Conventional continuous mandatory mechanical ventilation relies on the passive recoil of the chest wall for expiration. This results in an exponentially decreasing expiratory flow.

Flow controlled ventilation (FCV), a new ventilation mode with constant, continuous, controlled expiratory flow, has recently become clinically available and is increasingly being adopted for complex mechanical ventilation during surgery.

In both clinical and pre-clinical settings, an improvement in ventilation (CO2 clearance) has been observed during FCV compared to conventional ventilation. Recently, Schranc et al. compared flow-controlled ventilation with pressure-regulated volume control in both double lung ventilation and one-lung ventilation in pigs. They report differences in dead space ventilation that may explain the improved CO2 clearance, although their study was not designed to compare dead space ventilation within the group of double lung ventilation.

Dead space ventilation, or "wasted ventilation", is the ventilation of hypoperfused lung zones, and is clinically relevant, as it is a strong predictor of mortality in patients with the acute respiratory distress syndrome (ARDS) and is correlated with higher airway driving pressures which are thought to be injurious to the lung (lung stress).

This trial aims to study the difference in dead space ventilation between conventional mechanical ventilation in volume-controlled mode and flow controlled-ventilation.

Inclusion Criteria:

• Adults [18-70] yrs
• General anaesthesia for elective surgery
• Arterial line, central venous line and endotracheal tube as part of standard of care
• Expected duration of controlled mechanical ventilation ≥ 60 minutes
• Supine position (0±10°)

Exclusion Criteria:

• One lung ventilation
• Known pregnancy
• Increased intra-abdominal pressure (pneumoperitoneum or obesity (BMI > 30kg/m2))
• COPD GOLD IV or home oxygen dependence
• Cardiac pacemaker, implantable cardioverter-defibrillator (ICD) or thoracic neurostimulator
• Skin lesions (e.g. injury, inflammation) at the level where the Electrical Impedance Tomography (EIT) band is to be applied
• Clinical signs of raised intracranial pressure
• Potential interference with the surgery due to the setup of the study instruments.
• Patient refusal to participate

During analysis the groups (flow-controlled ventilation and volume-controlled ventilation) will be blinded.

Participants know that they will be exposed to both modes of ventilation, but are unaware of the randomized sequence (FCV-VCV or VCV-FCV).

It is impossible to mask the investigators delivering the intervention.