Cerebral Oximetry in Lower Body Negative Pressure

The study aims to describe hemodynamic effects of lower body negative pressure (LBNP). 1. If and how changes in cerebral oxygen saturation (StO2) measured by near infrared spectroscopy (NIRS) relate to changes in blood flow in the carotid arteries during progressive LBNP. 2. If and how mean systemic filling pressure (MSFP) can be measured by brief occlusion of blood flow to an arm and if this relates to LBNP-level. 3. If and how pulse pressure variations and photoplethysmographic amplitude variations during positive expiratory pressure (PEP) and continuous positive airway pressure (CPAP) relate to LBNP-level.