In this investigation the researchers explore whether different types of breathing procedures can improve the peripheral oxygen saturation to reduce the risk of becoming a acute mountain sickness or a high altitude pulmonary edema.

Acute mountain sickness (AMS) is a pathological effect of high altitude on humans caused by acute exposure to low partial pressure of oxygen at high altitude. It commonly occurs above 2500 meters of altitude. AMS appears as a collection of nonspecific symptoms acquired at high altitude or in low air pressure resembling a case of "flu, carbon monoxide poisoning, or a hangover".

It is caused by a drop in pressure and lowering partial pressure of oxygen during increasing altitude. The direct consequence of those changes is a hypoxic pulmonary vasoconstriction (Euler-Lijestrand-mechanism). In addition a rise in pulmonary blood pressure (Hypertonia) can occur so that there is a higher risk of developing a high altitude pulmonary edema (HAPE).

In this investigation the investigators are exploring whether different types of breathing procedures can improve the peripheral oxygen saturation. We are comparing breathing with no regulation with two different procedures of hyperventilation during trekking in different altitudes. Procedure 1 (hyperventilation 1) describes inhalation during one step and exhalation during the next step. Procedure 2 (hyperventilation 2) describes inhalation and exhalation during one step.

The effect of the different breathing procedures can be quantified measuring the peripheral oxygen saturation. In addition the investigators are comparing the breathing rate and the minute ventilation as well as the expiratory end-tidal CO2-partial pressure of the three different breathing procedures.

Furthermore, the investigators are examining the ability to concentrate in order to quantify the effect of AMS on organ functions.

Exclusion Criteria:

• acute clinically significant inter-current diseases