During any general anaesthetic which involves muscle relaxation artificial breathing is required. This is most commonly provided by pushing air under pressure (positive pressure ventilation) into the lungs via a tube in the airway (the tracheal tube). It has been observed for many years that with this form of breathing the distribution of gas within the lungs differs from that seen during 'natural' breathing: more of the gas goes to the upper parts of the lung than lower parts. This change in how the gas is distributed can lead to problems with how well oxygen is taken up by blood and carbon dioxide removed from the body. Previous work using mathematical modelling has found that the position of the tracheal tube might affect air distribution, but this has previously been difficult to study in 'real life', requiring the use of radioactive dyes and computerised tomography (CT). However a bedside test is now available which allows us to study these changes rapidly and non-invasively, using electrical impedance tomography (EIT). The EIT device is commercially available (PulmoVista®, Draeger UK) and is used in hospitals worldwide as a bedside monitor of lung ventilation.