Effect of moderate altitude and nocturnal oxygen therapy on cerebrovascular function in patients with COPD: A randomized, crossover trial at 2048 m.

We investigated whether nocturnal oxygen therapy improves next-day cerebrovascular function in lowlanders with chronic obstructive pulmonary disease (COPD) staying at moderate altitude. This randomized, placebo-controlled single-blind crossover trial was performed in moderate-to-severe COPD patients [forced expiratory volume in 1 s (FEV₁)/forced vital capacity (FVC) <0.7; FEV₁ 30%-80% of predicted], living at <800 m a.s.l. and arterial oxygen saturation ( $S_{p{O}_2}$ ) measured with pulse oximetry ≥92%. Patients underwent assessments at 490 m and during two separate sojourns of 2 days at 2048 m, receiving either 3 L min^-1 nocturnal oxygen therapy or placebo in a randomized crossover design. At both altitudes, $S_{p{O}_2}$ , cerebral tissue oxygenation (CTO, measured by near-infrared spectroscopy), mean arterial blood pressure (MAP, measured by finger plethysmography) and middle cerebral artery systolic peak blood flow velocity (sMCAv, measured by transcranial Doppler ultrasound) were assessed while patients were quietly breathing with fraction of inspired O₂ ( $F_{I{O}_2}$ ) 0.21, with $F_{I{O}_2}$ 1.0, voluntarily hyperventilating, voluntarily hyperventilating with $F_{I{O}_2}$ 1.0, and during a head-up tilt. Overall, 18 patients (8 women) aged (mean ± SD) 65 ± 5 years, with FEV₁ 54.7% ± 13.9% predicted were analysed. At 2048 m under $F_{{\mathrm{IO}}_2}$ 0.21, patients became hypoxaemic ( $S_{p{O}_2}$ 90.3% ± 1.6%), while MAP, CTO and sMCAv remained unchanged compared with 490 m. All ventilatory manoeuvres at 2048 m induced greater increases in $S_{p{O}_2}$ compared with 490 m, while changes in MAP, CTO and sMCAv were similar. Head-up tilt induced a similar decrease in blood pressure, whereas sMCAv changed less in response to systemic hypotension (ΔsMCAv/ΔMAP 0.9 ± 1.3 vs. 2.3 ± 1.7 cm s^-1 mmHg^-1) at 2048 m. No effect of nocturnal oxygen therapy was observed during any manoeuvres. This randomized clinical trial in moderate-to-severe COPD patients ascending to 2048 m showed that moderate hypoxaemia does not translate to daytime cerebral hypoxia or cerebrovascular autoregulatory impairments while at rest or during ventilatory or orthostatic challenges.