Sildenafil increased exercise capacity during hypoxia at low altitudes and at Mount Everest base camp: a randomized, double-blind, placebo-controlled crossover trial - Ghofrani, Ha; Reichenberger, F; Kohstall, M G; Mrosek, E H; Seeger, T; Olschewski, H; Seeger, W; Grimminger, F (Ann Intern Med, 2004)