Outcome assessment The primary outcome is occurrence of respiratory insufficiency defined as arterial oxygen pressure (PaO2) <8 kilo pascal (kPa) measured via arterial blood gas sample, or peripheral oxygen saturation (SpO2) <90%, measured peripherally with a pulse oximeter. Secondary outcomes are changes in arterial carbon dioxide pressure (PaO2), arterial oxygen pressure/fraction inspired oxygen (PaO2/FiO2) (arterial blood gas sample) over time and between groups; lung function assessed as inspiratory vital capacity (IVC), forced vital capacity (FVC), forced expiratory flow in the one second (FEV1) and peak expiratory flow (PEF) by a micro spirometer; number of patients with respiratory insufficiency, defined as SpO2 <90%, or PaO2 <8kPa and/or PaCO2 ≥6.5 kPa measured without oxygen supply ; postoperative pneumonia and total length of stay at the wards and at the hospital.