Earlier studies has shown a correlation between older age and longer time for the rise in the end-tidal O2 concentration during pre-oxygenation. In this study the investigators aim to analyse this correlation more closely and investigate if the arterial partial pressure of oxygen (PaO2) as measured before the start of pre-oxygenation, with the patient breathing air, is a better predictor than age for estimating the time necessary for achieving the goal of pre-oxygenation.

Before induction of general anaesthesia pre-oxygenation is a recommended procedure to enhance the time available for solving a problem with the airway or intubation during induction. This can be accomplished by letting the patient breath normal tidal volumes with 100% oxygen for 3 to 5 minutes during pre-oxygenation. If there is airway closure during tidal volume breathing, pre-oxygenation needs longer time to achieve a maximum effect. Airway closure increases with advanced age and a few earlier studies has also shown a correlation between older age and longer time for the rise in the end-tidal O2 concentration during pre-oxygenation.

Pre-oxygenation is a strongly recommended safety procedure before starting anaesthesia but has at least one major side affect: it is the main reason for the development of atelectasis in the lungs during anaesthesia. Atelectasis impairs oxygenation during anaesthesia and probably increase the risk for postoperative pulmonary complications.

Postoperative complications are more common with advanced age and it is important to understand the mechanisms and risk factors involved.

In this study, the investigators hypothesise that the PaO2 as measured before the start of pre-oxygenation, with the patient breathing air, is a better predictor than age for estimating the time necessary for achieving the goal of pre-oxygenation. If this assumption is correct it might have implication for how pre-oxygenation should be adjusted with advancing age.

Inclusion Criteria:

• Patients scheduled for general anaesthesia in American Society of Anesthesiology (ASA) physical status classification grade I-III.
• Body mass index over 18.5 and less than 30 kg/m2
• Peripheral oxygen saturation 92% or more on air.

Exclusion Criteria:

• ASA IV.
• Patients with active smoking and former smokers with a history of smoking more than 6 pack-years.
• Chronic obstructive lung disease.
• Bloodgas with PaO2 ≤ 8 kilopascal (kPa) or PaCO2 ≥ 6.5 kPa.
• Continuous positive airway pressure treatment at night.
• Heart failure or angina pectoris.
• Hemoglobin value less than 100 g/L.
• Pregnancy.
• Any neuromuscular or neurologic illness reducing ventilatory capacity.
• Any endocrine disease influencing basic metabolic demand.
• Not possible to achieve a tight mask seal.
• Patient unable to follow instructions.

Patients scheduled for elective surgery in general anaesthesia.