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Permanent residence at high-altitude and chronic mountain sickness (CMS) may alter the cerebrovascular homeostasis and orthostatic responses. Healthy male participants living at sea-level (LL; n = 15), 3800 m (HL3800m; n = 13) and 5100 m (HL5100m; n = 17), respectively, and CMS highlanders living at 5100 m (n = 31) were recruited. Middle cerebral artery mean blood flow velocity (MCAv), cerebral oxygen delivery (CDO2), mean blood pressure (MAP), heart rate variability and spontaneuous cardiac baroreflex sensitivity (cBRS) were assessed while sitting, initial 30 s and after 3 min of standing. Cerebral autoregulation index (ARI) was estimated (ΔMCAv%baseline)/ΔMAP%baseline) in response to the orthostatic challenge. Altitude and CMS were associated with hypoxemia and elevated hemoglobin concentration. While sitting, MCAv and LFpower negatively correlated with altitude but were not affected by CMS. CDO2 remained preserved. BRS was comparable across all altitudes, but lower with CMS. Within initial 30 s of standing, altitude and CMS correlated with a lesser ΔMAP while ARI remained unaffected. After 3 min standing, MCAv, CDO2 and cBRS remained preserved across altitudes. The LF/HF ratio increased in HL5100m compared to LL and HL3800m from sitting to standing. In contrary, CMS showed blunted autonomic nervous activation in responses to standing. Despite altitude- and CMS-associated hypoxemia, erythrocytosis and impaired blood pressure regulation (CMS only), cerebral homeostasis remained overall preserved. [ABSTRACT FROM AUTHOR]

New Findings What is the central question of this study? Cerebral hypoxia impairs cognitive function and exercise performance and may result in brain damage. Residents at high altitude, in particular those with high-altitude pulmonary hypertension, are prone to hypoxaemia due to the exposure to reduced barometric pressure and impaired pulmonary gas exchange. Whether highlanders have a reduced cerebral oxygenation has not been studied., What is the main finding and its importance? We found that despite a reduced arterial oxygen saturation, healthy highlanders and even those with pulmonary hypertension have a similar cerebral oxygenation to healthy lowlanders, suggesting that compensatory mechanisms protect long-term residents at high altitude from cerebral hypoxia., High-altitude pulmonary hypertension (HAPH), a chronic altitude-related illness, causes hypoxaemia and impaired exercise performance. We evaluated the hypothesis that haemodynamic limitation and hypoxaemia in patients with HAPH are associated with impaired cerebral tissue oxygenation (CTO) compared with healthy highlanders (HH) and lowlanders (LL). We studied 36 highlanders with HAPH and 54 HH at an altitude of 3250 m, and 34 LL at 760 m. Mean(±SD) pulmonary artery pressures were 34(±3), 22(±5) and 16(±4) mmHg, respectively ( P < 0.05, all comparisons). The CTO was monitored by near-infrared spectroscopy along with pulse oximetry (peripheral arterial oxygen saturation [ABSTRACT FROM AUTHOR]