Brief Reports Open camera or QR reader and scan code to access this article and other resources online. Global REACH 2018: Characterizing Acid–Base Balance Over 21 Days at 4,300 m in Lowlanders Andrew R. Steele, 1 Philip N. Ainslie, 1 Rachel Stone, 2 Kaitlyn Tymko, 3 Courtney Tymko, 1 Connor A. Howe, 1 David MacLeod, 4 James D. Anholm, 5 Christopher Gasho, 5 and Michael M. Tymko 1,6 Abstract Steele, Andrew R., Philip N. Ainslie, Rachel Stone, Kaitlyn Tymko, Courtney Tymko, Connor A. Howe, David MacLeod, James D. Anholm, Christopher Gasho, and Michael M. Tymko. Global REACH 2018: characterizing acid–base balance over 21 days at 4,300 m in lowlanders. High Alt Med Biol. 23:185–191, 2022. Introduction: High altitude exposure results in hyperventilatory-induced respiratory alkalosis, followed by metabolic compensation to return arterial blood pH (pHa) toward sea level values. However, previous work has limited sample sizes, short-term exposure, and pharmacological confounders (e.g., acetazolamide). The purpose of this investigation was to characterize acid–base balance after rapid ascent to high altitude (i.e., 4,300 m) in lowlanders. We hypothesized that despite rapid bicarbonate ([HCO 3 - ]) excretion during early acclimatization, partial respiratory alkalosis would still be apparent as reflected in elevations in pHa compared with sea level after 21 days of acclimatization to 4,300 m. Methods: In 16 (3 female) healthy volunteers not taking any medications, radial artery blood samples were collected and analyzed at sea level (150 m; Lima, Peru), and on days 1, 3, 7, 14, and 21 after rapid automobile (*8 hours) ascent to high altitude (4,300 m; Cerro de Pasco, Peru). Results and Discussion: Although reductions in [HCO 3 - ] occurred by day 3 ( p < 0.01), they remained stable there- after and were insufficient to fully normalize pHa back to sea level values over the subsequent 21 days ( p < 0.01). These data indicate that only partial compensation for respiratory alkalosis persists throughout 21 days at 4,300 m. Keywords: acid–base balance; high altitude; hypoxia; metabolic compensation Introduction T here are several important physiological changes that occur to facilitate successful acclimatization in individuals of lowland descent (i.e., born <1,500 m) to high altitude. Among the first and most significant respon- ses to high altitude is an increase in alveolar ventilation. Although this hypoxia-induced hyperventilation attenuates the decrease in arterial blood oxygenation, it causes a con- comitant decrease in the partial pressure of arterial carbon dioxide (PaCO 2 ). Respiratory-induced alkalosis results in an obligatory excretion of bicarbonate ([HCO 3 - ]) following hypocapnia to ‘‘normalize’’ arterial blood pH (pHa) back to sea level values. 1 Centre for Heart, Lung and Vascular Health, School of Health and Exercise Sciences, University of British Columbia—Okanagan, Kelowna, British Columbia, Canada. 2 Department of Kinesiology, University of Windsor, Windsor, Ontario, Canada. 3 Department of Kinesiology and Recreation Management, University of Manitoba, Winnipeg, Manitoba, Canada. 4 Human Pharmacology and Physiology Lab, Duke University Medical Center, Durham, North Carolina, USA. 5 Division of Pulmonary and Critical Care, Department of Medicine, School of Medicine, Loma Linda University, Loma Linda, California, USA. 6 Faculty of Kinesiology, Sport, and Recreation, University of Alberta, Edmonton, Alberta, Canada. HIGH ALTITUDE MEDICINE & BIOLOGY Volume 23, Number 2, 2022 ª Mary Ann Liebert, Inc. DOI: 10.1089/ham.2021.0115 185 Downloaded by 3.89.210.72 from www.liebertpub.com at 09/09/23. For personal use only.