Rescue services in mountainous regions are frequently called to missions at altitudes >3000 m. Under the difficult conditions of acute exposure to altitude, the crews then have to undertake demanding medical and rescue measures. Previous studies in non-medical personnel, such as astronauts, aircraft pilots, and military helicopter pilots have found that the lack of oxygen associated with acute exposure to altitude may impair cognitive functions. No data exists on the effect this may have on the performance of medical staff in terms of patient examination, communication, decision-making, planning, and overall patient care. This study aims to close this knowledge gap. The investigators of this study aim to make rescue missions to high altitude safer for both the patients and the rescuers.

To assess the effect of high altitude on patient care, the investigators recruit highly trained medical specialists who will perform patient care in simulated scenarios both at high altitude and at low altitude. These scenarios will be recorded and the performance of the medical specialists judged by independent reviewers.

The medical specialists will also perform in simulated scenarios at high altitude two more times: once with supplementary oxygen, and once after spending a night at high altitude. the investigators do this to evaluate whether supplementary oxygen improves their performance, and whether symptoms of acute mountain sickness (which usually develop after spending the first night at high altitude) decreases their performance further.

Rescue services in mountainous regions are frequently called to missions at altitudes >3000 m. Under the difficult conditions of acute exposure to altitude, the crews then have to undertake demanding medical and rescue measures, such as an emergency induction of anaesthesia, resuscitation, treatment of polytraumatized patients, or a winch manoeuvre by helicopter in exposed, fall-prone terrain.

The exponential decrease in barometric pressure at altitude results in hypobaric hypoxia (HH), leading to a reduction in the partial pressure of oxygen at every point along the oxygen transport chain from the ambient air to tissue mitochondria. If the body's adaptive mechanisms fail to compensate for the lack of oxygen, symptoms like headache, nausea, fatigue, and dizziness may occur. In addition, HH may impair higher cortical functions. Individuals affected by high altitude frequently do not recognise a decline in cognitive function and overall performance, which can lead to incidents and even fatal consequences.

Previous studies in non-medical personnel, such as astronauts, aircraft pilots, and military helicopter pilots have explored the influence of HH on multiple cognitive domains. Impairment of working memory was described during hypoxia awareness trainings and in pilots while others reported no effects. Some studies have reported reduced psychomotor vigilance, while others found no effects. To counteract these impairments, the European Union Aviation Safety Agency (EASA) mandates simulated hypoxia training for pilots flying rescue missions to above 4000 m.

Studies in medical personnel are few and mostly focused on the quality of cardiopulmonary resuscitation (CPR) at (simulated) high altitude. High-quality CPR, which can be physically demanding for rescuers even under normoxic conditions, leads to rescuer fatigue faster under HH. Two recent studies have shown that HH leads to a lower quality of CPR at simulated and natural high altitude. A smaller study found simulated altitude to have a negative effect on the quality of ventilation but not on chest compression. A recent study reported a slower reaction time in medical personnel at simulated high altitude. Particularly noteworthy is the fact that rescuers did neither notice the reduced cognitive function nor the decreased quality of CPR they provided under HH, even though they were highly trained helicopter emergency medical services personnel. To the investigators' knowledge, CPR at high altitude has only been studied as an isolated skill. No data exist on the performance of medical staff in terms of patient examination, communication, decision-making, planning, and overall patient care. This study aims to close this knowledge gap. The findings of this study may help to broaden the understanding of HH, and lay the ground for further research in high altitude rescue.

As the primary endpoint of this study, the investigators evaluate medical performance and patient care using validated scores for medical skills and non-technical skills (Modified Simulation Team Assessment Tool (STAT), Concise Assessment of Leader Management (CALM), Team Emergency Assessment Measure (TEAM). These three scores are averaged (25% STAT, 25% CALM, 50% TEAM) to form a composite score. Scores are assessed by an analysis of video recordings of the simulated scenarios by independent outside assessors.

