Pilot trial of positive expiratory pressure for acute mountain sickness.

1 single arm with 3 phases:

• 1: Baseline Breathing (No PEP) Participants breathe normally without any device for 10 minutes in supine position, serving as control.
• 2: Positive Expiratory Pressure (PEP) Breathing Participants breathe through a face mask with a mechanical PEP device set at 10 cmH₂O expiratory pressure for 10 minutes supine. Inspiration is unrestricted; expiration occurs against resistance to improve oxygenation and reduce AMS symptoms.
• 3: Post-PEP Breathing (No PEP) After PEP, participants breathe normally for 10 minutes supine.

Each phase is consecutive at altitudes 4,300 m, 5,560 m, and 6,000 m

The intervention under investigation will be the application of positive expiratory pressure (PEP) at 10 cmH₂O using an Ambu mask. The mask will allow participants to inspire freely while generating a controlled expiratory pressure. The PEP device is a non-pharmacological medical device whose uses in both hospital and pre-hospital settings are part of everyday medical practice.

The Lake Louise Score (LLS) is a standardized tool to diagnose Acute Mountain Sickness (AMS). It combines self-reported symptoms (headache, gastrointestinal upset, fatigue/weakness, dizziness/lightheadedness) rated from 0 (none) to 3 (severe). AMS is diagnosed when headache is present plus at least one other symptom, with a total score ≥3. Higher scores reflect greater symptom severity: 3-5 mild AMS, 6-9 moderate, ≥10 severe. The LLS is used in research and expeditions to quantify altitude illness and monitor response to acclimatization or treatment.