Impact of nocturnal oxygen enrichment on high-altitude acclimatization

Nocturnal oxygen enrichment improves sleep at high altitudes but may impair acclimatization. Our purpose was to determine if nocturnal oxygen enrichment impacts acclimatization. A 7-day acclimatization protocol occurred at a field-based research site between 0 and 4,200 m. Participants were housed at 2,800 m and slept with (O₂^þ , 32.3 ± 2.5% O₂) or without (O⁻₂ , 20.8 ± 0.1% O₂) nocturnal oxygen enrichment. Resting and steady-state cycling (5-min, 1.75 W·kg⁻¹) tests occurred on Day 0 (0 m) and Days 1, 4, and 7 (2,800 m). Sleep, vastus lateralis muscle oxygenation [oxygenated hemoglobin (O₂Hb), deoxygenated hemoglobin (HHb)], arterial blood oxygen saturation (Sp_O2), heart rate (HR), and expired gases were measured. Five daily hikes from 2,800 to 4,200 m were also completed. Sleep was longer (P = 0.028) and overnight Sp_O2 higher (P < 0.001) in the O₂^þ (452 ± 63 min, 96 ± 1%) than the O⁻₂ group (427 ± 63 min, 91 ± 2%). The O₂^þ and O⁻₂ groups did not differ at rest in DO₂Hb (−1.47 ± 0.99, −1.46 ± 1.30 A.U., P = 0.901), DHHb (0.78 ± 0.84, 0.51 ± 0.96 A.U., P = 0.202), Sp_O2 (93 ± 3, 93 ± 3%, P = 1.000), HR (59 ± 6, 64 ± 13 beats·min⁻¹, P = 0.229), respiratory exchange ratio (RER, 0.81 ± 0.07, 0.79 ± 0.06, P = 0.274), and ventilation body temperature pressure saturated (BTPS) (10.56 ± 2.12, 10.80 ± 1.96 L·min⁻¹, P = 0.717). The O₂^þ and O⁻₂ groups also did not differ while cycling in DO₂Hb (−2.96 ± 3.03, −1.70 ± 3.46 A.U., P = 0.278), DHHb (7.59 ± 4.65, 6.34 ± 3.21 A.U., P = 0.451), Sp_O2 (90 ± 6, 89 ± 6%, P = 0.875), HR (113 ± 10, 118 ± 16 beats·min⁻¹, P = 0.408), RER (0.89 ± 0.06, 0.89 ± 0.07, P = 0.756), and ventilation BTPS (54.00 ± 15.42, 60.18 ± 18.42 L·min⁻¹, P = 0.371). Sp_O2 while cycling returned toward Day 0 (0 m) values by Day 7 (2,800 m) in both groups (P < 0.001) indicating short-term acclimatization. Nocturnal oxygen enrichment improves sleep but does not impair short-term acclimatization when completing daily prolonged exercise.