Time course of cerebral oxygenation and cerebrovascular reactivity in Kyrgyz highlanders. A five-year prospective cohort study

Introduction: This prospective cohort study assessed the effects of chronic hypoxaemia due to high-altitude residency on the cerebral tissue oxygenation (CTO) and cerebrovascular reactivity.Methods: Highlanders, born, raised, and currently living above 2,500 m, without cardiopulmonary disease, participated in a prospective cohort study from 2012 until 2017. The measurements were performed at 3,250 m. After 20 min of rest in supine position while breathing ambient air (FiO(2) 0.21) or oxygen (FiO(2) 1.0) in random order, guided hyperventilation followed under the corresponding gas mixture. Finger pulse oximetry (SpO(2)) and cerebral near-infrared spectroscopy assessing CTO and change in cerebral haemoglobin concentration (cHb), a surrogate of cerebral blood volume changes and cerebrovascular reactivity, were applied. Arterial blood gases were obtained during ambient air breathing.Results: Fifty three highlanders, aged 50 +/- 2 years, participated in 2017 and 2012. While breathing air in 2017 vs. 2012, PaO2 was reduced, mean +/- SE, 7.40 +/- 0.13 vs. 7.84 +/- 0.13 kPa; heart rate was increased 77 +/- 1 vs. 70 +/- 1 bpm (p < 0.05) but CTO remained unchanged, 67.2% +/- 0.7% vs. 67.4% +/- 0.7%. With oxygen, SpO(2) and CTO increased similarly in 2017 and 2012, by a mean (95% CI) of 8.3% (7.5-9.1) vs. 8.5% (7.7-9.3) in SpO(2), and 5.5% (4.1-7.0) vs. 4.5% (3.0-6.0) in CTO, respectively. Hyperventilation resulted in less reduction of cHb in 2017 vs. 2012, mean difference (95% CI) in change with air 2.0 U/L (0.3-3.6); with oxygen, 2.1 U/L (0.5-3.7).Conclusion: Within 5 years, CTO in highlanders was preserved despite a decreased PaO2. As this was associated with a reduced response of cerebral blood volume to hypocapnia, adaptation of cerebrovascular reactivity might have occurred.