebral metabolism during apnea. The cerebral metabolic rate of oxygen, measured from the product of cerebral blood flow and the radial artery- jugular venous oxygen content difference, was reduced by ~29% at the termination of apnea. However, there was no change in the cerebral non-oxidative metabolism, calculated from the ratio of oxygen and carbohydrate metabolism. Study 4 examined the cerebral metabolic response in three apneas eliciting separate levels of hypoxemia and hypercapnia. Apneas generating the most severe hypercapnia, irrespective of hypoxia, elicited the largest reduction in the cerebral metabolic rate of oxygen. Moreover, apneas generating the most severe hypoxia, irrespective of hypercapnia, caused a cerebral net release of lactate, suggesting astrocyte glycogenolysis. Together, the findings of this thesis provide new insight into the determinants of an extreme apnea breaking point, and the observations of hypercapnic-induced reduction in oxidative cerebral metabolism provides a tenable mechanism for cerebral protection against prolonged apnea. iii Preface The University of British Columbia Clinical Research Ethics Board approved all experimental chapters (4 through to 7) - ID: H14-00922 Some aspects of Chapter 2 have been published: Bain et al., (2015) Cerebral vascular control and metabolism in heat stress. Compr Physiol, 5(3): 1345-80. Co-authors Lars Nybo, and Philip Ainslie helped with revisions and drafting sections (not included in this Thesis) of the manuscript. Anthony Bain drafted the majority of the manuscript, was responsible for all figures / tables, interpretations, and is contact author. The study presented in Chapter 4 has been published: Bain et al., (2015) Peripheral chemoreflex inhibition with low-dose dopamine: New insight into mechanisms of extreme apnea. Am J Physiol Regul Integr Comp Physiol, 309(9): 1162-71. Co-authors Zeljko Dujic, Ryan Hoiland, Otto Barak, Dennis Madden, Ivan Drvis, Mike Stembridge, David MacLeod, Douglas MacLeod and Philip Ainslie helped with data collection and manuscript revisions. Philip Ainslie, Ivan Drvis and Anthony Bain conceived the study idea and design. Alongside data collection and study design, Anthony Bain performed all data analysis, drafted the manuscript, and is contact author. The study presented in Chapter 5 “Forced vital capacity, but not central chemoreflex predicts the hyperoxic breath-hold duration in elite apnea divers” is being prepared for publication as a short-communication in an academic journal yet to be determined. Co- authors are Otto Barak, Ryan Hoiland, Ivan Drvis, Damian Bailey, Zeljko Dujic, David MacLeod, & Philip Ainslie. Philip Ainslie, Zeljko Dujic, Ivan Drvis and Anthony Bain conceived the study idea and design. Alongside data collection and study design, Anthony Bain performed all data analysis, and drafted the manuscript. The study presented in Chapter 6 “Cerebral oxidative metabolism is decreased with extreme apnea in humans; impact of hypercapnia” is published in J Physiol: Bain et al., (2016). doi: 10.1113/JP272404 [Epub ahead of print]. Co-authors Philip Ainslie, Ryan iv Hoiland, Otto Barak, Marija Cavar, Ivan Drvis, Mike Stembridge, Douglas MacLeod, Damian Bailey, Zeljko Dujic and David MacLeod, helped with data collection and manuscript revisions. Philip Ainslie, Zeljko Dujic, Ivan Drvis and Anthony Bain conceived the study idea and design. Alongside data collection and study design, Anthony Bain performed all data analysis, drafted the manuscript, and is contact author. The study presented in Chapter 7 “Hypercapnia is essential to reduce the cerebral oxidative metabolism during extreme apnea in humans” has been prepared for publication consideration in Journal of Cerebral Blood Flow and Metabolism. Co- authors are Otto Barak, Ryan Hoiland, Ivan Drvis, Damian Bailey, Zeljko Dujic, David MacLeod, & Philip Ainslie. Philip Ainslie, Zeljko Dujic, Ivan Drvis and Anthony Bain conceived the study idea and design. Alongside data collection and study design, Anthony Bain performed all data analysis, and drafted the manuscript. Chapter 4 was presented at the 2016 Okanagan Cardiovascular and Respiratory Symposium. Chapter 6 was presented at the 2015 International Hypoxia Symposium in Lake Louise. A report of all presentations from the conference is published in Kayser B., (2015), High Alt Med Biol. 16(3):261-266. Other first author academic publications from Anthony Bain, not included in this thesis, but performed while conducting the PhD dissertation under supervision of Prof Ainslie at the University of British Columbia, include: Bain AR. (2015) Patent foramen oval: a leaking radiator? J Physiol, 593(20): 4513-4. Bain AR, Ainslie PN. (2014) On the limits of cerebral oxygen extraction. J Physiol. 15(592): 2917-2918. v Bain AR, Morrison SA, Ainslie PN. (2014) Cerebral oxygenation and hyperthermia. Front Physiol. 4: 5:92. Bain AR, Smith KJ, Lewis NC, Foster GE, Wildfong KW, Willie CK, Hartley GL, Cheung SS, Ainslie PN. (2013) Regional changes in brain blood flow during severe passive hyperthermia: effects of PaCO2 and extracranial blood flow. J Appl Physiol 115(5): 653-9. vi Table of Contents Abstract .............................................................................................................................. iii Preface ................................................................................................................................ iv Table of Contents .............................................................................................................. vii List of Tables ...................................................................................................................... x List of Figures .................................................................................................................... xi List of Symbols and Abbreviations .................................................................................. xvi Acknowledgments .......................................................................................................... xviii Chapter 1 - Introduction ...................................................................................................... 1 1.1 Objectives ................................................................................................................. 2 1.2 Specific aims and hypotheses ................................................................................... 3 1.2.1 Study 1 (Chapter 4) ............................................................................................ 3 1.2.2 Study 2 (Chapter 5) ............................................................................................ 3 1.2.3 Study