Secondary endpoints are described in detail elsewhere. They involve the measurement of reaction speed, risk assessment ability, cognitive function, the presence/absence of acute mountain sickness (AMS), the self-assessment of cognitive capacity, and basic vital functions. The concrete tests and measurements the investigators imply are:

• Psychomotor Vigilance Test (PVT)
• Balloon Analogue Risk Task (BART)
• Digit Symbol Substitution Test (DSST)
• Lake Louis Score (LLS), AMS is present at a score of three or higher
• Self-Assessment of Cognitive Capacity on a scale of 1-10
• Heart rate, Blood-pressure, peripheral blood oxygen saturation (SpO2) as measured by non-invasive means

The investigators will assess medical performance (and all other tests and measurements mentioned above) at four time points (baseline, interventions 1-3).

Testing at 30 minutes after arrival by train at high altitude (the High Altitude Research Station Jungfraujoch, 3450 m, "intervention 1") and at low altitude (Bern, 540 m, "baseline") are performed to answer the main hypothesis; whether acute rescuer exposure to high altitude impairs patient care.

To answer the additional research questions, additional testing will be performed at high altitude 4 hours after arrival with supplemental oxygen (4 litres/min, nasal) ("intervention 2") and after spending the night at 3450 m ("intervention 4").

At the four time points of measurement, the participants will have to run through one of the following four scenarios of simulated patient care:

1. A polytraumatised 35 y.o. patient who fell 12 metres. Adequate treatment will entail: anamnesis, immobilisation of the cervical spine, treatment of a tension pneumothorax, installation of analgosedation.
2. A 64 y.o. patient with acute abdominal pain and nausea due to an acute myocardial infarction. Adequate treatment will entail: Anamnesis, performing and interpreting a 12-lead electrocardiogram, resuscitation.
3. A 35 y.o. mother with her 5 m.o. child who is experiencing a seizure. Adequate treatment will entail: Anamnesis, treatment of the epileptic seizure, recognition of impeding respiratory failure and consequent assisted ventilation.
4. A 19 y.o. hypothermic patient. Adequate treatment will entail: Anamnesis, recognition of likely hypothermia and taking measures to avoid afterdrop, resuscitation according to hypothermia protocols, adequate care after return of spontaneous circulation.

All four scenarios are designed to be of equal difficulty. However, the order of the scenarios is randomised electronically before the start of the study, so that different participants go through different scenarios at all points of measurements (baseline, interventions 1-3). This is done to avoid skewing of performance due to some scenarios being inadvertently more difficult or easier than others.

Before each scenario, the participants undergo all tests and measurements outlined above (PVT, BART, DSST, LLS, Self-Assessment of Cognitive Capacity on a scale of 1-10, Heart rate, Blood-pressure, peripheral blood oxygen saturation).

Supplemental application of oxygen during intervention 2 might bias the participants. Therefore, they will receive air via nasal cannula at the same flow rate during all other scenarios (baseline, interventions 1 and 3) in the sense of a "placebo administration".

Inclusion Criteria:

Medical doctors with certification and experience in preclinical emergency medicine, defined as (all must apply):

• 2 or more years of training in anaesthesiology
• 3 or more months of preclinical work
• 50 or more cases as the lead preclinical physician with potential danger to the patient's life (with a National Advisory Committee for Aeronautics score (NACA) of 4 or more)
• A valid preclinical certification (Swiss "Notarztkurs" or similar)
• Written informed consent to participate in the study

Exclusion Criteria (none must apply):

• Any medical condition known to place the participant at higher risk for hypoxia-induced adverse events (cardiovascular, pulmonary, neurological, otherwise).
• Pre-acclimatisation to high altitude, defined as travel to above 2500 m in the 4 weeks previous to the study

No individual participant data (IPD) will be shared in the primary publication(s). IPD can be made available at a reasonable request.

Participants cannot be masked to whether they are at high or low altitude and whether they have spent a night at high altitude. However, they will not know whether they receive air (baseline, interventions 1 and 3) or supplementary oxygen nasally at the same flow rate (intervention 2).

Outcome Assessors will be fully blinded. They assess the medical performance on video recordings of the simulated scenarios, and will not know which time point a specific scenario has taken place (baseline, interventions 1-3) and whether participants receive air or supplementary oxygen.