At high-altitude, increases in haematocrit (HCT) are achieved acutely through altitude-induced diuresis and chronically through erythropoiesis, both of which increase arterial oxygen content (CaO2). Given the impact of alterations in HCT on CaO2, haemoconcentration during acclimatization has been hypothesized to partly explain the reductions in cerebral blood flow (CBF) at high-altitude. However, the degree to which this response mitigates the decrease in CBF has not been experimentally explored. To test this hypothesis, healthy males (n=13) ascended to 5050 m over nine days without the aid of prophylactic acclimatization medications. Following one week of acclimatization at 5050 m, participants were haemodiluted by rapid saline infusion (2.10±0.28 L) to return HCT towards pre-acclimatized levels. Arterial blood gases, HCT, global CBF (duplex ultrasound), and haemodynamic variables were measured following initial arrival to 5050 m and after one week of acclimatization at high-altitude, prior to and following the hypervolemic haemodilution protocol. Following one week at high-altitude, HCT increased from 42.5±2.5 to 49.6±2.5 % (P<0.001) and was subsequently reduced to 45.6±2.3 % (P<0.001) following haemodilution. Global CBF decreased from 844±160 to 619±136 mL/min (P=0.033) following one week of acclimatization and increased to 714±204 mL/min (P=0.045) following haemodilution. Despite the significant changes in HCT, and thus CaO2, cerebral oxygen delivery was unchanged at all time points. Further, these observations occurred in the absence of any changes in mean arterial pressure, cardiac output, pH or partial pressure of arterial oxygen pre and post haemodilution, highlighting the influence of HCT in the regulation of CBF. These findings are the first to experimentally demonstrate, via manipulation of HCT, that haemoconcentration contributes to the reduction in CBF during acclimatization to high-altitude.
Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCA181
Poster Communications: Hemoconcentration reduces cerebral blood flow during acclimatization to high-altitude in humans
C. A. Howe1, R. L. Hoiland1, D. Macleod2, J. C. Tremblay3, H. H. Carter4, A. Patrician1, E. Delorme1, M. G. Rieger1, M. Tymko1, M. Stembridge5, C. Gasho6, A. Williams7, A. Santoro2, D. Green8, P. Ainslie1
1. Centre for Heart, Lung and Vascular Health, University of British Columbia Okanagan, Kelowna, British Columbia, Canada. 2. Human Pharmacology and Physiology Laboratory, Department of Anesthesiology, Duke University, Durham, North Carolina, United States. 3. Cardiovascular Stress Response Laboratory, School of Kinesiology and Health Studies, Queen's University, Kingston, Ontario, Canada. 4. Department of Nutrition, Exercise and Sports, University of Copenhagen, Copenhagen, Denmark. 5. Cardiff Center for Exercise and Health, Cardiff Metropolitan University, Cardiff, United Kingdom. 6. Loma Linda Healthcare System and Loma Linda University School of Medicine, Loma Linda, California, United States. 7. University of British Columbia, International Collaboration on Repair Discoveries, Vancouver, British Columbia, Canada. 8. Sport and Exercise Science, School of Human Sciences, Faculty of Science, The University of Western Australia, Crawley, Western Australia, Australia.
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Where applicable, experiments conform with Society ethical requirements.