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.

This study aims to investigate the effect of tracheal intubation on regional ventilation of the lungs by comparing measurements before and after the patient is anaesthetised and intubated. The investgiators aim to show whether altered patterns of ventilation are caused by patients simply being asleep and ventilated, or whether these changes are due to the use of a tracheal tube itself. The exact effect of tube position will also be studied by measuring ventilation as the tube is deliberately advanced until it enters one of the lungs. This will give us information about the ideal position for a tube within the trachea to promote optimal ventilation patterns within the lungs

Inclusion Criteria:

1. Willing and able to consent
2. undergoing a surgical procedure for which the anaesthetic technique includes muscle relaxation, intubation and ventilation for clinical care.

Exclusion Criteria:

1. Unwilling or unable to consent
2. co-existant respiratory disease associated with known abnormalities of ventilation/perfusion matching. This includes chronic obstructive pulmonary disease, asthma, or abnormalities of the chest wall or abdomen affecting ventilation e.g. morbid obesity
3. Requirement for double-lumen tracheal tube for clinical care
4. increased risk of regurgitation/aspiration e.g. pregnancy, symptomatic acid reflux, structural abnormalities of the gastro-intestinal tract (e.g. hiatus hernia)

All patients attending for a surgical procedure are eligible and will be approached.