Endurance Athletes Are at Increased Risk for Early Acute Mountain Sickness at 3450 m.

INTRODUCTION: Acute mountain sickness (AMS) may develop in nonacclimatized individuals after exposure to altitudes ≥2500 m. Anecdotal reports suggest that endurance-trained (ET) athletes with a high maximal oxygen uptake (V˙O2max) may be at increased risk for AMS. Possible underlying mechanisms include a training-induced increase in resting parasympathetic activity, higher resting metabolic rate (RMR), and lower hypoxic ventilatory response (HVR). METHODS: In 38 healthy, nonacclimatized men (19 ET and 19 untrained controls [UT], V˙O2max 66 ± 6 mL·min·kg vs 45 ± 7 mL·min·kg; P < 0.001) peripheral oxygen saturation (SpO2), heart rate variability, RMR, and poikilocapnic HVR were assessed at 424 m and during 48 h at 3450 m after passive ascent by train (~2 h). Acute mountain sickness was evaluated by AMS cerebral (AMS-C) score. RESULTS: On day 1 at altitude, ET presented with a higher AMS incidence (42% vs 11%; P < 0.05) and severity (AMS-C score: ET, 0.48 ± 0.5 vs UT, 0.21 ± 0.2; P = 0.03), but no group difference was found on days 2 and 3. SpO2 decreased upon arrival at altitude (ET: 82% ± 6% vs UT: 83% ± 4%; ptime <0.001) with a significantly different time course between ET and UT (ptime × group = 0.045). Parasympathetic activity decreased at altitude (P < 0.001) but was always higher in ET (P < 0.05). At altitude RMR increased (P < 0.001) and was higher in ET (P < 0.001). Hypoxic ventilatory response increased only in ET (P < 0.05) and was greater than in UT after 24 and 48 h (P < 0.05). CONCLUSIONS: Endurance-trained athletes are at higher risk for developing AMS on the first day after passive and rapid ascent to 3450 m, possibly due to an increased parasympathetic activity and an increased RMR, while HVR appeared to be of minor importance. Differences in AMS time course and physiological responses should be taken into consideration when ET are planning high-altitude sojourns.