A 28-year-old woman at high altitude (4500 m) develops acute mountain sickness with headache, nausea, and mild dyspnea. Her ABG shows: pH 7.48, PaCO₂ 28 mmHg, PaO₂ 55 mmHg, HCO₃⁻ 20 mEq/L, SaO₂ 82%. Vital signs: RR 24/min, HR 110/min, BP 118/82 mmHg. She is conscious and alert. What is the most appropriate immediate management?