Residence at moderate altitude improves ventilatory response to high altitude

Background: This study compared the distribution of arterial oxygen saturation (SaO₂) and susceptibility to Acute Mountain Sickness (AMS) in moderate altitude residents (MAR) and low altitude residents (LAR) following rapid ascent to 4056 m. Methods: Resting PETCO₂ and SaO₂ were measured in 38 subjects residing for > 3 mo near Colorado Springs, CO (MAR group), at 1940 m (USAF Academy), and after ∼1 h at 4056 m on the summit of Pikes Peak, CO, following ascent by car. SaO₂ was also measured at 610-m elevation intervals during the ascent. Of the LAR (50 m) group, 39 subjects were exposed to a similar ascent profile in a hypobaric chamber. Results: At 1940 m the MAR SaO₂ and PETCO₂ were 94 ± 1% (X̄ ± SD) and 33.6 ± 2.8 mmHg, respectively. At 3048 m and higher, MAR SaO₂ decreased, reaching 86 ± 2% (p < 0.001) at 4056 m, and PETCO₂ (32.1 ± 4.5 mmHg) decreased (p < 0.05). At 50 m the LAR SaO₂ and PETCO₂ were 98 ± 1% and 38.7 ± 2.7 mmHg, respectively. At 1940 m and higher, LAR SaO₂ decreased (p < 0.001), reaching 82 ± 5% at 4056 m, and PETCO₂ (36.4 ± 3.5 mmHg) decreased (p < 0.05). Above 2438 m, the MAR SaO₂ was higher (p < 0.001) than the LAR. Only one MAR subject, but nine LAR subjects reported AMS symptoms. Conclusions: Ventilatory acclimatization developed during moderate altitude residence substantially enhances arterial oxygenation during rapid ascents to higher altitudes. Compared with prior studies, the level of ventilatory acclimatization achieved at moderate altitude is similar to residing at 4056 m for approximately 5-9 d.